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Tachi
13-May-2006, 17:09
Just came across this product in eBay, anyone tried it before:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=29981&item=7579836625

David Karp
14-May-2006, 16:57
I have seen the ad, but was always afraid to use it because it goes under the Fresnel. It seems like even if it is very thin, the plane of focus will be off by the thickness of the Fresnel.

Anyone tried it?

N Dhananjay
14-May-2006, 19:45
I have seen the ad, but was always afraid to use it because it goes under the Fresnel. It seems like even if it is very thin, the plane of focus will be off by the thickness of the Fresnel.

Anyone tried it?

A fresnel is a field lens and can be used on either side of the ground glass. Fresnels used between the lens and ground glass will shift the plane of focus by about 1/3 the thickness of the fresnel. Thus, cameras using that design need to compensate for that by shifting the position of the GG - in other words, the GG-film registration will change when one is using/not using a fresnel and this needs to be compensated for. There is, of course, nothing to stop you from using a fresnel on the outside of the GG (i.e., between the GG and your eye), unless you worry about scratching the fresenl etc..

This fresnel should work fine when placed between the GG and the eye. If placed between the GG and lens, there should be the shift in plane of focus but given how thin the fresnel is, I would imagine that it might be swamped by other factors (1/3 of 18 thou works out to about 0.15mm, and I find I cannot reliably focus to that level of accuracy). Others might argue differently, of course.

Cheers, DJ

Robert A. Zeichner
15-May-2006, 04:19
While it is true that .018" is thin, you must consider that the total displacement is .018" for the physical thickness and another .006" for the optical shift. That adds up to .024" of rearward shift. The ANSI spec. for film holder depth (in 4x5) is +/- .007" which is markedly less total error than you would be starting out with. Why start out with any error that can be avoided in the first place? In fact, our propensity to make focusing errors would be good reason to make certain the gg/film plane coincidence is dead on, would it not? That way, we are not adding human error to equipment error.

N Dhananjay
15-May-2006, 19:00
While it is true that .018" is thin, you must consider that the total displacement is .018" for the physical thickness and another .006" for the optical shift. That adds up to .024" of rearward shift. The ANSI spec. for film holder depth (in 4x5) is +/- .007" which is markedly less total error than you would be starting out with. Why start out with any error that can be avoided in the first place? In fact, our propensity to make focusing errors would be good reason to make certain the gg/film plane coincidence is dead on, would it not? That way, we are not adding human error to equipment error.

Oh, no question that I'd agree with that. I should have been more clear - I was saying that the fresenel would be perfectly viable when used between the GG and the eye (and when used between the GG and lens, if it was being swapped out for another fresnel of somewhat different thickness, the error would be small but there nonetheless). I should also add that Robert's article on ensuring GG-film plane registration makes it fairly easy to ensure accurate registration if the thickness of the fresnel being exchanged does vary - using his methodology, you can shim out the GG so that the registration with the film plane is correct. Hope that clarifies things. Cheers, DJ

Michael S. Briggs
16-May-2006, 03:02
While it is true that .018" is thin, you must consider that the total displacement is .018" for the physical thickness and another .006" for the optical shift. That adds up to .024" of rearward shift. The ANSI spec. for film holder depth (in 4x5) is +/- .007" which is markedly less total error than you would be starting out with. Why start out with any error that can be avoided in the first place? In fact, our propensity to make focusing errors would be good reason to make certain the gg/film plane coincidence is dead on, would it not? That way, we are not adding human error to equipment error.

I think you have a sign error in your calculation. The optical effect is to reduce the convergence of the rays and shift the focus point to the rear -- so if you put the Fresnel in front of the ground glass (as instructed by the eBay seller) you want to move the ground glass back by approx 1/3 the thickness of the Fresnel. If you lay the Fresnel on the stops that formely positioned the ground glass, and then the ground glass behind the Fresnel, you have moved the ground glass back by the full thickness of the Fresnel. The difference between the actual and desired position is 2/3 the thickness of the Fresnel (i.e., 0.12 inches), not 4/3 (i.e., 0.24 inches).

So the position error is about twice the ANSI spec for 4x5 filmholders.

I totaly agree with "Why start out with any error that can be avoided in the first place?"

Another way to look at the error is to compare it to the depth of focus, which is not doubt how the ANSI spec was derived. The equation for depth of focus (for reasonable approximations for most photography) is 2CN, where C is the diameter of the circle of confusion and N is the f-number. I prefer to think of it as +/- CN, placing the depth of focus symmetrically about the plane of best focus. A typically recommended value for C for 4x5 is 0.1 mm. At f16 this makes the depth of focus 1.6 mm or 0.063 inches, which is 3.5 times larger than the error from using this Fresnel. If you want to use a smaller circle of confusion, say 0.03 mm, perhaps because you make large enlargements, the depth of focus would become 0.5 mm = 0.018 mm and using this Fresnel at f16 as instructed by the seller would become problematic.

If anyone is feeling deja vu from this discussion, it might be from http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=00CIgP

David Karp
16-May-2006, 10:07
. . . afraid to use it because it goes under the Fresnel.

Oops, but you guys caught that I meant under the ground glass.

Thanks for the interesting discussion on this. I knew that some cameras were made to take the accessory Fresnel on top of the glass. I never thought about doing it as a matter of course with any Fresnel. Always learning something new. I love this forum.

Thanks again.

Robert A. Zeichner
16-May-2006, 16:42
"I think you have a sign error in your calculation".

Michael, thanks for spotting that error. I never seem to get that right until I sit down with pencil and paper and draw it all out. The error is indeed only .012" and not .024" as you have explained. Still, the point remains that if any error that is correctable is allowed to go unchecked, shallow film holders and the inevitable human error can throw the end result beyond the acceptable limits and result in soft negatives.