Fresnel lens for Linhof Technika

[dazdraperma]

Hi all,

I've purchased this fresnel lens that theoretically should brighten up the groundglass on my Linhof Technika III.

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

The instructions suggest installing in FRONT of the groundglass (as in, between GG and the lens). However, some worry that by doing this one might mess up the focusing. What do you think - shall I install in on the BACK of the groundglass?

[Bob Salomon]

Unless you know someone that can properly reset the shims for the gg you would then mount it on top of the gg. That’s where LInhof has put fresnels for the past 40 or 50 years.

[dazdraperma]

Unless you know someone that can properly reset the shims for the gg you would then mount it on top of the gg. That’s where LInhof has put fresnels for the past 40 or 50 years.

Thanks, Bob!

[diversey]

http://www.afterness.com/vico222/fre...ass_harris.pdf

[abruzzi]

http://www.afterness.com/vico222/fre...ass_harris.pdf

Correct me if I’m wrong, but as I read that he says that the fresnel optics cause a focus shift due to the focal length of the fresnel lens, and the whole thing should be shimmed back 1/3 the thickness of the fresnel. But what isn’t clear, and the author glosses over very quickly, is that 1/3 focus shift assume the ground glass is in the correct position. So if the fresnel is placed where the GG usually goes, then the GG is placed on top of it, it is already being shifted back 1x the thickness of the fresnel, so in fact, you’d want to shift the the two forward by 2/3 the thickness of the fresnel, something many cameras can’t do.

For this reason, the rear mount, between the glass and you, seems a more practical solution. It also has the benefit of being easily removable. I’ve only found fresnels to be useful on wide lenses. Normal and long lenses, even f9 lenses, seem fine to me without fresnels.

[Bob Salomon]

Correct me if I’m wrong, but as I read that he says that the fresnel optics cause a focus shift due to the focal length of the fresnel lens, and the whole thing should be shimmed back 1/3 the thickness of the fresnel. But what isn’t clear, and the author glosses over very quickly, is that 1/3 focus shift assume the ground glass is in the correct position. So if the fresnel is placed where the GG usually goes, then the GG is placed on top of it, it is already being shifted back 1x the thickness of the fresnel, so in fact, you’d want to shift the the two forward by 2/3 the thickness of the fresnel, something many cameras can’t do.

For this reason, the rear mount, between the glass and you, seems a more practical solution. It also has the benefit of being easily removable. I’ve only found fresnels to be useful on wide lenses. Normal and long lenses, even f9 lenses, seem fine to me without fresnels.

Anything that you place behind the lens; filter, fresnel, etc. causes a shift equal to 1/3rd the thickness of whatever you put back there. Plus, in this case, the amount you displace the gg.

[abruzzi]

Anything that you place behind the lens; filter, fresnel, etc. causes a shift equal to 1/3rd the thickness of whatever you put back there. Plus, in this case, the amount you displace the gg.

thanks for confirming.

[dazdraperma]

OK, I'm puzzled now.
The Frenel goes either on one side of the GG, or on the other. Either between GG and the lens, or between GG and the photographer. In any case, the geometry will change:there will be a sandwich of 2 glasses instead of one piece. So, from my understanding, the change in geometry will inevitable result in the change of focus.

If I do it as Bob suggests - by mounting Frenel on top of GG (I assume that means between GG and photographer), the Frenel will push GG forward, towards the lens.

Where am I wrong?

[abruzzi]

OK, I'm puzzled now.
The Frenel goes either on one side of the GG, or on the other. Either between GG and the lens, or between GG and the photographer. In any case, the geometry will change:there will be a sandwich of 2 glasses instead of one piece. So, from my understanding, the change in geometry will inevitable result in the change of focus.

If I do it as Bob suggests - by mounting Frenel on top of GG (I assume that means between GG and photographer), the Frenel will push GG forward, towards the lens.

Where am I wrong?

On the technika the glass is pinned to a mounting "shelf" and there may or may not be shims between the shelf and the GG. If you put the fresnel behind the GG (between your eyes and the GG), it doesn't change the position of the ground glass which is still sitting on the shelf or shims, so should not affect focus. If you place the fresnel on the shelf, then the GG behind of it, the GG changes position and the light passes through the fresnel before hitting the GG, making two things you need to compensate for--the position of the GG and the optical effect of the fresnel (see diversey's link.)

I'm not 100% sure, but I believe that early technika's had an optional fresnel that was between the lens and the glass, and if you have one of those, you need to leave the fresnel in place. but on mine (1974 Master Technika), there is a little rotating tab on either side of the ground glass that can be rotated to hold the fresnel in place, then rotated away to remove it.

EDIT: not the greatest, but look at the diagram of the linhof back here:

https://linhof.com/en/fresnelscheibe-4x5/

You can see the rotating clips.

EDIT2: found a photo. Ignore the fact that it is mounted on a sliding back but the pictures here show you how the gg mounts (the 4 inch long black pieces with three screws each that run the entire short edge of the glass.) The middle screw also holds the tab that can be rotated to release or hold the fresnel in place:

http://www.laflexcamera.com/serviced...s-6x7-6x9-rare

[dazdraperma]

OK, please look at this image:


(and yes, I did break the gg today )
So, this is the inside part of the frame, lens-side.
By unscrewing the 4 screws, I can lift up the two strips which hold the gg in place above 2 springs. If I put the Fresnel inside this assembly, it will change the geometry - right?

This is the other (photographer's) side of the assembly: .

It would be awesome to put the Frenel right here, only one problem:Frenel is the same size as the gg and cannot fit outside.


So, the dilemma is: Frenel cannot go inside the assembly, as it will change the geometry of the focusing plane. It can also not go outside of the assembly, since it's too big, and also there's not mounting for it to stay in place.