Exposure Compensation

In the last post I said that once you get a feel for reading histograms, you can start to visualize what a histogram for a particular scene will look like. I'd like to start off this post with a bit of practice doing just that. Pictured above are 3 pieces of construction paper—one white, one gray and one black. In the next set of images, I'm going to put my camera in Program mode and take individual photos of each piece of paper. The paper will be the only thing in the frame. The lighting will be uniform, or at least as uniform as I can get it, shooting here in my kitchen/photo studio. I want you to take a minute now and picture in your mind what the image and the histogram for each of these photos will look like. 

Ok? Got it?

Here they are...

Is this what you were expecting? Probably not. 

If you're thinking that I'm playing a trick on you, I assure you, I'm not. You can try this experiment with your own camera. Your results will be very similar. This little exercise is meant to demonstrate how certain scenes can fool our camera's auto exposure modes. Have you ever noticed that whenever you take a picture of a document it comes out dark? Have you ever noticed that your images of black fire debris are often gray or silver looking? It's because the camera is trying to expose the scene for a middle tone. Averaging light meters operate on the principal that most scenes reflect about 18% of the light falling on them. On a gray scale, that 18% reflectance means around a middle gray, about halfway between black and white, like the gray you see in the three images above. Modern light metering systems are more sophisticated than the old averaging light meters of the film era but they still are still designed to achieve a good exposure for an "average" (i.e. 18% reflectance) scene. That's why my camera's program mode adjusted the exposures so that all three pieces of paper came out as middle gray.  

Scenes that are brighter than average* will tend to come out underexposed. That's why pictures of white paper documents come out looking gray. Scenes that are much darker than average will tend to come out overexposed. The camera in these cases will brighten the exposure so that the blacks and shadows come out around middle gray instead. The latter effect can sometimes work in our favor at fire scenes. When we shoot a pile of fire debris or an object blackened by soot or char, overexposing slightly can make textures and details more visible. (See Expose to the Right- ETTR below). However, overexposure can cause us problems as well. If we raise the blacks and shadows to middle tones we are much more likely to clip highlights, as you can see in the following example.

  *Bright and dark in the context of this discussion refer to color and reflectance not to the amount of light present.

Overexposure in Darker than Average Scenes

Underexposure in  Brighter than Average Scenes

Adjusting Exposure in the Field

Now that you know when and why your camera metering system can get fooled, lets talk about what we can do about it. If you take a shot and the camera's metering system doesn't give you the exposure that you want, there are several ways of correcting it. One way to go about it is to switch metering modes. Center-weighted metering allows us to be selective about what part of the scene we meter from. Spot metering allows us to select the precise spot in the frame we want to meter. However, if the subject we are shooting is much brighter or much darker than average, we will still have an exposure problem. If there is a mid-tone object or area in the scene, we can try the focus/meter-and-recompose technique to achieve proper exposure. We have to be careful with this technique though. Since the metering point and the focus point are commonly tied together,* metering off of an object will also set the focus distance. If that distance is very different from your camera to subject distance, your subject will be out of focus. 

 You can also adjust the exposure of your shot by switching to manual shooting mode. In this case you could take the meter reading off of the grass or another handy reference surface to set the exposure and then shoot the transformer.  While in manual mode, the strong backlighting won't affect the camera settings. If the first shot in manual mode doesn't turn out exactly as you want it, the exposure can be adjusted either by changing the shutter speed or the aperture. This works fine, but it's not my go-to solution.

While it is possible to adjust exposure by changing the metering or the shooting modes, these techniques can be slow and a bit finicky. That's why, for adjusting tricky exposures, I prefer to use exposure compensation.

* It is possible on higher end cameras to change this arrangement so that auto exposure and autofocus are assigned to different buttons. There are pros and cons to this approach.

Using Exposure Compensation

Exposure compensation works with Program, Shutter Priority, and Aperture Priority, the three shooting modes we are most likely to use for forensic and investigative work. Exposure compensation is typically engaged by pressing a button on the camera body to turn it on, then using a command dial to adjust the amount of compensation. The button is usually identified by a +/- symbol as shown in the photo here. Turning the command dial one direction will increase the exposure, typically in 1/3-stop increments. Turning the dial the other way will reduce the exposure. The amount of exposure compensation will be displayed on the rear LCD, in the viewfinder, and on the top LCD if there is one.

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