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Digital as Polaroid®

Are you considering using your digital camera as a Polaroid® proof substitute? If you’re a medium or large format photographer you probably already have a Polaroid® film back to evaluate lighting ratios, exposure, filter effects and composition. Polaroid is expensive, $2.50 per sheet of 4X5 B&W, and often has “processing “ flaws (offset by trading 20 of the flawed prints and negatives for a box of new Polaroid® film at your friendly dealer). Digital cameras are becoming less expensive with more and improved features, such as manual focusing, black & white capability and manual control over focusing, exposure mode and metering. The following discussion may help you with using the digital camera as a Polaroid® substitute.

The first thing to consider is the format aspect ratio, that is, how does the digital frame compare to the film camera’s? Many CCDs have 4:3 aspect ratios (1.33:1), that is, the image width is 1.33 times longer than its height. This is important in previsualizing the composition. The table below gives aspect ratios for several common formats. Note that even with Polaroid® one is unlikely to get a perfect match. The next thing to consider is how does the digital camera lens focal length compare to the film camera’s? My digital camera has a focal length range of 7.2-50.8 mm, equivalent to 28-200 mm in 35 mm format or about ¼ the 35 mm focal equivalent length. The table gives the normal lens focal length, based on diagonal measure, for various common formats.

FormatDigital35 4.5X6 6X7 4X5 5X7 “4X5” Polaroid®
Aspect ratio (l/h) 1.331.44 1.33 1.18 1.25 1.4 1.28
Normal FL(mm) 11 44 75 85 162 220 144

Suppose, for example, you have a 210 mm lens on the 4X5 camera, what is the digital camera FL that you should set for equivalence? The taking lens to normal lens ratio is 210/162 or 1.3. Multiply this by 44 (most digital cameras use 35 mm equivalents) to get 57mm for the digital camera equivalent focal length setting. The digital camera must of course be used at the same location as the film camera, best done by using a tripod for both. The lens focal length equivalence thus becomes key to making this work.

One place you will have trouble with is previewing depth of field with the digital camera. Digital camera lenses have much shorter focal lengths than large and medium format cameras and as pointed out above they are about 4 times shorter than those for 35mm camera lenses. This is because the CCD is much smaller than film. As it turns out depth of field is not a function of lens focal length, but of image size on the film/CCD. If the image taken with a long lens is made the same size on the film as an image made by a wide-angle lens, by moving the long lens/camera further from the subject and the same f-stop is used then the depth of field (dof) will be the same with both lenses. If the lenses are of different focal lengths and the digital camera, with its shorter focal length lens, is used at the same subject to camera distance as the longer lens/camera combination, the depth of field of the digital image will be much greater than that of the longer focal length system.

For example, comparing an 11 mm focal length lens at f8 with a 100 mm lens, to got the same depth of field you would have to set the 100 mm lens to f660! Trying to use your digital camera to preview dof is not going to work!

If your digital camera has a black and white mode you may be able to preview the effect of B&W color filters both for tone adjustment and detecting tonal mergers. In the B&W mode many cameras allow you to select “internal” digital color filters a substitute for the real thing. This feature may or may not give you a filter effect that will occur with real film. If the camera is too smart it will automatically try to compensate for the “funny” light it “see” through a red or yellow filter placed in front of the lens. You may be able to defeat this automatic function. Split filters will also present difficulties because they should be used at stopped down apertures and as seen above, what is seen with a short focal length digital camera lens is not the same as what’s seen with the longer film camera lens. It’s worth exploring how your digital camera copes with filters.

The digital camera is thus best used for compositional previewing, checking that the wayward branch is out of the frame, evaluating some filter effects, checking lighting ratios and visual note taking in the field. If you require depth of field information you will still need Polaroid®.