Depth of Field: Is more always better?

Note: This photo tip page requires a firm grasp of the concepts of depth of field and how it is correlated to focal length and aperture. I'm assuming the reader has a basic knowledge of these concepts. Any good photography book (not just underwater ones) will cover this in enough detail to bring readers up to speed for this photo tip. (Yes, this article was written before the digital age, but the concepts are still 100% accurate.)

Obviously, I hang around a lot of underwater photographers. I lead photo trips, teach photo classes, give out advice (sometimes unwanted!) and hear a lot of underwater photographers talk photography. I'm often surprised how most people discuss depth of field. It is obvious to me that many of them understand the concept but not the application of depth of field in underwater photography.

When we're talking about having money, more is always better, but when we're talking about depth of field, more is not always better. Often it is better, yes. Always? Hell no. Many photographers never shoot a macro lens any wider than f/16. They buy the largest flash units in the world so they can blind everything shooting at f/22, or in the case of an extreme macro lens, effectively at f/44 or f/64. Why? "Because more depth of field is better" they say. Why?

I'm here to tell you that more is not always better. Before I argue the point, let me start with a story. Once upon a time there was a fantastic underwater camera called a Nikonos. Unfortunately, it was not an SLR, so the photographer could not actually see through the same lens that the film would be exposed through. It's what we call a rangefinder camera, meaning that the photographer looks through a window on the top of the camera that approximates the view of the camera. Because the viewfinder is always in focus (your eye does the focusing), you can't tell by looking through the viewfinder if the lens is focused correctly on the subject, so you have to guess. If you guess that something is 4 feet away and it's actually 5 feet away, the picture might be out of focus. If you guess 4 feet and it's actually 4.5 feet, it might be close enough that the focus will be okay. With the Nikonos, everyone wants as much depth of field as possible to compensate for the inaccuracy in focusing. When shooting with my Nikonos and 15 mm lens in open water and ambient light (as I do with large marine mammals) I like to have enough depth of field that I won't have to refocus too often. Sometimes animals are moving too fast and all I can hope to do is shoot away and concentrate on composition. If I know that I have good depth of field from 3 feet to infinity, I really don't need to worry at all about focus unless the animal comes really close. This is handy.

Unfortunately, the Nikonos has taught generations of underwater photographers that more depth of field is better. This has carried over into macro photography where most people are dealing with extension tubes or SLR's with autofocus. In this case, we no longer need depth of field to compensate for the inaccuracy of focusing. The Nikonos framer tells us how far away to put the camera for proper focus, and the SLR can focus accurately on the subject.

For extreme macro work, (let's pull a number out of the water here...say 1:3 or closer) the reproduction ratio of the lens limits the depth of field significantly, and most people want to shoot at f/16 or f/22 to be sure that the subject is in focus from the front to the back. It can be kind of annoying to have a fish eye in focus but the lips (closer to the camera) out of focus. This is a good time for maximum depth of field.

However, at reproduction ratios somewhat less than 1:3, shooting with maximum depth of field can be a detriment to the image. Why? Having a lot of depth of field can make the background very sharp and distract the viewer from the subject. Furthermore, lenses are sharpest in the middle of their aperture range (around f/8). Due to diffraction, at f/22 most lenses are somewhat softer than at f/8, even though they give more depth of field.

This huge interest in maximum depth of field is very common in underwater photographers. It is much less common in surface photographers. Portrait photographers rarely shoot at f/22. They know that a person should stand out from a background. If you shoot at f/4 to f/8 (depending on the reproduction ratio), you can get enough depth of field to keep the entire face in focus but throw the background out of focus. This is important because the eye goes to the sharpest thing in a photograph. If the idea is to draw one's attention to the face, you don't want the trees in the background to be sharp. They distract the viewer and clutter the photograph. Same thing underwater. You want the fish in focus and the background a little soft so the fish stands out and the background doesn't interfere.

A common trick in television is to run closing titles of a film over video that is ever so slightly out of focus, shot that way on purpose. This makes the titles very sharp in comparision to the video, and makes them jump out better and be easier to read as they scroll by. It's the same concept.

Now here's a real life example. Figure 1 is the cover of Sport Diver featuring a photograph of mine taken in Fiji. It's a blue ribbon eel sticking out of the reef. Many people commented on how this eel seemed to jump off the page. I got several questions when this cover came out from people who wanted to know how I made it look so three dimensional. I sold several prints of this image. What was the trick?

No trick. I shot the image at f/8, f/11 and f/16, knowing that if I shot too stopped-down, the reef in the background would be in focus and distracting. I didn't know exactly which f-stop would be best, so I experimented. When I got the film back, the f/16 image was too sharp in the background. I sent the f/8 and f/11 shots to the magazine and the editor chose the f/8 version (a horizontal) and blew it up and cropped to a vertical for the cover. With the face of the eel razor-sharp, the image was able to be enlarged and cropped to a vertical no problem, and with the background nicely out of focus, the text on the magazine cover seemed to jump right off the page. Most photographers when faced with a situation like this simply set it on f/22 and blast away, because "more depth of field is better". I can tell you for certain that the f/16 versions of this image were not nearly as stunning. They were not cover material. Without the f/8 version of this shot, I never would have gotten the cover of this issue. Furthermore, at f/8 I was only shooting at 1/4 power on the strobes, so they recycled instantly. Gotta love that!

People often think of composition in terms of 2 dimensions. Think of depth of field as your 3rd dimension in composition and think about how much you need to make the impact you want. I'm not saying that f/22 is bad. Often f/22 is the best answer to the depth of field question...but not always.

How about another example? The other effect you get by opening the aperture (in addition to a reduction in depth of field) is an increase in the amount of ambient light that reaches the film. Most underwater macro shots have a black, underexposed background, even when shot in broad daylight, because at f/22, there is simply not enough ambient light to register on the film. By opening up to f/11 or f/8 and shooting upward slightly, you can get some nice blue water in the background of a shot rather than a boring black background.

Well, that's tip #1. I hope you enjoyed it!


More Photo-tips>>

Figure 1: Blue Ribbon Eel at f/8. Note that the background is out of focus, thus making the face of the eel and the text on the page jump out in a 3D-like effect.

Here's a bunch of feather duster worms in the Bahamas shot with a 50 mm macro lens at f/16. This shot was taken near noon, but the background is dark because at f/16 there is not enough background light to register on the film.

It's a subtle difference, but at f/5.6 the background light registers on film and the background is a nice pleasant blue. Isn't that better? At f/5.6 there is still enough depth of field for all the feather dusters to be in focus.

Here's another example. The nice blue background really makes this image pop. You can't get it at f/16! This was shot at f/6.3.

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Last Update 12/20/02