Originally Posted by
Ivan J. Eberle
Robert, point well taken that the amount that the GG moves is not necessarily the thickness of the Fresnel lens as a shim. But the only ANSI standard that I've yet found is the median for the film holders, have yet to see a number for the distance from the gate to the GG. I've assumed it's either .190" or .192", the correct number being somewhat variable based upon the thickness of the specific emulsion in use (at least with filmholders like the standard 2-up darkslide having a film slot and no pressure plates).
While compensating for average film thickness is certainly a legitimate factor when aligning gg placement, I would still be very cautious about using the same +/-.007" tolerance for error as is used in the ANSI spec for (4x5) film holders. Suppose gg placement is .007" shallow and you insert a film holder that is .007" deep. Both technically meet the +/- .007" ANSI spec, but combined as a system, your film is now .014" away from the gg position. This, I assure you will be noticeable.
Second is, how much difference does a couple of thousandths actually make at the film plane in practical use-- under what specific circumstances is it going to show up? I'd certainly expect it's much more critical with an Aero Ektar f/2.5 used wide open than with the typical f/5.6 or slower view camera lens stopped down to a taking aperture of at least f/11 (more likely f/22 for adequate DOF) for landscape. Can anybody point to a chart or give a formula for calculating depth of critical focus at the film plane for a given focal length, focusing distance, and aperture?
In a word, plenty. A lot will depend on what focal length of lens you are using. Shorter focal lengths with converging rays at a greater angle to each other than with long lenses, have a shallower depth of focus requirement to achieve satisfactory sharpness. While closing the aperture down to smaller f stops will increase depth of field, it will not fully compensate for poor gg/film plane coincidence and I have the negatives to prove it. A "couple of thousandths" beyond the range of tolerance will throw you into noticeable softness.
Realistically, can wooden field cameras or holders be produced in volume to within a repeatable tolerance of .007"? Agreed that stacked errors of say, a wooden back that's on the far side of the tolerance, with some swelling from humidity, coupled with a moisture-swelled wooden holder, could really begin to add up. It's not too hard to see why mid to late 20th Century technical designs began to be manufactured from more dimensionally stable materials like aluminum as soon as became practicable.
While a bit more of a challenge, by using aged hardwoods, properly sawn and sealed and precision milling equipment, the expert makers can achieve surprisingly accurate results. I've never personally compared the dimensional stability of wood to aluminum, but I can tell you with a fair degree of certainty that aluminum is not the most dimensionally stable material when it comes to changes in temperature. This I have measured.
Also mentioned in another thread recently and still begging a satisfactory answer: Why are Fresnels ever put inside the optical path in the first place, if putting them on the outside (on top of the GG) achieves the same brightening effect and allows easier interchange for different focal length lenses?
Bookmarks