Index by Subject

Note: Unusual subjects, times of day, or lighting will of course influence the histogram. You should not attempt to always get a nice well-balanced curve positioned in the middle range of the graph (such as the histogram above). For instance, a snowy scene outdoors will have the majority of tones in the graph positioned on the right side of the histogram. A foggy morning will show a limited range of tones, perhaps centered in the middle. A shot in a darkened theatre will have more than normal tones concentrated on the left part of the graph.

The point is that these are what you would expect and want, in order to render the image as you normally would see it. My advice is not to try to change these natural contrast results, unless you have a specific reason to do so --

Histogram Calibration Target

Histogram from the Calibration Target

 

Canon 10D Parameters MenuParameter settings - for high contrast in this case

When the image contrast exceeds your camera's contrast range, save the highlights!:
If the contrast range in the scene is larger than the camera’s dynamic range, and none of the previous suggestions for physical measures (adding more light, etc.) or timing/ viewpoint modification (shooting at a different time, etc.) are sufficient, then the preferred alternative is to expose in a manner so that the highlights are preserved. A surprising amount of shadow detail can often be recovered later in post-exposure image proces-sing, particularly if shooting in RAW mode, but blown-out highlights are irretrievably gone.
If your LCD panel shows a blinking out-of-gamut display, look closely at where it is indicating that highlights will be lost. Highlight areas that are NOT detailed (overcast sky, specular “mirror-like” reflections, etc.) can be sacrificed if necessary to retain important shadow detail.

 

Contrast Control in Digital Photography

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Contrast Control: Camera Settings and Exposure Methods

In addition to physical measures and timing or position measures, photographers also have a range of camera tools available for trying to match camera dynamic range with image contrast. Many of these suggestions will depend upon the particular features of your digital camera. I have tried to make them broad general guidelines that would be useful to any photographer, but inevitably, there are some that are specific to the cameras that I know. I work primarily with the Canon line of cameras, but Nikon, Fuji, Pentax, Minolta, and others frequently have a similar feature, though it may be called something else.

Use your histogram!:
If you have a histogram display on your camera, take low-resolution test shots with different exposure levels (using exposure compensation or program shift to override the camera’s default exposure setting) and then view the resulting histograms. Try to achieve an exposure that is well centered between the light and dark extremes of the histogram graph, and that there are no detailed highlight or detailed shadow pixels which are cut off (shown as a band at the left or right extreme of the graph) by the exposure. This is the ideal exposure for a normally lit scene. However, given the lighting situation, it you may not be able to achieve that with any exposure.

Don't throw away your grey card when you switch to digital:
The 18% grey card has a long history of helping film photographers to achieve good exposures. By filling the camera’s sensors with a known grey value that corresponds with the camera’s assumptions, a photographer can ensure that darker and lighter tones will be placed correctly. Digital photographers can also make use of this technique, with a few twists. In complex or difficult lighting situations, place the grey card (or often a grey cloth that is the same darkness is easier to use) in the same light as the scene you want to shoot. Either move the camera close to it, or zoom the lens to mostly fill the frame with that shade of grey. Make sure that your camera is set to display as much information as possible on the screen, and when it shows the recommended aperture and shutter speed, remember or write it down. Then remove the card, and focus on your scene. If the meter shows a different setting, use your exposure compensation controls or set your camera on Manual mode, and modify the exposure setting until it matches the grey-card reading. This will help to guarantee the best exposure level, given the lighting. You can, of course, change this somewhat if you want the scene to be lighter or darker than it actually is.

Use a histogram calibration target:
You can also make use of a clever idea to optimize the contrast in your digital exposure, if your camera includes a histogram display. Instead of a grey card, this technique makes use of a white/grey/black card to help you assess and modify exposure to match the lighting that you have. The idea is that you first take a close-up picture of a three-part card that has a total black section, an 18% grey section, and a pure white section. You can either make this yourself, or purchase one (I’ve seen them advertised on Adorama under the name “Digital Calibration Target”. Rather expensive for what you get, though -- $99 !)
When you review the histogram of the test shot, the white and black sections should show on your histogram display as spikes (since they are essentially single values), while the grey should show as a bump in the middle of the graph. By the placement of these three markers, you can judge whether your exposure is going to be well-calibrated. If, for instance, the white spike is positioned at the right extreme of the chart, spilling over and cutting off, you would know that there is a good chance your highlights will burn out on the actual shot. Similarly, if the spike representing the black section is pinned against the left axis of the graph, it would indicate that you could potentially be in danger of losing shadow details in your image.
Depending on your review, you would then modify your exposure up or down to hopefully center the values and ensure that both the white spike and black spike are entirely visible and that there is a little room left on either side of the histogram. If BOTH spikes are pinned to opposite ends of the graph, it is a good indication that the lighting you have is simply too contrasty. In that circumstance, you would try some of the methods described in this article to lessen the contrast somewhat, through different lighting, moving position, changing the camera’s contrast settings, and so on.

Does your camera have parameters?
If your camera allows you to manipulate image contrast parameters directly (as with the Canon 10D), then this can be an extremely valuable tool for matching your camera’s dynamic range with the scene you are photographing. This custom setting enables you to override the default sensor settings, and to record images at higher or lower contrast levels. Note: If you are shooting in RAW mode, this contrast parameter is applied when you “develop” the RAW digital negative, rather than at the time of exposure. For all other shooting modes, however, you can specify settings ranging from +2 to -2 where contrast is enhanced (for flat lighting) or minimized (for hard, contrasty light). This can make all the difference when working with challenging lighting. Remember to do a test shot and look at the histogram to verify that the curve has been expanded or compressed by the proper amount.

The best of both worlds?
If the scene that you are shooting is static, and the light is not changing rapidly (at dawn, dusk, or on cloudy/stormy days with fluctuating light), there is one more suggestion, which depends upon a tripod. You DID bring along your tripod, didn’t you? From my own 25 years experience, and that of many other photographers, simply shooting more and more of your images from a tripod (especially if the lighting is not really bright) is the single most valuable suggestion for improving all of your images. By using a tripod, you can respond to the challenge of a very wide dynamic range by taking two shots instead of one. The first shot can be optimized to capture the highlights most completely, without worrying about the shadows. In the second shot, of course, you expose for the shadow details and let the highlights blow out. Photoshop or other image processing programs are then used to combine the two exposures, so that you get the best of both worlds for the difficult lighting conditions.

I hope that this guide to digital contrast control will prove useful to you - it is the distillation of many years of photographic learning. Please feel free to print it out, and drop me a line at my website email address: ed@edaites.com . I always like getting feedback, and I'd also be interested in hearing any further suggestions from photographers who've developed additional methods of handling digital dynamic range.

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1: Contrast Control Overview
2: Filters and Lighting Methods
3: Timing and Position Methods
4: Camera Settings and Exposure Methods


 
 

 

  
 

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