I have an 1898 5x7 Rochester Premo Rev Back that is in perfect working order. I plan to place a thin (40 mil) Fresnel lens between the ground glass and the lens (positioned exactly like my Graphic View Ektalite focusing screen) to improve the luminosity of the focusing panel.

I have heard that this could interfere with focus accuracy due to disparity in the location of the film plane vs ground glass. Frankly this doesn't make sense to me. First, wouldn't depth of field take care of any slight difference in the location of the focusing screen vs. film plane? Secondly, even if you were at maximum aperture with minimal depth, as in extended bellows close ups, I can't imagine 40 mil difference being significant.

Comments?

A friend of mine met Dr. Rudolf Kingslake, the last lens pioneer. He ran the Hawkeye works and Kodak for decades. They brought him back during the cuban missle crisis. His written several books on lens history and lens design.

My friend asked him with a convertible lens, does it really make a difference if the single cell is in front or behind the aperture?

He said, "Why don't you try it and find out."

You've got very good logic behind your reasoning. I could argue that the depth of field at the film plane isn't large even stopped down. But the truth is in the film. Try it and see what happens.

You are thinking of depth of focus, depth of field's evil twin. When one is large, the other is small. At infinity, depth of field is large, depth of focus is nil.

If you must add a fresnel, put it behind the ground glass. That is, between you and the ground glass, not between the ground glass and the lens.