Assessing Your Camera s Capabilities for Shooting Raw


Assessing Your Camera's Capabilities for Shooting Raw

In Chapter 6, you learned a lot about how image information is stored in a raw file. To recap: In a raw file, half of the information your camera captures is used to record the highest exposure value (brightest parts) in your scene. Half of what's remaining from that is used for the next highest EV, and so on, for about 8 to 10 stops worth of light. This means that the bright areas of your image will have tremendous detail, while the shadow areas will be represented by very little information.

The practical upshot of this is that if you try to choose exposures that capture lots of bright tones, you'll have a lot of image data to work with when you edit. This will allow you to make edits with far less risk of encountering posterization or tone breaks. If you expose to capture shadow detail, the resulting image will have less data compared to one produced when you expose for the highlights, simply because of the way information is represented in digital form. If you try to brighten those shadow details, the image will become noisy and possibly posterized.

It's much more effective to darken the data-rich bright areas in a raw file than it is to brighten the low-data shadow areas.

The risk, of course, is that if you overexpose, you may clip your highlights and end up with an image with lots of useless white space in it. Fortunately, there are ways to avoid this problem.

Your camera probably provides several methods for controlling exposure, and you'll want to use all of these in conjunction with your camera's histogram display to determine a good exposure for your scene.

Before you go on, you need to determine what exposure tools your camera provides. Raw-capable cameras are usually well stocked with features, so your camera will most likely provide some variant of the capabilities discussed here.

In-camera histogram

Because a histogram provides such an easy way to judge over- or under-exposure, many cameras now provide the ability to view a histogram of any image that you've shot.

On most cameras, to see a histogram of an image, you start by using the camera's playback controls just as you normally would. If your camera has a histogram capability, you will be able to cycle through a variety of information displays, one of which will contain a histogram (Figure 7.3).

Figure 7.3. When playing back images that you've shot, the Canon EOS 20D can superimpose several screens of information. In addition to basic exposure settings, the 20D can display a histogram.


Reading an in-camera histogram is no different than reading the histograms provided by Photoshop. Like Camera Raw, some cameras provide three-channel histograms that let you view separate histograms for the red, green, and blue channels.

Many cameras that provide a histogram display also include special clipping displays. A clipping display indicates clipped highlights in the thumbnail display of your image by flashing any highlight pixels that are clipped (Figure 7.4).

Figure 7.4. In this image, the camera is indicating clipped highlights by flashing clipped pixels in black.


The image in your camera's LCD screen

While the histogram provides an easy way to spot whether you've over- or underexposed, when you shoot raw your camera's histogram is not entirely accurate. The image that's shown on your camera's LCD screen is a JPEG image generated from the raw file. Because it's a JPEG image, it has had a gamma correction curve applied. The histogram that your camera displays is a histogram of this gamma-corrected data, which means that it's not the same as the histogram that you'll see when you edit the image in Camera Raw (Figure 7.5).

Figure 7.5. Unfortunately, when your camera displays a histogram of a raw file (left), it won't necessarily match the histogram you see in Camera Raw (right).


In addition to the discrepancy caused by the gamma-corrected data that your camera is working with, the fact that Camera Raw displays single-channel information accounts for some of the other disagreement that can occur between the two displays.

In general, you'll probably find that your camera's histogram shows shadow values that are fairly accurate, but that it is very conservative about highlight values. Highlight tones that are very close to the right edge of your camera's histogram may have more headroom when viewed in Camera Raw. This is good news, actually. After all, if the histogram on your camera is going to be inaccurate, you'd rather it be overly cautious.

To be certain of how your particular camera works, do a little experimenting. Transfer some images to your computer, but leave copies of them on your camera's storage card. You can then compare the camera's histogram side by side with the Camera Raw histogram. If you find out that your camera's histogram shows highlights as being brighter than they really are, then you should be able to push your overexposures farther than what your histogram indicates.

The effect of your settings on the LCD image

Most cameras provide settings for altering contrast, saturation, sharpening, and a few other parameters. As you learned in Chapter 2, these parameters are used for processing JPEG images and are not applied to any raw files that you shoot. However, your camera may apply these settings to the JPEG previews of your raw files that it builds for display on the camera's LCD screen. Since your camera bases its histogram on these JPEG previews, these parameter settings can have an impact on the accuracy of your in-camera histogram (Figure 7.6).

Figure 7.6. Your choice of in-camera parameter settings can affect your camera-generated histograms. Though the differences are slight, changing the camera's settings affected the histograms of these two images, shot moments apart. In addition to a slight difference in shape, the data in the right image is shifted a little to the right.


In raw mode, shoot some test shots with different parameters and notice how both the image review and the histogram change. Most cameras provide a range of 2 to +2 for each editable parameter. However, 0 usually doesn't mean no correction; it actually means more correction than 2, but not as much as +2, so you may find that a setting of 2 yields the most accurate histogram when you're shooting raw, because it results in the least alteration of your image. However, you probably won't be able to completely eliminate the discrepancy between your camera histogram and Camera Raw's histogram, because your in-camera histogram will still be gamma corrected.

LIVE HISTOGRAM

Some cameras offer the ability to display a histogram while you're shooting. Live histograms are not possible on single-lens reflex cameras, but any camera in which the LCD screen can be used as a viewfinder is capable of supporting a live-histogram feature (Figure 7.7).

Figure 7.7. With a live-histogram feature, you can see a histogram of the scene you're currently shooting, updated in real time as you recompose and change your exposure settings.


A live histogram is automatically updated as you compose your scene and make any changes to your exposure settings. For tricky lighting situations, this can help you zero in on a correct exposure. However, it can be difficult to see the histogram while it's superimposed on a live scene and constantly updating. Personally, I find it much easier to use a regular playback histogram.


Shooting with a histogram

If you're simply taking snapshots or quickly documenting an event or scene, then you're probably not going to stop and take a look at the histograms for every image that you shoot. The tools and procedures we're discussing here are for composing shots that are a little more complicatedshots where you've visualized a particular image that you will shoot and then finish in your image editor.

For these types of images, your shooting workflow will usually be something like this:

  • Shoot an initial image. In most cases, you'll simply use your camera's auto exposure setting for this shot.

  • Review the histogram of this initial exposure and assess whether the shot needs any type of exposure adjustment.

  • Make the adjustment and reshoot.

  • Review the new image's histogram.

If you're shooting in a rapidly changing situation or don't have time to shoot and review and adjust, then you'll want to consider bracketing your shots, which we'll discuss shortly.

Exposure compensation

Obviously, the histogram display is essential for the workflow just described. But you'll also need controls for adjusting the exposure of your image. Your camera may provide several options.

Almost all cameras provide exposure compensation controls. Exposure compensation lets you tell your camera to over- or underexpose by a certain number of stops (or fractions of a stop). You have no control over how the camera achieves this adjustmentit may make changes to shutter speed or aperture or both. In most cases, the way the adjustment is made is irrelevant, so exposure compensation is ideal for making adjustments to your camera's default exposure.

Some cameras provide physical dials or buttons for setting exposure compensation, along with a readout that indicates the current exposure compensation setting (Figure 7.8).

Figure 7.8. To adjust exposure compensation on this Canon SLR camera, you simply turn a wheel on the back of the camera. The exposure compensation display on the camera's status LCD indicates the amount of compensation you've dialed in.


On other cameras, you access exposure compensation through the in-camera menuing system (Figure 7.9).

Figure 7.9. Some cameras require you to use a menu option to change exposure compensation.


In most cases, exposure compensation controls let you over- or underexpose by up to two stops in either one-third-stop or one-half-stop increments (many cameras let you choose the size of the exposure compensation interval).

All exposure compensation moves are based on the camera's initial metering of a scene. So, for example, your camera may meter a scene and recommend an exposure of 1/125 of a second at f/16. If you then dial in an exposure compensation setting of +1, the camera will change to an exposure that's one EV above that initial metering.

To determine how much exposure compensation to use, you can consult your histogram, as you'll see later in this chapter.

Shutter and aperture priority

In addition to exposure compensation, your camera may have special priority modes. Shutter and aperture priority also let you adjust your exposure settings, but with more control than you get with exposure compensation.

In shutter priority mode, you select the shutter speed that you want, and the camera automatically selects a corresponding aperture that yields a good exposure.

Aperture priority works the opposite way: select the aperture that you want, and the camera will automatically select an appropriate shutter speed.

To over- or underexpose in either mode, you'll still need to use your exposure compensation control. However, your camera will try to preserve your priority setting.

So, for example, if you're shooting a portrait and want to blur the background of the image, you might switch to aperture priority mode and select a wide aperture. If you then decide to over- or underexpose with exposure compensation, the camera will try to preserve your aperture setting. (You must use exposure compensation to create the overexposure, because if you open the aperture to try to overexpose, the camera will simply switch to a faster shutter speed to maintain what it thinks is the correct exposure.)

Similarly, if you're shooting a football game and want to ensure that you can freeze the motion of the players, you would choose shutter priority mode and set a fast shutter speed. If you decide that you need to overexpose your shots, you can dial in a positive exposure compensation setting, and the camera will try to achieve that new exposure without altering your chosen shutter speed setting.

Manual mode

NOTE

As with exposure compensation, some cameras provide dedicated buttons for setting priority or manual modes, while others use a menu-based selection mechanism. Whatever scheme your camera uses, you'll want to check your camera's manual to find out how to activate these modes and how to change their settings.


Some cameras offer a full manual mode that gives you control of both shutter speed and aperture. Your light meter still will work as usual and will tell you what it thinks the correct exposure should be, but you're free to adjust the camera's settings any way you want.

Consequently, when you want to over- or underexpose in manual mode, you don't need to use exposure compensation (although you can). Instead, you can choose precisely how you want to achieve your exposure change. This control allows you to be absolutely certain that you preserve the shutter speed or aperture setting that you want for your intended image.

As you've already seen, your camera's exposure compensation readout indicates how much you've altered your exposure with the exposure compensation control. On most cameras, this same readout shows how much you are over- or underexposing when you make changes from the camera's initial metering using priority or manual mode.

ISO: YOUR THIRD EXPOSURE PARAMETER

One of the great advantages of digital photography is that you can change your ISO setting from shot to shot. The ability to alter the ISO setting gives you an additional exposure parameter for times when you can't quite get the shutter speed or aperture that you want.

Just as doubling the size of the aperture doubles the amount of light that strikes the focal plane, or doubling the shutter speed halves the amount of light that strikes the focal plane, doubling the ISO setting makes the camera twice as sensitive to light. This can often buy you some latitude when making exposure choices.

For example, say you're shooting a landscape and you want to use a small aperture to ensure a deep depth of field. Unfortunately, it's late in the evening and the light is dim. Your camera is set to ISO 100. With the small aperture, your camera has recommended a shutter speed of 1/15 of a secondmuch too slow for handheld shooting. If you increase your ISO setting to 200, then you'll gain one stop and be able to shoot at 1/30 of a secondbetter for handheld, but still possibly a little slow. Increase the ISO setting to 400, and you'll be able to use a shutter speed of 1/60 of a second.

Depending on the quality of your camera, the increased ISO setting may yield more noise in your final image, but a little more noise is better than a blurry picture.


Auto bracketing

Bracketing is the process of shooting multiple shots of the same image using different exposures. Bracketing is a great way to safeguard against ending up back at home with a poorly exposed photo. If you're in a tricky exposure situation, or if you're not sure what the best exposure is for shooting raw, then bracketing is a great way to ensure that you get a usable shot.

You can easily bracket using your exposure compensation control. Just shoot a shot at your regular exposure, then use exposure compensation to overexpose, shoot another shot, and then take another underexposed. Or if you know that you don't need any underexposure, shoot several shots overexposed by varying degrees.

How much of an interval to use in your bracket varies depending on the situation. Sometimes you may want to bracket by an entire stop, and other times you may merely want to bracket by a third of a stop.

If your camera has an auto bracketing feature, it can perform these exposure adjustments automatically. Most auto bracketing modes let you select the bracketing interval that you want to use. When you press the shutter, the camera will take an exposure at the default metering. When you press the shutter again, the camera will automatically shoot with your selected underexposure. The next shot will shoot your bracketed overexposure. If your camera has a drive feature, (sometimes called burst or multishot), you can activate it to shoot a burst of automatically bracketed exposures with a single shutter press (Figure 7.10).

Figure 7.10. In auto bracketing mode, my camera automatically adjusted its exposure to shoot three shots: one underexposed, one normal, and one overexposed.


Note that auto bracketing modes work by using exposure compensation to effect different exposures. If you're very concerned about maintaining a particular shutter speed or aperture, then you'll want to use auto bracketing in conjunction with a priority mode, or perform your bracketing by hand in manual mode.

Putting it all together

When shooting raw, you want to capture as much information as you can from the bright areas of your imagewithout clipping your highlightsbecause these are the areas where your camera captures the greatest number of tones. With a histogram at your disposal, it's very easy to ensure that you've done this. If your histogram doesn't have data on the right side, then you're capturing more information from the darker areas of your sceneareas that the camera represents with very few tones.

At the same time, you want to capture an image with a good range of contrast. This means that you don't want all of the information pushed over to the right side of the histogram. Ideally, you want it spanning the entire range. If your scene has a lot of dark tones in itlike the playground image in Chapter 6then your histogram will have a lot of data on its left side. This is not bad. You just want to ensure that the bright parts of your scene are being exposed more toward the right.

Figure 7.11 shows the display from a camera after shooting a flower. The +/0 readout indicates that the camera was shooting at its recommended metering. The histogram shows an image with a good range of contrast and a lot of shadow data (which may actually be clipping), but it doesn't show a tremendous amount of data on the right side. With a little overexposure, we may be able to capture more of the bright areas of the scene, where the camera can capture a greater number of tones.

Figure 7.11. This image has a good amount of data, but it's pretty weak on the highlights and heavily exposed in the shadowstime to try an overexposure to see if I can capture more in the highlights.


Using the exposure compensation control, we make a +2/3 exposure adjustment and shoot again. This time, the histogram shows more information in the bright highlights (Figure 7.12). It also shows some highlight clipping, revealed on the histogram and by flashing black pixels on the clipped areas of the scene.

Figure 7.12. My first overexposure attempt does yield more highlight datatoo much, according to the camera's histogram.


Because we know that the camera's histogram will indicate highlight clipping before it actually occurs in the raw file, we decide to push our luck and do one more overexposure. We already have a couple of well-exposed images to fall back on, and as long as we don't erase them, trying this extra experiment poses no risk to getting a good final exposure.

For Figure 7.13, the exposure compensation was set to +11/3. Now the histogram and clipping display indicate a heavily clipped image, but let's stick with the hunch that things probably aren't as bad as they seem.

Figure 7.13. My camera says that my second overexposure attempt has gone way too far in terms of highlight clipping. I'm going to trust that the camera's histogram is overcautious, and that things will be okay back in Camera Raw.


Back in Camera Raw, the second overexposure does, in fact, have a little highlight clipping that's unrecoverablenot as much as what the camera indicated, but still enough to be annoying (Figure 7.14).

Figure 7.14. My second overexposure was too aggressive. No matter how far back I pull the Exposure slider, there are some blown highlights that I just can't recover.


If we go to the first overexposure, we find a little bit of single-channel highlight clipping, but nothing that can't be recovered. More important, its data is shifted a little more to the right than in the initial default exposure. This means that we've exposed for the bright parts of the image, where the camera is capable of capturing more levels (Figure 7.15).

Figure 7.15. This image has lots of useful data in the highlights and only single-channel clipping, so I can easily recover the little bits of overexposed flower.


The rest of the edits proceed normallyfirst a negative exposure adjustment to recover the highlights, and then slight brightness and highlight tweaks. Finally, some cropping and then sharpening in Photoshop, and the image is complete (Figure 7.16).

Figure 7.16. The final image has a lot of data to work with, so editing is quick and easy, with little concern over artifacts or tone breaks.





Getting Started with Camera Raw(c) How to make better pictures using Photoshop and Photoshop Elements
Getting Started with Camera Raw: How to make better pictures using Photoshop and Photoshop Elements (2nd Edition)
ISBN: 0321592131
EAN: 2147483647
Year: 2006
Pages: 76
Authors: Ben Long

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