Originally Posted by TracyStorer
Not sure what the t-dimension is for Wisner holders. It is a tad different for S&S and AWB. However, depth of focus of long lenses used at the apertures typically seen in ULF work is very great. I did the math on this on APUG at some point in the past but can not find the post now.
Depth of focus is the zone of sharpness surrounding the film plane, and is different from depth of field, which is the zone of sharpness surrounding the plane of focus. There is a good discussion of the topic, with relevant formulas that allow you to calculate depth of focus, in Way Beyond Monochrome by Ralph Lambrecht and Chris Woodhouse.
If you take your large ULF camera out in the field with a lens of 300mm or more and use the back focus mechanism you will find that at an aperture of f/11 or so it is very hard to see any difference in sharpness over a span of several millimeters. The difference in t-dimension measured in most ULF holders is on the order of .1 - .3 millimeters.
From Lambrecht and Woodhouse.
"Depth of focus increases with the circle of confusion and magnification. It decreases with lens aperture and is at its minimum when the lens is focused at infinity."