I need some assistance from an optical guru in the audience. This is a specific problem requiring an optical calculation for depth-of-focus of a specific enlarging lens.
Background
In the course of getting an LED enlarger head built for my Omega D2, the latest issue -- and (fingers crossed) the last one -- is getting the light even across the full 4x5 field with my 135mm lens (my only lens for 4x5). It's very close with 120 (80mm) and essentially perfect with 35mm (50mm). It's not quite there yet with 4x5, and I won't go into all the specifics here.
The (almost) working model of this handsome LED head already has a piece of white acrylic permanently (i.e., irretrievably) inset for diffusion, inset a bit, so that the distance from the bottom acrylic surface to the negative in the carrier is 3/16 inch. I just measured it.
In order to improve evenness, we are going to add a second diffuser above this one, spaced between it and the LED panel. Rather than cutting the light down as much as white would do, I have ordered a sheet of a frosted acrylic material from TAP Plastics called Satinice, of which I have a tiny sample. It is 1/8th-inch thick (the minimum made) and frosted on both surfaces. The texture will make no difference in this application above the white diffuser, but I would like to know if I can use it also, in a second iteration of the head, in place of the white acrylic. The value of this, if it evens the light sufficiently, is simply more light getting through, to help keep exposure times in a good range.
Problem at hand
Therefore, I need to determine whether, given the 3/16 of an inch from the lowest acrylic surface to my negative, my 50mm lens at f16 (please see note at bottom) will bring the texture of the Satinice into sufficient focus to render the texture in a print. I recognize that there are variables in here that I can't specify, principally the fineness of frost texture and size of the print, assuming my lens is pretty good (1970s Nikon). I really don't want to get into minutiae about how close the viewer is. For the sake of common sense, let's assume a 9x enlargement of the negative showing nice clear sky areas.
I am using the 50mm lens for this problem, because I am assuming that the shorter focal length (than my 80 and 135) will naturally have more depth-of-focus. I have also stipulated f/16, even though I rarely use an aperture smaller than f/8, and more typically f/5.6, simply because I would like to know. Again, someone could start bringing up diffraction limitations, etc., but I'd like to keep this within the realm of slightly broader considerations.
Thank you in advance to anyone taking the time to run the calculation. I am very grateful. It may save considerable testing time and expense
Bookmarks