The Proper Exposure Triangle

Exposure Stops

Exposure Variables

Reciprocal Exposures

The Reflective Light Meter

Normal Versus Non-normal Scenes

Tools for Determining Proper Exposures with Tricky Scenes

Exposure Modes

Bracketing

The F/16 Sunny Day Rule

Common Filters

Chapter Summary

Explain the four exposure variables and their interrelationships

Explain that shutter speeds are not only exposure controls but also motion controls

Explain the concept of the Theory of Reciprocity

Explain how a reflective light meter works

Explain what “tools” are available to help determine the proper exposure for a tricky scene

Explain the typically encountered “non-normal” scenes that require exposure adjustments

Explain the various exposure modes available with different cameras

Explain how to bracket in manual and automatic exposure modes

Explain the F/16 Sunny Day Rule

Explain the basic uses of filters as lens protection, reflection removers, and “sunglasses”

The Proper Exposure Triangle

Images can be considered properly exposed, overexposed, or underexposed. For the most part, it should be a goal to have most images properly exposed. Why just “most” of our images? There will be times when we intentionally “bracket” a particular image.

To bracket an image means that after taking what we consider to be a properly exposed image of a particular subject, we immediately take two or more additional shots of the same subject from the same point of view; these additional shots are intentionally overexposed and underexposed. Usually, there are two good reasons for following one photo with a series of brackets, as are shown in Figures 2-1, 2-2, and 2-3.

One is that the lighting at the scene might be particularly problematic for any number of reasons, and we are actually unsure if our exposure setting on the camera will truly produce a “proper exposure.” In this case, it is just being smart to take additional exposures, in the hopes that at least one of them may truly be “properly exposed.” Think of it as taking “insurance shots.” We won’t get the opportunity to capture this image of the subject ever again, and we want to be assured that at least one of this series of bracketed shots has a properly exposed image among them.

Another good time to take a series of bracketed shots of the same subject is when that subject is a candidate for examination-quality images. As discussed in the previous chapter, these are extremely important images that will be either analyzed or examined in a forensic laboratory by an expert who will be comparing the image to something else to try to establish identity. For example, an image of a fingerprint from the crime scene will be compared to known fingerprints from a suspect. Or an image of a shoe print from the crime scene will be compared to a suspect’s shoe. Or an image of a bite mark on a victim will be compared to the known dentition of a suspect. In these situations, it is extremely important that the examiner be able to distinguish all the details in the image. Even if the image is “properly exposed,” some of the details necessary to make the identification may be in an area that is a bit more brightly lit than the rest of the image. Additionally, some of the details necessary to make the identification may be in an area that is a bit less brightly lit than the rest of the image. In these situations, in order to provide the best opportunity for the examiner to clearly distinguish all the markings within the image that are there, having additional brackets of the “properly exposed” image might help ensure that the examiner can see all that is present in the series of images necessary to make the determination of either an identification or an exclusion.

Exactly how to change the exposure settings to capture a series of brackets of an important item of evidence will be explained immediately after the exposure variables themselves have been explained.

There are four variables that affect an image’s exposure:

If any of these four variables changes, the exposure of the image is changed. To help us remember the four variables that affect an image’s exposure, we group them into what is referred to as the proper exposure triangle. Although a triangle has just three sides, adding the “inside” of the triangle as an element of proper exposure practices allows us to group these four variables together. A typical proper exposure triangle looks like this:

Exposure Stops

Before discussing the individual exposure variables, it is best to understand gross exposure changes first. An image may be described as being properly exposed, overexposed, or underexposed. Of course, any one of the three can happen accidentally, or they can happen because the camera’s exposure variables were carefully chosen to create that effect.

When intentionally choosing to alter the camera’s exposure, it is possible to increase or decrease the exposure by precise amounts. When we discuss exact exposure amounts, the first concept to understand is the idea of an exposure stop. It is possible to take any properly exposed image and to increase its exposure by precisely one stop or decrease its exposure by precisely one stop. A +1 exposure stop means that you’re doubling the lighting of the original image. A –1 exposure stop means that you’re halving the lighting of the original image. You may hear a photographer talk about “opening up one stop,” or “increasing the exposure by +1.” Conversely, you may hear that they intend to “close down one stop,” or “decrease the exposure by –1.”

Many years ago, cameras offered the photographer controls that altered exposures only by a full stop. Today’s cameras, both film and digital, usually allow a photographer to vary exposures by different incremental amounts. Most cameras today include menu selections to change exposures by full stops, 1/2 of a stop or 1/3 of a stop. This point is very important to remember, because depending on how the camera’s menu has been set, changing a camera’s exposure by pushing a button or rotating a dial may result in either a full stop alteration with just one push of a button or one click when rotating a dial—or pushing the same button once or rotating the dial to the next click may only be a 1/3 stop or a 1/2 stop adjustment. You might be trying to increase or decrease exposure by +1 or –1, but if your camera has been set previously to provide 1/2-stop increments or 1/3-stop increments, one push of a button or one rotation click of a dial may not provide you with a full stop change. The solution is to check your camera’s menu selections, especially if the camera you are using has been used by another photographer who prefers different increments of bracketing.

Exposure Variables

Now that you understand how an exposure can be altered in precise amounts with both overexposures and underexposures, it is now time to learn how this information applies to each of the exposure variables.