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Saturday, 22 October 2011

Adobe Photoshop Lightroom Tutorial - Part X - White Balance

Part I - Introduction
Part II - Lightroom Workflow
Part III - Organizing the Library
Part IV - Keywording and Metadata
Part V - Using Filters
Part VI - Importing Your Images
Part VII - Basic Editing Tools
Part VIII - Developing Your Images, The Basics
Part IX - Reading and Interpreting the Histogram - Basic Adjustments
Part X - White Balance
Part XI - The Tonal Scale
Part XII - Presence Controls
Part XIII - Coming Soon!

White Balance

White balancing an image means adjusting the relative intensities of the colors (in this case, of the three primary channels) to correct the drift. This process is usually performed over white or grey (two neutral colors).

Color Temperature

When adjusting the white balance of an image, you often see the concept of color temperature. This term, borrowed from Physics, describes a characteristic of light. As much as it concerns a photographer, it's sufficient to know the following:

Color temperature is usually measured in Kelvin degrees.
The lower its value, the warmer the color.
The higher its value, the cooler the color.

A photographer would often know the color temperature of well known light sources, to dial into the camera the correct color temperature in advance.

If an incorrect color temperature is used, or if the camera fails in autodetecting it, the resulting colors in the image will have drifted and will appear with an incorrect hue: typically yellow or blue depending on the side of the drift. In the following picture, you can see a strongly incorrect white temperature in a shot taken indoor with a flash:

Incorrectly White Balanced Photo

In the previous picture, the temperature of a tungsten bulb (about 2800 K) has been dialed in the camera and the shot was taken indoor with a bounce flash: you can clearly see how the hues have drifted towards the blue. Now: if you're asking why it's blue and not yellow, it's just because white balance compensates the lightning conditions used to take the shot. 2800 K corresponds to a hot color (a hue towards the yellow), so the compensation is made towards the other side (the blue).

When editing RAW files, Lightroom lets you use the Kelvin color temperature scale to set the color temperature of your image, otherwise the slider will use a [-100,100] range to adjust the temperature. Fortunately, (yet) another advantage of shooting RAW is that you can correct the white balance in post-production without affecting the image nor losing any kind of information; on the other hands, adjustments on a non-RAW file are more limited and they cannot achieve the same level of accuracy that they can achieve on RAW files. This means that, although you should always try to get it right straight out of the camera, you could always go Auto and tune it in post-production.

With Lightroom you can correct the white balance of a shot using the following methods:

You can use the color picker to sample the color of a matrix of 5x5 pixels: Lightroom corrects the white balance against the color of the chosen pixels. To use this method, you have to make sure that some neutral color is present in your shot.
You can use a preset: Lightroom offers some presets with the temperature of many well known lightning conditions (daylight, cloudy, flash, tungsten, etc.)
You can manually dial the color temperature.

In the following picture, you can see the relevant controls in the Basic panel:

White Balance Control

To use the first method, I could choose the color picker (on the left side of the panel) and choose some pixels on the images with neutral color. In this case, I remembered the door (where the two ribbons are hanging from) to be a pretty neutral light gray. If I sample those color, as shown in the following picture, Lightroom would choose a temperature of 5750 K that, indeed, is very close to the correct one (about 5500 K). The 200K difference in the resulting image cannot be appreciated very easily.

White Balance - Color Picker to Choose a Neutral Color

The resulting image is:

Correctly Balanced Photo

If you cannot use the color picker because your image doesn't contain any neutral color, you can balance it manually using your own judgement. The Temp slider is colored, as usually, with a hue scale from blue to yellow, as you can see in the picture of the Basic panel above. If you need to make the colors drift to blue just move the slider to the left and if you need to make the colors drift to yellow just move the slider to the right. The colored slider is a good mnemonic.

As I promised in the previous part of this series, we will check on the histogram what's going on to have a better understanding of each develop setting. In the following picture you can see the histogram of the balanced image and the picture of the unbalanced image with a -1000K compensation and a +1000K compensation:

Histogram of the Balanced Image

Histogram of the Unbalanced Image (-1000K)

Histogram of the Unbalanced Image (+1000K)

The first histogram is the histogram of the balanced image. As you can see, the three channels overlap very well on the right half of the histogram (if you recall what we've seen in the the previous post, Lightroom use the gray color where the three color channels overlap in the histogram). The corresponding pixels are those contained in the big area of the door, which is a neutral gray.

Pushing down the Temp slider adds a blue hue and we can see from the second histogram that the blue channel starts to expand to the right while the green and red channels start to compress to the left. On the other hand, when the Temp slider is pushed up and a yellow hue is added we can see that the blue channel starts to compress to the left and the green and red channels start to expand to the left.

Tint

As we've seen, the effect of changing the temperature is "shifting" (in reality, compressing or expanding) the blue channel apart from the other two channels. Intuitively you could argue that, since the relative position of the green and red channels hasn't changed so much, this process would be insufficient to correctly balance the white, at least in certain circumstances. In fact, you would be right. That's why there's another slider called Tint.

The tint is an adjustment used to correct the green or magenta tint of the image. Lowering the tint value raises the green tint of the image while raising the tint value raises the magenta tint of the image. Why were green and magenta chosen? Let's return to the histogram and see what happens.

The tint adjustment has an effect on the green channel very similar to the one that the temperature adjustment has on the blue channel: it expands or compresses the green channel away from the other two:

When the tint slider is moved to the left, the green channel expands to the right, compressing the blue and the red channels to the left.
When the tint slider is moved to the right, the green channel compresses to the left, expanding the blue and the red channels (whose sum is magenta) to the right.

In the following pictures you can see what happens compensating the tint of the previous image:

Histogram of the Unbalanced Image (Tint -20)

Histogram of the Unbalanced Image (Tint +20)

The tint setting is seldom used, much less than the temperature setting is. However, it's important for you to know how it works since it can really help you fine tune your images when tweaking the temperature setting is insufficient. Here's an example of a picture I took recently (contrast in the mid-tones has been increased with Lightroom):

Picture with a Green Cast

Can you see the green tint in the halo around the silhouette of the building? Well, that's a green cast that must be corrected. You can see how the tint setting has been used, adding a +17 compensation, to remove the green cast from the light. The result can be seen in the following picture:

Green Cast Corrected (Tint +17)

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Adobe Photoshop Lightroom Tutorial - Part IX - Reading and Interpreting the Histogram - Basic Adjustments

The Image Histogram

The image histogram is a graphical representation of the number of pixels of an image with a specific luminance value. The histogram is shown in the Histogram panel that is available both in the Library and Develop modules.

Lightroom - Histogram Panel

The Lightroom histogram is the superposition of the histogram of the three RGB channels using the following color codes:

The histogram is a primary RGB color when the channel histogram of such color is not overlapping any other channel histogram.
The histogram is gray when all of the three RGB channels histograms are overlapping.
The histogram is a secondary RGB color when the two channel histograms corresponding to the primary colors whose mix is the specified secondary color are overlapping:

The histogram will be yellow when the red and green channel histograms are overlapping.
The histogram will be cyan when the green and blue channel histograms are overlapping.
The histogram will be magenta when the red and blue channel histograms are overlapping.

The histogram is an useful tool to inspect the tones used in a photo and it can provide useful insights about the quality of photo. Remember that there's not a good histogram and a bad one. The histogram is just an analytically computed representation of the colors that are present in a photo. Depending on the result you want to achieve, you can use the image histogram to evaluate whether the tones that are present in the photo are the ones you were looking for or not.

The bottom line is: the histogram is a tool and you should learn how to use it. However, use your eyes and your feelings to judge your photo, not the histogram.

The Luminosity Scale is Logarithmic

One of the keys to correctly understanding the histogram is this: the luminosity scale (the horizontal axis) is logarithmic. This may sound confusing at first but it needs not be: photographers are using logarithmic scales so often that many don't even realize they're doing so.

As we've seen in another post, the basic assumption is that our own eyes behave as logarithmic sensors on most of the spectrum range we're dealing with. If fact, you soon realize that other scales we often use are logarithmic as well: the zone system, shutter speeds and aperture values.

How to Read and Interpret an Histogram

Instead of thinking about a logarithmic scale, let's make it simple and just think about f-stops. The f-stop scale is logarithmic as well with respect to the light quantity we're letting in into our camera sensor): an f-stop increment (resp: decrement) doubles (resp: halves) the quantity of light that will hit our sensor. If you're used to think about f-stops, you can conveniently use them when reading and interpreting a luminosity logarithmic axis such as the one you find in histogram, levels and curves graphs. This way, you're life will be easy.

When looking at a graphic with the luminosity in a logarithmic scale (most of which you're using: histograms, levels, curves), just think: equal distances in the logarithmic scale correspond to an equal difference in terms of f-stops. This is also the reason why, usually, such graphics are often divided into a number of squares: to let you think about f-stops and zones. If you look at the screenshot of the Lightroom Histogram panel, you'll notice that it's horizontally divided into 4 segments of equal width. If you consider the histogram to be 8-stops wide (which it often a good approximation, but you should really be aware of your camera dynamic range), every segment is 2-stop wider. Half of a segment will be 1-stop wide. You don't need any more technicality to proceed and use Lightroom proficiently. However, the web is full of detailed information related to dynamic range, gamma correction, tone mapping and so forth.

In the following posts, when interpreting histogram to understand the effect of a develop setting, we'll always use this trick to make things easier.

Basic Adjustments

The editing workflow is where your real Lightroom artist comes out, so that there's not such a thing as a "right way" to proceed. However, the odds that you'll start using the editing controls in the Basic panel are very high, so that's the place we'll start from. Afterwards, we'll describe every panel in detail in the same order it appears in the Lightroom user interface.

The Basic panel contains the most basic image adjustments. Namely, they are:

Treatment: color, black & white.
White balance: temperature and tint.
Tone: exposure, recovery, fill light, blacks, brightness and contrast.
Presence: clarity, vibrance, saturation.

Don't be deceived by their name, though. Basic doesn't mean powerless. On the contrary, they're pretty powerful and they will be fundamental adjustments for your image more often than not.

I also found that their own meaning is deceiving and many people find it difficult to understand what their purpose exactly is. To make sense of them and unleash all of their power, we're going through a detailed explanation of what's their effect and what they're meant for.

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Adobe Photoshop Lightroom Tutorial - Part VIII - Developing Your Images, The Basics

The Develop Module

If you're still reading this series it means that:

You found it interesting, and I'm really happy to hear that this is helping you somehow.
It's now time to start editing your images.

The Develop module is the module in which most of the post-production editing will be done. The structure of the Develop module window is very easy to understand:

Most of the editing tools are located in the panels in the right palette.
The left side palette contains the panels used to manage the presets, the change history and the collections.

The Basics

One big advantage of using a tool such as Lightroom, is that it is a non destructive editing tool. What does this mean and why should it matter to you? Well, this is a very important characteristic that unleashes your creativity without ever worrying about damaging your original files.

When you edit an image in a Lightroom catalog, you're really never modifying the original file. Instead, Lightroom is tracking down each and every modification you apply and stores them in the catalog database. The advantages of this approach are manifold:

You can experiment without restraint. Lightroom is protecting your master files and you won't touch them. Ever.
You can create as many virtual copies of the master as you need, if you want to apply different settings to the same images.
Lightroom will keep track of the complete modification history of each image in the catalog. If, at a later time, you want to review it, it's there for you.
The modification history can be snapshotted at any time. If, for example, you take snapshots of an image at different stages of your work, you will be able to reload a different snapshot at any time in case you want to start over from a previous point of the image history.

Virtual Copies

A virtual copy of an image is another version of the same image to which you can independently apply a set of adjustments of any kind. Although a virtual copy is undistinguishable from a regular image, it's most often used to apply a different set of develop settings to an image.

A virtual copy of an image can be created both in the Library and in the Develop module, selecting the Create Virtual Copy item of the Lightroom Photo menu or of the image contextual menu. In the Library module, a virtual copy is easily identifiable by the "twisted corner" of its thumbnail, as shown in the following picture:

Lightroom - Virtual Copies

In the previous screenshot, the first thumbnail is a regular image, and the other two are virtual copies of the same image with (slightly) different develop settings and metadata (the first is not rated, the second has a rating of 4 and the third has a rating of 5).

You can create a virtual copy at any time: Lightroom will create a new virtual copy with the same settings as the source image at that point in time. After creating a virtual image, you can take advantage of its modification history, copied from the original file as well, to roll back to a different point in time or to select one of its snapshots (if available). And if you feel like starting over again, you can reset an image state using the Reset item of the Develop Settings submenu.

Virtual copies are an useful tool to maintain different copies of the same image without duplicating the image itself. Let's suppose that you want to maintain three versions of an image: a color one, a black and white one and a duotone one. Just create two virtual copies, apply the settings you desire and you'll be able to maintain the three images as if they were completely unrelated. Under the surface, however, Lightroom will only store one image (the master file you imported in the catalog), and you won't suffer any space consumption overhead.

History

Almost every program you use every day supports at least a basic form of command history. When you undo an action, for example, you're rolling back one step in the modification history of your file. Lightroom history, however, is different: it's persistent. This means that the history is stored alongside any other image setting and it will survive Lightroom restarts. Thus, you will be able to examine the history and roll it back at any time you wish. If you're an Adobe Photoshop user, you will recognize in Lightroom this well known Photoshop feature.

The history of an image is stored in the History panel, on the left side palette of the Develop module, as shown in the following picture:

Lightroom Develop Module - History Panel

In the previous picture, you can see an example of the history of an image. The selected item, a modification of the Tint setting, is not the top of the list but it's the third in time: this means that I'm inspecting the state of the image as it was two modification steps ago. In this case, the new value of the Tint setting is 27 and it's a +20 modification from its previous value.

If I continued to work from this state, and applied a new modification, the following history entries would be deleted and the history would start reflecting the image state since where I am.

This is the kind of situation in which you would consider creating a snapshot.

Snapshots

There are times where you where you'd like to be able to freeze an image settings at a point in time. Lightroom will let you do it and this feature is called a snapshot. When you create a snapshot, you're saving the state of the image at the time the snapshot is taken. An image, moreover, can have as many snapshots as you need and, once more, they are a feature that do not introduce any overhead. If you're experimenting different settings, as sure you will, snapshots are the quickest way of pinning those you like them and marking them as candidates for the final image version.

Snapshots can be managed using the Snapshots panel in the Develop module, as shown in the following picture:

Lightroom Develop Module - Snapshots Panel

In the figure above, you can see that the currently selected picture has got just one snapshot, called Low contrast and corrected green cast.

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Adobe Photoshop Lightroom Tutorial - Part VII - Basic Editing Tools

Basic Editing Tools

Before switching Lightroom into the Develop module and start using its powerful editing tools, you should be aware that basic editing tools are available in the Library module as well.

These tools are very simple and you probably won't be using them so often. However, I think it's good for you to know they exist since they can help you perform some quick adjustment in one or multiple images without leaving the Library module. These tools, especially when used with custom develop presets (more on this on the following parts), can save you a lot of work.

The Quick Develop panel (and probably the Histogram panel as well) is the panel that contains the basic editing tools available in the Library module:

Lightroom Library Module - Histogram and Quick Develop Panels

In the current Lightroom version, the Quick Develop panel will let you:

Apply a saved develop preset.
Apply a crop preset.
Apply a color treatment preset: color or black and white.
Apply a white balance preset or modify the temperature and tint incrementing or decreasing their values by fixed steps.
Modify the tones of the images incrementing or decreasing their values by fixed steps. The tonal control available are: exposure, recovery, fill lights, blacks, brightness, contrast, clarity and vibrance.

If, for example, you need to quickly adjust the exposure of an image by an f-stop, you can do it in the Quick Develop panel without leaving the library module.

You can also apply modifications to multiple images in the following ways:

You can select multiple images and apply a modification.
You can copy the modification from one image and paste it into other images using the Sync Settings button.
You can copy metadata from one image and paste it into other images using the Sync Metadata button.

When using the Sync Metadata or the Sync Settings features, Lightroom will also ask you about which metadata or settings have to be copied from the source image, as shown in the following pictures.

Lightroom - Sync Metadata Window

Lightroom - Sync Settings Window

In some circumstances, I often use these features to speed up my workflow. If, for example, I voluntarily underexposed a bunch of images by 1 f-stop to keep the desired ISO, aperture and shutter speed values (if you shoot RAW, you often can), I can correct the exposure with just one click in the Library module. If the correction looks right, I can propagate it to all of the images with just a couple of clicks. Then, I'll continue fine tuning every image using the Develop module.

If you want to help me keep on writing this blog, buy your Adobe Photoshop licenses at the best price on Amazon using the links below.

Adobe Photoshop Lightroom Tutorial - Part VI - Importing Your Images

Importing Your Images

The first step in your workflow with Lightroom will often be importing images into the catalog. Images can be imported from any accessible location from your operating system, including cameras connected to your computer.

To import images into your catalog, use the Import button in the lower left corner of the Library module and the import window will appear, as seen in the following picture:

Lightroom Import Window

In the left side palette, the user can choose the source folder or device from which images must be imported. Especially in the case of entire block devices or big folder hierarchies, it's important to know that subfolders can be automatically imported selecting the Include Subfolders checkbox (it appears only when a suitable source is selected). When the source has been selected, Lightroom will begin scanning it and showing previews of the images it found in the main window.

After choosing the source, you must select how to deal with imported images. Lightroom gives you four choices:

Copy as DNG: Lightroom will convert the images to DNG in a new location and add them to the catalog.
Copy: Lightroom will copy the images to a new location and add them into the catalog.
Move: Lightroom will move the images to a new location and add them into the catalog.
Add: Lightroom will add the images to the catalog.

The option you'll choose will depend on what you need to do, on how you're used to work and on the type of storage system you own. If you're importing files from your camera, you'll probably copy them into your catalog (and optionally convert them to DNG).

As I already explained in the first part of this blog post, I usually copy into the catalog the new photos and the photos I'm working on and that need to always have with me. Over time, when the images are ready to be moved to long time storage, I move them to a different folder of the catalog, usually located in another storage device.

After choosing how Lightroom has to handle the files to be imported, in the case you chose to have Lightroom copy or move them, you have got to choose their destination using the file chooser in the right column of the window.

Depending on the import operation to perform, on the right palette there will be a series of panels where you can tune the parameters of the import operation:

File Handling: This panel lets you tweak the basics of the import operation. You can choose how Lightroom will generate previews of the imported files, whether Lightroom should check for suspected duplicates not to import the same image multiple times and whether Lightroom should perform a secondary copy (a backup) of the imported imaged to a location specified by the user. This panel is always available.
Apply During Import: This is a very useful panel that lets you specify whether Lightroom should apply a develop setting and/or a metadata setting to all of the imported images. It also lets you add a set of keywords to apply to all of the imported images. This is a very handy feature, and you will get used to it very quickly. More information on develop settings will be given in the following parts. This panel is always available.
File Renaming: If you configured Lightroom to copy or move images, this panel can be used to have Lightroom rename the copied files according to the rules you establish. This panel is only available when Lightroom copies or moves files.
Destination: This panel is used to specify the target directory where files will be moved or copied to. The user can instruct Lightroom to organize the files into subdirectories according to date an image was taken. This panel is only available when Lightroom copies or moves files.

Finally, selecting the Import button will start the import operation.

Tweaking Preview Rendering

By default, Lightroom will generate minimal previews of the imported files and will only generate full screen previews when needed. While this is a benefit in terms of space consumption and CPU usage during import, whether is a good choice for you depend on how you work on your images.

In my specific case, I found that generating 1:1 previews during the import operation is beneficial when I'm editing images and continuously jumping from one to another. For the same reasons, I configured my catalogs to purge previews after 30 days.

Depending on your usage patterns, you could find a preset that's more beneficial in your case. If your goal is importing as faster as you can and you rarely inspect imported images, then the default settings or the Standard previews will be sufficient.

On the other hand, if you closely inspect most of the images you import, it might be wise to have Lightroom generate full size preview while importing your images. It will save you time and frustration and won't have to wait for Lightroom to generate a preview every time you open an image for editing.

The deal, as usual, is a tradeoff, and in this case it's a tradeoff between responsiveness and used space. If you don't mind sacrificing some disk space to store full size previews, the user experience will be much better while editing your images. Also, you don't have to wait for Lightroom to generate the previews, either. Indeed, Lightroom will first generate standard previews and you'll be able to use your catalog while Lightroom will generate the remaining previews in the background.

On the other hand, if you're running short of space, you're left only with the choice of using minimal previews and having Lightroom generate them on demand.

However, you can have Lightroom generate previews for an entire folder anytime you need it, using the Render Standard Previews and Render 1:1 Previews items into the Library/Previews menu.

Last but not least, if you want to configure the standard preview size for your catalog, you can do it in the Catalog Settings window, as shown in the following picture:

Lightroom - Catalog Settings

DNG Versus Proprietary RAW Files Formats

DNG is a file format developed by Adobe whose goal is to be a fully compatible, full featured, provider independent digital negative file format. The benefit of converting your RAW files to DNG is that, hopefully, you will be using a standard file format that's being embraced by more and more digital camera producers. You can think of DNG, for simplicity's sake, as the PDF analogous for digital cameras.

The downside of using DNG is that some information found in proprietary RAW file formats cannot be imported into DNG files and may be lost. You can search the Internet for further information about the difference between DNG files and the specific RAW file format you're currently using.

If you want to help me keep on writing this blog, buy your Adobe Photoshop licenses at the best price on Amazon using the links below.

Adobe Photoshop Lightroom Tutorial - Part V - Using Filters

Filters

As explained in the previous part, metadata is useful because it lets you organize your catalog and search for images using an incredible variety of criteria.

The Lightroom Library module is the where most of your searches will take place and it offers plenty of controls where you can start your search from.

As we've seen in the previous part, you can start a quick metadata search using:

The arrow control that is shown at the right side of a metadata value in the Metadata panel (only available for selected headers).
The arrow control that is shown at the right side of a keyword in the Keyword List panel.
Using the Attribute search bar in the filter bar.

The Attribute search shown in the previous part is just one of the kind of searches that the Lightroom filter bar will let you perform. The filter bar, in fact, is the place where you will perform any non-trivial metadata filtering.

Search criteria can also be saved for future use and quick access to frequently used search criteria.

The Filter Bar

The filter bar functionality is pretty intuitive:

It lets you build a query using the search criteria that it offers.
It applies the filter to the images contained in the currently selected folder and all of its sub-hierarchy.
It will display the results in the Lightroom main window (see Part I).

An inactive filter bar is shown in the following picture (if the filter bar is not visible, just activate it using the Show Filter Bar item in the View menu):

Lightroom Library Module - Filter Bar

The filter bars lets you build your query using criteria coming from the following sets, whose interface you can activate choosing the corresponding control in the filter bar:

Text: This interface is used to build a plain text search in one of the searchable text fields.
Attribute: This interface is used to perform a search using one of the special attribute metadata (rating, flag, label, kind) described in the previous section.
Metadata: This interface is used to perform a search using whichever available metadata field.
Saved filter: This menu is used to open previously saved filter or to save the current search criteria into a new filter.
None: the None control is just used to turn filtering off.

The following pictures show the interface of the filter bar interfaces described so far:

Lightroom Library Module - Filter Bar - Text

In the Text type search, the user can make a free text search specifying the text fields to search, the text match criteria (Contains all, starts with, etc.) and the text to search.

Lightroom Library Module - Filter Bar - Attribute

In the Attribute type search, as detailed in the previous part, the user can filter the images using flags, rating, label and kind controls, as can be seen in the previous image.

Lightroom Library Module - Filter Bar - Metadata

In the Metadata type search, the user can build a criteria made up multiple metadata filters, one per column. By default, Lightroom provides 4 columns but the user can add them or remove them at will. To change the metadata a column is associated with, it's sufficient to choose another metadata field clicking on the column label. Each column will contain the available filter values for each metadata field, depending on the values that can be found in your catalog.

Lightroom Library Module - Filter Bar - Multiple Criteria

A filter can be made up of different types of criteria. As you can see in the previous image, different types of filters can be stacked up together to build a complex query.

Lightroom Library Module - Filter Bar - Filter Menu

Lightroom provides a set of prebuilt filters with the most commonly used criteria, such as the Flagged filter, one of the filters I use most. If you want to save a commonly used search criteria into a filter, just select the Save Current Settings as a New Preset... from the filter menu and the filter will be available for future use.

A reduced filter functionality is present in the viewer bar, as it can be seen in the following picture:

Lightroom Viewer Bar

In its upper right corner, in every module where this bar is present, you can select a saved filter using the filter menu and apply a quick attribute filter using flags and ratings.

If you want to help me keep on writing this blog, buy your Adobe Photoshop licenses at the best price on Amazon using the links below.

Adobe Photoshop Lightroom Tutorial - Part IV - Keywording and Metadata

Metadata

Metadata simply means "data about data". In the case we're interested in, images metadata, we're referring to a set of additional data that may be attached to your images to provide further detail about them. Image metadata has been sort of standardized, and the most common sets of metadata you're probably going to deal with are EXIF and IPTC. However, depending on your gear, additional proprietary metadata can be attached to your image. Especially in the case of RAW images, for their own nature, the files must contain the settings that were active in your camera at the time of shooting, so that the RAW developing tool is able to apply them in post-production. Such metadata are highly dependent on the gear you're using and there's no easy way of providing a comprehensive list. However, common examples of such metadata are:

Detailed information about the camera and the lens you were shooting with.
White temperature.
Exposure compensation.
Flash status.
Flash exposure compensation.
Metering mode.
ISO speed.

Some metadata are meant to be provided by the user and IPTC metadata falls almost completely in this category. The user can use IPTC fields to provide additional information about a photography such as:

General information about the image (title, caption, etc.).
Information about the image content (subject, models, etc.)
Information about the image location.
Information about the image author.
Information about the image copyright status.

As far as it concerns image organization, metadata is a powerful tool to help you organize and filter your data. Lightroom gives the possibility of performing searches using whatever metadata header your images may have. That's a pretty useful tool to build your smart collections or dig into your gigantic catalog looking for a specific photo whose location you cannot remember.

Very often, just a few metadata helped me restrict the search scope to a small bunch of them. For example, the camera I used, the lens and the focal length are one of the first criteria I use when digging into my catalog. And if this is not sufficient, then I rely on keywords, as explained in the next sections.

To inspect and modify an image metadata, you can use the Metadata panel, a screenshot of which is shown in the following picture.

Lightroom Library Module - Metadata Panel

If you're interested in a specific set of metadata headers, such as EXIF or IPTC, you can select it in the listbox control in the upper left corner of the panel (in the image above the EXIF set is chosen).

You will be able to introduce custom values in writable metadata headers, such as IPTC. If you want to quickly filter your images using a specific metadata value, you can use the arrow control shown on the right side of the corresponding header. This control is only available on selected metadata headers.

The Importance of Well Known Metadata Headers

Metadata is important because you can take advantage of it at any stage of your workflow, without the need of artificially tweaking it to your advantage unconsciously introducing redundancies and potential inconsistency problems. Metadata can be used, for example:

To organize your images: location or subject metadata can be used to search and build collections.
To edit your images: camera and lens information can be used to apply specific corrections, such as applying camera calibration profiles or lens correction effects.

I've seen many people manually organizing their catalogs duplicating information that could easily be stored in commonly used metadata headers such as the IPTC location headers. There's little use in creating custom keywords to do such a task:

If there's a standard header, just it. Chances are your tool will give you additional feature built around it than what you can do with a plain keyword. A tool could, for example, build a map based on location headers or use the copyright status of your image to choose a suitable watermark, while it could hardly do so using your own custom keywords (that it's unable to interpret).
Custom keywords can be confusing, at times, since they're only labels.

My advice is: take your time exploring EXIF and IPTC headers and learn what they're meant for. They're there for a good reason.

Custom Metadata

Well known metadata such as EXIF and IPTC are such because they're the most widely used metadata headers out there. However, they're not sufficient for most practical organization purposes and you will rely on applying your own (custom) metadata to your images.

Lightroom lets your add the following kind of metadata to your images:

Flags: flagged or rejected.
Colored labels: 5 different colors currently available.
Ratings: a rating from 0 to 5.
Keywords.

Flags, labels and ratings are very intuitive tools and are distinct in that they have a direct effect on how Lightroom shows your image thumbnail in the library module.

Lightroom Library Module - How Some Metadata Is Shown in the User Interface

In the previous image, you can notice the following:

The first two images are flagged, as indicated by the white flag icon on the upper left corner of each thumbnail.
The first image was rated with 4 stars and the second with five, as indicated by the black star icons just below the image thumbnails.
The third image was flagged as rejected, as indicated by the black flag icon on the upper left corner of the thumbnail. Additionally, Lightroom grays out the thumbnail of rejected images, making easier to quickly identify them into the film strip.
The fourth image as been labelled with a yellow label, as indicated by the yellow icon on the bottom right corner of the thumbnail and by the yellow shade of the thumbnail itself.

This metadata is very useful in your everyday workflow because they let your rate your image, reject shots you don't like and flag the shots you are interested to. Lightroom also comes with prebuilt filters that let you quickly filter your images using these metadata.

Lightroom Image Module - Attribute Filters

As you can see in the previous screenshot, the Lightroom filter bar lets you quickly filter in a graphical and intuitive way using the metadata we just talked about:

Using the three flag icons, you can filter flagged, unflagged and rejected images.
Using the five star icons, you can filter images with the specified minimum rating.
Using the seven colored box icons, you can filter images using the specified colored label.
Using the three kind icons, you can filter images based on their types: images, virtual copies and videos.

Keywording

When you need more than all of what we previously described, Lightroom lets you apply custom keywords to your images. Keywords are just text labels that you can attach to your images and every image can have as many keywords as you'd like.

I use many kind of keywords:

Keywords that specify the image type: landscape, portrait, etc.
Keywords that specify the kind of event: holiday, birthday, wedding, etc.
Keywords that specify the kind of post-processing I applied: levels, orton, crop, black and white, duotone, etc.

The keywords you choose to use are up to you, as well as it's only the user who decides how he wants their images to be organized.

The most important advice I can give about keywords is: being consistent and coherent. I surely am about the keywords I use and over the years I've built a keyword dictionary I'm pretty comfortable with. There's no use in using keywords you won't remember. I'm almost always able to figure out which keywords I might have applied to the picture I'm looking for simply remembering how it looked like, when it was taken or the kind of event it represents.

My advice here is: experiment with keywords. At the beginning you're probably going to refine your dictionary very often. Over time, however, you will get used to a well known subset of keywords you will be using over and over again.

Lightroom Library Module - Keywording

The Keywording panel is used to manage the keywords of your images. You can add keywords simply by typing them in the text box shown in the previous figure. Lightroom, using your keyword history and the keystrokes, will help you quickly identify existing keywords showing them in a combo box. Also, based on the keywords already applied to the image and your keyword history usage, will try to help you suggesting candidate keywords in the 3x3 matrix labelled Keyword Suggestions that you can see in the picture.

If you're a keyword power user and your dictionary is made up of hundreds of words, you can organize them in subsets. You can select the subset you're interested to in the Keyword Set combo box and Lightroom will show them in the 3x3 matrix just below it, as seen in the previous picture (in that case, the automatically built Recent Keywords set is shown).

Managing Your Keywords

Keywords themselves can be organized in a tree structure that's handy to use especially when your dictionary is becoming pretty huge.

Lightroom Library Module - Keyword List Panel

The Keyword List panel is used to filter, create and organize your keywords. As you can see in the previous screenshot, the activity and the animal keywords are parent keywords that have some children. Clicking on the white triangle on their left will open their subtree and will show you their sub-hierarchy.

This panel can also be used to know how many pictures are labelled with a specific keyword, as indicated by the number on the right side of the keyword itself.

To quickly filter the images based on a specific keyword, you can use the right arrow icon that appears when hovering your mouse pointer over them.

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