By Ron Eggers
One difference between a serious photographer and a casual photographer is the level of expertise honed from years of shooting experience. A variety of tools are available to help you sharpen photographic skills and insights. The depth-of-field guide, which has been around for some 30 years, has been revised recently with the introduction of ExpoAperture2 Depth-of-Field Guides from ExpoImaging.
For too many photographers, depth of field is a relatively vague concept of what's in focus and what isn't. In fact, you can determine depth of field very precisely, mathematically. Many fixed-focus lenses and some zoom lenses have depth-of-field guides marked on their barrels. Some cameras also have depth-of-field preview capabilities. These work well enough while shooting. But they aren't much help in planning a shoot.
ExpoDisc inventor George Wallace (not the governor, nor the comedian) devised a depth-of-field calculator in 1978, making it possible for photography students and other photographers interested in expanding their photographic abilities, to know exactly what would be in focus and what wouldn't.
That was a time in photography when there was a minor resurgence of the objectives of the Group f64, and the style of photographers like Ansel Adams and Edward Weston, where everything in a frame was as sharp as a tack. To ensure those results, some photographers turned to depth-of-field guides.
DOF guides were also employed to control depth of field creatively. With it, a photographer could determine what would be sharp and what would be softer in a composition and set the exposure accordingly.
The two ExpoAperture2 Depth-of-Field Guides are refined versions of that early DOF guide. Both are dual sided. The original disk covers lens focal lengths from 15mm to 135mm. The second covers 70mm to 600mm. With either disk, a good starting point is to find the aperture for the desired depth of field.
The first step is to set the sensor size on the lower side of the outer dial. Both disks include preset format sizes range from 1.7X (with 35 mm being 1X) to 6x6 cm. A reference card is included that lists many of the popular DSLR bodies and their sensor factor, which is the same as the lens conversion factor when shooting with that body and 35mm lenses. A sensor factor of 1X indicates that the sensor size is the same as a 35mm frame. The card, and the ExpoAperture2 guides, include half-frame 35mm, APS, 35mm, 120, 220 and 620.
Using the first disk, the 35mm setting is used here as an example. It works the same with either 35mm film or a full-frame sensor. After setting the format size on the outer ring, set the focal length of the lens being used in the yellow window on the inside of the dial. Since a 50mm lens is the standard lens of a 35mm camera, that focal length serves as a good example.
Then flip the disk over to the Distance Dial and find the distance that the primary subject is going to be away from the lens. For this example, try 7.5 feet. The next step is to figure out how much depth of field is required to come up with the desired composition. There might be numerous elements in a frame, but only a few of those elements should be sharply in focus.
That might be from 6 to 10 feet. Then count the number of alternating gray and white segments inside the outer distance dial. That would be two zones. Flip the disk over again. This time, on the gray inner dial, find the number of zones. The aperture that's on the blue dial above zone number, in this case f9, is the f/stop required to achieve the desired depth of field.
Conversely, it's also possible to find the resulting depth of field from a specific aperture. The first two steps of setting the sensor size and lens focal length are the same. Then find the f/stop that's going to be used on the outer blue ring. Find the number of zones beneath the aperture, on the gray dial. For f/9, that would be two. Flip the disk over and find the distance to the primary subject. Using 7.5 feet again, two zones indicate everything from 6 to 10 feet would be in sharp focus.
Be careful not to move the disks as you turn the guide from one side to the other to read the settings. It's particularly easy to accidentally shift the format adjustment without noticing it.
The ExpoAperture2 Depth-of-Field Guides are useful tools for any photographer interested in working more effectively or more creatively with depth of field. They are particularly useful as learning tools for photography students.