Fresnel screen focus distance is different from ground glass

[nicnilov]

I'm using a Chamonix 45N-1 4x5 camera which comes with a fresnel focusing screen. The screen is set up on the lens side from the ground glass and the grooves are also facing the lens. It is a plastic screen and it has a small round hole in it through which I can see the image on the ground glass itself.

When I focus by the image that comes trough the fresnel I get different focusing distance from when I focus through the small hole in fresnel by the image on the ground glass. From what I gather there should be no such difference, since with a narrow depth of field my focus would be off in one case or the other. I assume the ground glass focus would be more precise and this is what I'm using, but then the fresnel gets in the way of checking the focus throughout the frame.

Is this something expected or am I doing something wrong?

Thanks!

[Alan9940]

I don't use any fresnel lens on my LF cameras, but, since the fresnel does have a focal length to itself I imagine this could affect proper focus. Again, I'm no expert but I always heard that the fresnel should behind the ground glass to avoid this issue. Sure hope someone who knows that actual facts jumps in on this thread.

[Oslolens]

This is reported and was upgraded on the N2, and one of the reasons why a bought a N2 instead. Google it and read the "fix" http://www.chamonixviewcamera.com/cameras/45n2

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[Bob Salomon]

I don't use any fresnel lens on my LF cameras, but, since the fresnel does have a focal length to itself I imagine this could affect proper focus. Again, I'm no expert but I always heard that the fresnel should behind the ground glass to avoid this issue. Sure hope someone who knows that actual facts jumps in on this thread.

If a camera is designed for the fresnel to be between the gg and the lens then the gg is adjusted to compensate for that position.

If the camera is designed for the fresnel to be between the gg and the user then the gg is adjusted for that.

As an example early Linhof cameras were adjusted for the fresnel to be between the gg and the lens. Later one are adjusted for the fresnel to be between the gg and the user.

Should one have mounted the fresnel in the wrong place then the gg has to be adjusted by a service center to correct the positioning of the ground side of the gg.

[Greg]

Years ago took my Nikon Multiphot apart to seriously clean it. The back on the Multiphot has the fresnel lens placed in front of the GG. When putting it all back together, I put the fresnel lens in front of the GG but mistakenly reversed it. Since I was doing macrophotography and microphotography with the Multiphot and depth of field being practically non-existant, this caused my 4x5 chromes to be ever so slightly out of focus. Fortunately diagnosed my mistake with the first batch of processed chromes and corrected the orientation of the fresnel lens. If this reversal of the fresnel lens would matter with general photography, I don't know. Would be interesting if someone else could pipe in on this.

[Oslolens]

there should be no such difference

There is only three things that are crucial, the same plane of focus between film and ground glass, darkness, and no moving of camera from focusing to exposure. Time to to repair. Ask them (Chamonix) what to do, and check the result afterwards before doing something important

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[choiliefan]

IIRC the later Speed Graphics had the grooved side of the fresnel facing the ground glass.

[cowanw]

I suggest you arrange a yard stick or metre stick up on a 45 degree angle fairly close to your camera and at a wide open f stop focus first one option and then the other. Develop and see your negatives. Is one or the other sharp or both or neither.

[nicnilov]

This is reported and was upgraded on the N2, and one of the reasons why a bought a N2 instead. Google it and read the "fix" http://www.chamonixviewcamera.com/cameras/45n2

Thanks for pointing that out, I was not aware this was a design issue. I contacted the manufacturer and their recommendation is to remove the fresnel and focus on GG.

[Bob Salomon]

Thanks for pointing that out, I was not aware this was a design issue. I contacted the manufacturer and their recommendation is to remove the fresnel and focus on GG.

A fresnel does 2 things:
1 brightens the gg image
2 even outs the illumination on the gg edge to edge.

Ask the manufacturer how they propose that you do these 2 things?
Or
What they will do to your camera to achieve those 2 things?