Fresnel lens in front of the ground glass or behind the ground glass?

Ian Grant said:

One disadvantage of a fresnel under the GG screen is it can make focussing harder with quite wide angle lenses, it's less of a problem when rear mounted. I seem to remember at least one company offering two different fresnels one petter optimised for wide angle use.

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I wonder is thats the reason that some of the Nikon F/F2 screens have multiple versions optimised for different focal lengths?

Personally, I find it harder to focus through a fresnel, the concentric lines are very distracting under a 6x loupe, so I much prefer a removable fresnel in the rear so I can put it on for composition and remove it for focusing. But in reality I've found I only use a fresnel on 8x10. 6x9 and 4x5, even with darker lenses like my 150mm ƒ9 G-Claron is fully bright enough for my uses. On 8x10 I find wides especially are hard to see the corners.

abruzzi said:

I wonder is thats the reason that some of the Nikon F/F2 screens have multiple versions optimised for different focal lengths?

Personally, I find it harder to focus through a fresnel, the concentric lines are very distracting under a 6x loupe, so I much prefer a removable fresnel in the rear so I can put it on for composition and remove it for focusing. But in reality I've found I only use a fresnel on 8x10. 6x9 and 4x5, even with darker lenses like my 150mm ƒ9 G-Claron is fully bright enough for my uses. On 8x10 I find wides especially are hard to see the corners.

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Are you using a loupe with an adjustable eyepiece? If so you should be focused only on the grain of the gg and the circles on the fresnel would not be in focus.

Do all fresnels, from the low cost page reading ones at office supply stores to high end designed for photography ones, have the same optical focal length? Probably not.

Film holder specs for 2x3, 3x4, 4x5 sheet film holders film position is .197 ± .007 inch from the edge of the holder to the surface of the film. 4x5 sheet film is .007 inch thick (Kodak).
https://www.freelists.org/archives/pure-silver/08-2016/pdfCXYK8Hzbex.pdf

shutterfinger said:

Do all fresnels, from the low cost page reading ones at office supply stores to high end designed for photography ones, have the same optical focal length? Probably not.

Film holder specs for 2x3, 3x4, 4x5 sheet film holders film position is .197 ± .007 inch from the edge of the holder to the surface of the film. 4x5 sheet film is .007 inch thick (Kodak).
https://www.freelists.org/archives/pure-silver/08-2016/pdfCXYK8Hzbex.pdf

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No the Fresnel focal lengths differ, as mentioned above with the various Nikon fresnels, quite specific screens for Fish eye and Wide angle lenses, etc. I think there's an error in that PDF as the standard specifies the holders internal width and length needed to accept the specified film size and not the Exposure height/Exposure width.

Ian

Bob S said:

Are you using a loupe with an adjustable eyepiece? If so you should be focused only on the grain of the gg and the circles on the fresnel would not be in focus.

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no, and that could be the problem. I have the common fixed Toyo loupe.

abruzzi said:

no, and that could be the problem. I have the common fixed Toyo loupe.

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Then you are not focusing on the grain side. Get a good loupe.

shutterfinger said:

Do all fresnels, from the low cost page reading ones at office supply stores to high end designed for photography ones, have the same optical focal length? Probably not.

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Do these count?

https://en.wikipedia.org/wiki/Fresnel_lens#/media/File:Oita_sekizaki_lighthouse_old_lense.jpg



While I'm trying to be funny, the wiki page is actually very informative.

Bob S said:

Then you are not focusing on the grain side. Get a good loupe.

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well, technically I am because I never focus through a fresnel, but directly on the glass, and while there might be some convenience there, I’ve never found a fresnel remotely useful on 4x5 or smaller. The Linhof fresnel that came with my TK has never been used, though I do use the fresnel clips to attach masks.

abruzzi said:

well, technically I am because I never focus through a fresnel, but directly on the glass, and while there might be some convenience there, I’ve never found a fresnel remotely useful on 4x5 or smaller. The Linhof fresnel that came with my TK has never been used, though I do use the fresnel clips to attach masks.

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No, you’re not. You are focusing on the top surface of your screen. Not on the grain side which is where the film plane lies. You are always the thickness of your gg from optimal focus.
But your loupe is fine for use on a print.

Diverging off topic into loupes now for a while.

abruzzi,

While Bob is right about optimum focus for the loupe being the frosted surface of the ground glass, your loupe may or may not be doing the job for you. If your loupe has an adjustable eyepiece, you can usually set that so the focus is on the frosted side. To make sure, turn your loupe around and view a nice bright image through it, moving it slowly nearer and farther from the ground glass until you get the focus on the frosted side. It's easily recognizable; so much so that I use a loupe without a skirt and focus this way all the time.

Chances are that you are focusing with your loupe just fine. However, even if your loupe isn't precisely focused on the frosted side of the ground glass, the sharpest image you see through it will be the one that's the sharpest on the ground glass; it just won't be as sharp as it possibly could if the loupe was properly focused on the correct side of the ground glass. That still works for focusing a view camera, even though it's not optimal.

(Back on topic for a sentence ) Having the Fresnel screen mounted improperly, on the other hand, will always degrade focus.

FWIW, I find using the "floating loupe" easier when having to see into corners or find the right viewing angle when using really short lenses or lots of movements.

Best,

Doremus