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Aperture and Depth of Field

“In evidence photography, there is never a good reason to minimize the depth of field. The range of distances over which an image is acceptably sharp is one of the most important factors for the evidence photographer to learn[…].”

— Stanford Weiss, Handbook of Forensic Photography

[M]aximizing depth of field is a critical skill. The photographer must know the camera variables that affect depth of field and purposefully select the setting[s] so that it maximizes depth of field.”

— Edward Robinson, Crime Scene Photography

Photographers often use shallow depth of field to isolate a subject from the background. For instance, a portrait photographer may use a large aperture, like f/1.8, to make a model stand out against a blurred background. The technique draws attention to the main subject and tells the viewer what is important in the image. However, when we photograph a fire scene, the entire scene is our subject. All the details are important, even if we don’t realize it at the time that we capture an image. Have you ever reviewed scene photos and noticed something important in the background that made a difference in your case? If not, just wait. It’s bound to happen! You never know what small detail may be important. Therefore, the general rule for forensic imaging is to maximize the depth of field.

Providing adequate DOF in our images also ensures the admissibility of our images in court. According to the Federal Rules of Evidence, in order for a photograph to be admissible it must be a fair and accurate representation of the scene or the evidence it depicts. If large portions of the image are obscured (because of a shallow depth of field for instance) this may invite challenges to the image’s admissibility and provide fodder for an opposing attorney. “How can this be a fair and accurate depiction of the scene? The background is a blurry mess!” “Were you intentionally trying to hide evidence in the background of the photo? Or do you just not know how to take a good photograph?” It’s best to avoid questions like these by taking good quality forensic images. We can provide the greatest scene details and avoid potential admissibility pitfalls by shooting with a large f/number. Exactly how large that number needs to be depends on a couple factors.

DOF for a given aperture is not constant. Both the focal length of the camera lens and the camera to subject distance will affect the depth of field of an image. As a result, figuring out what aperture will achieve a desired result for a certain composition can seem challenging. But with some trial and error, and a little experience, the photographer develops an understanding of how these factors work together. This is a learning process that requires some experimentation. Take several photos at different apertures and view the results. After a while you will start to get a feel for it.

There are a couple of tools that can help the photographer to learn how to use depth of field. One is a depth of field calculator. The depth of field for a given scene can be calculated mathematically. There are numerous apps available that allow the user to enter the focal length of the lens, the focus distance, and aperture.

Factors that increase D.O.F.

Larger f/numbers (f/16, f/22)
Shorter focal lengths (wide angle lenses)
Greater subject to camera distance
Smaller sensor size

Factors that decrease D.O.F.

Smaller f/numbers (f/1.8, f/2)
Longer focal lengths (telephoto lenses)
Closer camera to subject distances
Larger camera sensor

The app will then calculate the depth of field. Do I recommend using a DOF calculator at the fire scene? No. Not even a photo nerd like me is going to take the time to do that. There’s too much else to do. But playing with a DOF calculator during downtime (aka commercials while watching TV) is a great way to gain a better understanding of depth of field. The second tool is simpler and a little more practical for on-scene use. Most interchangeable lens cameras have a depth of field preview button. Pressing this button allows the photographer to see what the depth of field will be in the camera’s viewfinder or on the LCD screen. The depth of field preview button can be helpful in situations where there are foreground elements and background elements that both need to be in focus.

While all of this may seem like a lot to consider, some simple guidelines will ensure adequate depth of field in most cases.

In cases where lighting is adequate, such as in bright outdoor scenes, use the lens’s smallest aperture.
For evidence photos where you want the greatest subject detail possible, set the aperture between f/8 and f/11. Better yet, determine the “sweet spot” aperture for your particular lens. See sidebar to the right.
In all situations, try to keep the aperture at f/8 or higher.
Try to avoid situations where the focus point is in the near foreground as this will drastically reduce depth of field.
Focusing about ⅓ of the way between the foreground and background will achieve maximum D.O.F.

Finding the "Sweet-Spot"

Due to the way that light interacts with the elements in a camera lens, images tend to be sharpest using the middle apertures. Images shot wide open tend to suffer from softness, particularly around the edges. Sharpness also decreases at the smallest apertures due to diffraction. Lenses often perform their best 2-3 stops below their widest aperture. For example, an f/4 lens will usually be sharpest around f/8 or f/11. However, every lens is different. The best way to find the sweet spot for your lens is to shoot the same subject at different apertures and check the results. The next time you want the maximum subject detail possible, you’ll know what aperture to use.

Tips for Macro Shots

The camera to subject distance during macro shooting is typically very small. As a result, the depth of field tends to be extremely shallow, even with large f/numbers. To compensate for this:

Use the largest f/number available.
This may require supplemental lighting and/or a long exposure.
A tripod can be helpful to steady the camera during long exposure macro shots.
Compose the image so that the evidence is parallel to the front of the camera lens. See the example below.
If your camera has focus stacking, try using it. The camera will take a series of shots, varying the depth of the focus point for each shot. The in-camera software then combines the images to create a stacked photo with a large depth of field.

In this example the evidence is angled away from the camera lens. Because the bottom contacts are farther from the lens, the shallow depth of field causes them to be blurry.

In this example the image is composed so that the evidence is flat and parallel to the camera lens. The shallow depth of field has less effect and both sets of contacts are in focus.

D.O.F. - Fire Scene Example:

In the example below we have two items of evidence in a hallway. The purpose of the photo is to show an overall view of the hallway and the spatial relationship between the evidence items. The scene was challenging in two ways. First, there was little ambient light so I had to rely on external flash and appropriate camera settings to achieve a good exposure. Second, the distance between the evidence items required a large depth of field for them both to be in focus.

I took two exposures of this scene. For the first exposure I set the camera in Automatic mode and fired down the hallway. The camera’s algorithm decided on the following settings:

Aperture = f/4.5

Shutter speed = 1/60 second

ISO = 3200

Hallway shot in Automatic Exposure Mode

In this shot, the camera’s automatic focus grabbed evidence marker 2, next to the lighter. As you can see in the cropped image of the evidence can, the combination of the large aperture size, short camera to subject focus distance, and the high ISO noise obscures the background, losing important details.

D.O.F Calculator- Auto Exposure Mode

Above are the results from a depth of field calculation for the first example. The distance from the camera to evidence marker 2 was about 4 feet. With this subject distance and an aperture of f/4.5 the calculator tells us that the depth of field is only about 8". That doesn’t give us much to work with.

Hallway shot in Aperture Priority Mode

For the second photo, to increase the depth of field, I set the aperture to f/16. I kept the 1/60 second shutter speed but decreased the ISO to 500. This ISO setting decreased digital noise, and improved image quality while still producing a well exposed photo. The camera’s flash provided the extra light needed to balance the exposure. I set the focus point halfway between the two evidence markers for a greater camera to focus point distance.

D.O.F Calculator- Aperture Priority Mode

Above we can see the effect of the smaller aperture and the greater camera to subject distance. The D.O.F. in this example goes from 4’ to 11’ ensuring both items of evidence are in focus.

Conclusions

We can see in the images above that the smaller diameter aperture, reduced ISO, and longer focus distance resulted in a much sharper background. It’s even possible to read the evidence label in the cropped image! Clearly, the second image is much better photo from a forensic standpoint.

Depth of field is one of the most important considerations when it comes to forensic work. Shooting with the right aperture ensures that we have enough depth of field so that the entire image is in sharp focus, from foreground to background. By shooting with D.O.F. in mind we can capture much more detail and produce much better images.

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