Fresnel lenses and ground glass focusing screens your worries answered

[Randy]

I absolutely understand that the frosted side of the ground glass screen must be exactly at the film plane. If you place the fresnel lens flat onto this frosted side of the ground glass screen without moving the screen from its position.

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OK, I understand now what you did...or I think I understand.
I think where my confusion came in was - if you take, like a standard 4X5 back that has no fresnel, is not made to use a fresnel, but you then remove the gg, put the fresnel in it's place, then place the GG on top of it, you have moved the GG past the film plane, and the result could be slightly out of focus images.

~ but ~

If you can mount a fresnel in front of the GG with out moving the GG, you will maintain the GG at the exact film plane...correct...?

[Neal Chaves]

I found that with a 4X5 Crown Graphic, when I removed the Fresnel I had to insert shims under the ground glass to maintain accurate focus. The shims were thinner than the Fresnel, about .0025". Recently I replaced the broken ground glass and Fresnel in a Toyo G with a plain ground glass. I expected that I would need shims, but I was surprised to find that focus was perfect. There must be differences in the Fresnels themselves.

[Greg]

OK, I understand now what you did...or I think I understand.
I think where my confusion came in was - if you take, like a standard 4X5 back that has no fresnel, is not made to use a fresnel, but you then remove the gg, put the fresnel in it's place, then place the GG on top of it, you have moved the GG past the film plane, and the result could be slightly out of focus images.

~ but ~

If you can mount a fresnel in front of the GG with out moving the GG, you will maintain the GG at the exact film plane...correct...?

In the 1980s I was doing a lot of Photomicrography and Photomacrography (shooting 4x5 chromes) with my Nikon Multiphot. On the Multiphot, the fresnel lens is mounted in front of the GG. One time I took the Multiphot apart to clean it. Shot some 4x5 chromes and they came back out of focus. I called Nikon's Instrument Division to trouble shoot my problem. First question they asked me was if I had put the fresnel lens in front of or behind the GG. Told them I put it in front of the GG (I had made that mistake once before). Then they asked me which direction was the fresnel facing. I told them towards the GG. Was told the fresnel should be facing towards the lens. If the fresnel was reversed (how I had mounted it), the result would be that my images would noticeably be out of focus. This wouldn't be noticeable if I were using a regular view camera (depth of field more than compensates), but since I was shooting between X1 and X100, with its essentially minuscule depth of field, my film images would be noticeable out of focus.

[Dan Fromm]

If you can mount a fresnel in front of the GG with out moving the GG, you will maintain the GG at the exact film plane...correct...?

Stands to reason but it ain't so.

[Randy]

Stands to reason but it ain't so.

Trying my best to translate that statement...I give up.

[Jim Jones]

Dan is right. The length of the optical path is shortened when transparent material is inserted between the lens and ground glass (or film). You commonly see a similar effect in the apparently broken straw in a water glass.

[john.l.barford]

There are obviously many factors in fresnel lens placement that I have failed to understand. Could someone explain to me that if a fresnel lens has to have a focal length related to the focal length of the camera lens

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[john.l.barford]

There are obviously many factors in fresnel lens placement that I have failed to understand. Could someone explain to me that if a fresnel lens has to have a focal length related to the focal length of the camera lens why camera manufacturers fresnel lenses seem suitable for all lens focal lengths?

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[xkaes]

Here's what Darkroom Techniques said about it in 1990:



www.subclub.org/groundglass.JPG

[john.l.barford]

Excellent article especially the "light spreading screen" definition.

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