Removing the fresnel lens......

[Jim Jones]

Doremus gives very good information for most cameras. If the position of the GG doesn't change when the fresnel is removed in the Horseman, the information in the link cited in post #6 is correct. In some cameras, the face of the GG frame may not contact the surface upon which the film holder seats. In this case, measuring the distance from the face of the frame to the GG will be incorrect.

The 1951 ANSI standard for film holders gives the distance from the face of a 4x5 film holder to the back of the film slot (the so-called "T" distance) as .197" (5mm) +/-.007". The film slot is a minimum of .012" with no maximum specified. Film thickness might vary from .004" to .010". Thus, the position of film might vary over .022" and remain within specs, not considering further error from film curvature. Despite such an appalling tolerance, we should try position the ground glass near its optimum distance for our own personal use. Remember, small apertures are the LF photographer's friend.

[Chuck P.]

Doremus gives very good information for most cameras. If the position of the GG doesn't change when the fresnel is removed in the Horseman, the information in the link cited in post #6 is correct.

Yes, the position of the GG does not change when the fresnel is removed. The pictures show the fresnel installed, hopefully one can tell that it is recessed. The aluminum ledge is where the GG sits. Hopefully it's visible that removing the fresnel does not change the position of the GG----only the removal of the shims will do that as the other picture illustrates with the fresnel removed. Just to repeat, my understanding is that those shims should be removed if the fresnel is removed.

I won't be able to make a negative until this weekend, so I'm going to make an exposure of a completely straight on and plumb flat surface with and without the shims in place, see which one is true in terms of the resulting focus. If anyone has a better way for me to test that, I'm glad to hear it, but I've no way to measure such small tolerances nor am I going to purchase anything for that purpose, so that part of the discussion is mute for me.

I'm appreciative of all the comments.
Chuck.

[Tim k]

Chuck,
You dont need to go buy anything special. All you need is something pretty straight about 2-3 inches long, a clothespin or some clamp, and a toothpick.

Just take a filmholder, lay the straight thing across a corner, and clamp the toothpick to it, so it just touches the surface of the film. You dont need to know what the exact measurement is. Now take the clamped toothpick, and use it to measure how deep the rough side of the glass is recessed. Forget about the fresnel, and shims. If the toothpick shows the same distance, your golden.

I saw this somewhere, cant remember where. I'd send you a link if I could remember.

[Tim k]

Chuck,
This is not the article, but it will give you the idea.

http://tonopahpictures.0catch.com/UL...Measuring.html

[Chuck P.]

Chuck,
This is not the article, but it will give you the idea.

http://tonopahpictures.0catch.com/UL...Measuring.html

That's neat-----------I could separate the spring loaded hinge of the back just enough to carry this out and by this assessment, the depth of the "standard" was dead on with the depth on the holder with a sheet of film inserted----this was done without the shims in place. Hopefully my little experiment this weekend will prove good. Thanks for that link, I never would have thought of that. I can clearly see that with the shims in place, the depth of the standard would have been deeper than the depth of the holder by the difference of the shim thickness. This exercise has helped me see the relationship between the position of the GG and the position of the film plane even better.

[Doremus Scudder]

Hi all,

Let me clarify:

I was under the impression that part of the reason the OP wanted to remove the Fresnel screen was that focus was NOT accurate with it in. I took this to mean that the position of the Fresnel/ground-glass sandwich was not correct, or at least was being questioned.

In cameras that are designed to have the Fresnel screen placed between ground glass and lens, the focus shift caused by the Fresnel screen is taken into account and the Fresnel/glass sandwich placed accordingly, i.e., at a different distance from the lens than just a plain ground glass would be. I understood that this was the case with the OP's camera.

In such a camera, if it was designed to take both regular ground glass and a Fresnel/glass sandwich, there is likely a way to easily adapt the back to take a plain ground glass, e.g., removing the shims when removing the Fresnel screen could place the ground glass in the proper position.

However, if the positioning is in question in the first place, and you are not sure if the camera was designed to take both regular ground glass and Fresnel/glass sandwiches, then all bets are off.

You need to measure and do the tests to make sure your newly-installed plain ground glass is in the proper position. Since the tests are not all that difficult and only use a couple of sheets of film, there is really no reason not to do them.

For cameras with the Fresnel screen positioned between the photographer's eyes and the ground-glass, there is no problem; simply remove the Fresnel screen and you are good to go.

Hope this is more understandable.

Oh, of course, when measuring the position of the ground glass and the film plane, you need to start from a point on the camera that is the same distance relative to both. For all cameras I am familiar with, the surface of the removable back that faces the front of the camera is always this fixed distance from the film plane/ground-glass plane. If there are cameras that this does not apply to, then, of course, one would have to find somewhere else appropriate to take measurements from.

Best,

Doremus

[Chuck P.]

I probably did not describe the situation as accurately as it needed to be in the beginning, thanks for all the insight folks.

I visited your website Doremus, nice.

[Doremus Scudder]

.... I visited your website Doremus, nice.

Thanks!

Doremus