Validating Your Depth-of-Field Scale
The following article applies to lenses that have depth of field scales engraved or painted on the lens barrel. If you're shooting a AF zoom like I do these days, you probably don't have depth of field scales.
Some time ago I noticed that a lot the photos that I had taken using the depth-of-field scales on my Mamiya 645 lenses weren't as sharp as I expected at infinity. At first I thought this might indicate a mechanical problem such as a variation between the lens to film plane distance and the lens to focusing screen distance.
I tested this idea by setting the camera low to the ground on a tripod. I then stretched a 25 foot tape measure away from the camera, with the zero end located at the film plane. Using a magnifier, I focused precisely on the 15 foot mark on the scale. I checked the focusing mark on the lens barrel and found that it indeed read right at 15 feet. I then shot a frame at each aperture.
When I examined the processed film with a loupe, I found that the point of sharpest focus was at 15 feet, just as it should have been. What I also found was that areas within the range of - but toward the extremes of - the depth-of-field marks on the barrel were less sharp than I was comfortable with.
A lens actually only focuses a sharp image from a finite plane in the field of view to a finite plane at the film plane. For a technical discussion of what this means and how it relates to depth-of-field, please visit this page regarding the "circle of confusion". When it comes to placing the depth of field makes on the lens barrel, an engineer has to make a subjective decision about what "acceptable sharpness" means.
After examining the results of my tests, I decided that for my purposes I would deduct two f-stops from the depth-of-field scales marked on my lenses. In other words, if I am shooting at f/11, I use the d-o-f marks for f/5.6 instead of f/11. This makes shooting much more challenging in many situations, but I get the tack sharp results that I want.
Mamiya apparently used the same sharpness standard for all their lenses, because I learned that by appying this 2-stop deduction to all my lenses, I began to consistently get the sharpness front to back in the focusing range that I wanted sharp.
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