Removing the fresnel lens......

[Chuck P.]

I'm increasingly disliking the fresnel lens when it comes to focusing and I want to do without it for a while. I have a 4x5 HorsemanLE, the camera back contains the fresnel between the lens and the GG. The fresnel and the GG are sandwiched together with the long dimensions of the pair fitted into metal clips, more like guides than actual clips. Beneath this is a "shim" (I suppose) that the whole arrangement appears to rest on. I am either having difficulty in simply focusing with the fresnel or there is a focus error with the alignment of the inner surface of the fresnel and the film when the holder is inserted. I want to remove the fresnel lens from the metal guides and use just the GG to see if things get better. I can see how it can be done, but am concerned that I may mess up the alignment, if indeed there is no issue with it already. Any guidance is appreciated, will check back when i get home from work today, thanks.

Chuck

[Jim Jones]

In most cameras, removing a fresnel from between the GG and the lens will move the GG closer to the lens, and thus change the focusing. To compensate for this, the fresnel should be replaced by a shim about 2/3 the fresnel thickness. That 2/3 factor compensates for the index of refraction of the fresnel material, which may vary somewhat.

[Chuck P.]

Wow, something that is 2/3 the thickness of the fresnel, what in the world would that be? The fresnel is the original one, I suppose, it has a Horseman logo etched into it at one of the corners.

[fecaleagle]

Just make a little plastic spacer or strip that is 2/3 the thickness of the fresnel to sit on the edges of the GG and "shim" the GG out by that thickness. Not too bad an operation if you really hate the fresnel that much.

-William

[Tim k]

Its really quite simple.

The emulsion side of the film, in a loaded inserted holder, should be in the same place as the rough side of your ground glass while focusing.

Now if your going the other way, putting in a fresnel, your on your own. I think there is some witchcraft involved.

[Chuck P.]

I found some good info as well, it seems, from this (http://www.largeformatphotography.info/forum/showthread.php?49328-Focusing-problems-with-the-fresnel-groundglass-on-the-Chamonix-45N-1/page2&highlight=removing+fresnel) thread.

[Chuck P.]

I took the fresnel out and according to info in another thread, the shims on the left and right sides should also come out to sufficiently move the GG forward enough to maintain the GG/Film Plane "coincidence". After having done it, I see clearly what that means, but my only question now is whether or not the top and bottom guides should be put back with the GG. You can see they are made for the thickness of both the GG and the fresnel, but without the fresnel, it did not look right at all, so I took the guides back out. The GG is secure by the left and right retaining clips, but it can be slid slightly up or down with pressure.

In doing this I did get a little finger tip oil on the rough side of the GG, how should that side of the glass be cleaned?

[Steve Smith]

In a film holder, the emulsion is 4.8mm away from the outer surface of the holder. The ground side of the glass needs to be the same distance away from the surface of its frame where it is held in contact with the camera back.


Steve.

[Doremus Scudder]

I took the fresnel out and according to info in another thread, the shims on the left and right sides should also come out to sufficiently move the GG forward enough to maintain the GG/Film Plane "coincidence". After having done it, I see clearly what that means, but my only question now is whether or not the top and bottom guides should be put back with the GG. You can see they are made for the thickness of both the GG and the fresnel, but without the fresnel, it did not look right at all, so I took the guides back out. The GG is secured by the left and right retaining clips, but it can be slid slightly up or down with pressure.

If you were unsure that the focus was correct with the Fresnel screen installed, then you are now even less sure that the focus will be correct. You have removed shims that are approximately 2/3 of the thickness of the Fresnel screen, which might have positioned the ground glass correctly if the shims were indeed the right thickness to do so if the Fresnel screen had been correctly installed...

Now that you have the ground glass where you think it might be right, you need to make some measurements and do some tests to make sure the focus is okay.

Measuring is a bit tricky; if you have a depth micrometer or other precision instrument that will allow you to measure both the position of the frosted surface of the ground glass and the surface of the film when the holder is inserted and darkslide pulled, you are at a starting point. The inaccuracy lies in the fact that the ground glass deflects some during measurement, especially in the middle. You need to be very careful not to put pressure on the ground glass during measuring. Also, micrometers need a practiced touch to be used to their best advantage. Measure from the machined surface on the inside of the back of the camera. Film plane and ground-glass plane need to be very, very close to the same, plus or minus 0.007" seems to be the number I see quoted most. You can also adjust by testing and shimming, using trial-and-error to get the ground glass in the right position.

Test 1: Focus on a ruler laid horizontally on a tabletop, with the camera pointed down at about 45°. Use a medium to long focal length and focus using no movements and with the lens wide open. Focus on an easily-identifiable mark in the middle. Shoot, develop and inspect the negative using a 10x loupe or more and see if the mark you focused on is really in focus. If not, shim as needed (make sure you're shimming the right direction) and repeat till you are happy with focus. I do this regularly from time to time when I am printing. It's easy to set up, shoot and then just toss the negatives in the print developer for three minutes. I check them wet and then toss them.

Test 2: After having done test 1, set up again, this time using a long lens and focusing on the horizon at infinity. Some hills with trees in the far distance works well. Shoot wide-open, develop and inspect as above. Adjust if needed using shims.

If you do the two tests and shim till you are satisfied, it is often more reliable than measuring unless you have practiced with micrometers/calipers and have a feel for them.



In doing this I did get a little finger tip oil on the rough side of the GG, how should that side of the glass be cleaned?

Windex works fine, but use a soft cotton cloth if you don't want to have to clean the paper-towel residue off the glass later.

Hope this helps,

Doremus

[Chuck P.]

Doremus,

The statement below is from post #17 from the thread I referrenced above in my post #6, and it describes my situation exactly. The fresnel in my Horseman is suspended, or recessed, in front of the gg, and the shims were between the gg and the mounting pads of the frame. From your post it sounds like you disagree with that----that the shims should remain, if I understand you.

The author of that post #17 is saying (if I understand him correctly) that where the gg is without the fresnel, is not going to be the same as where the gg is with the fresnel, presumably, because the fresnel is a lens itself and will push the focus backward. So, removing the fresnel, means that the focus will now be forward a little, and removing the shims will move the gg forward the needed amount-----just trying to figure it out, difficult sometimes, but putting the shims back in is simple enough though if I need to.

"In cameras where the Fresnel is actually suspended in front of the GG as opposed to being sandwiched between the GG and mounting pads (Horseman 45FA is another example, although they do it by attaching the Fresnel with clips to the GG), you still have to move the position of the GG to get it in the correct position for use without a Fresnel. In the Horseman, the designer had the presence of mind to include shims between the GG and the mounting pads as part of the design so that if you wanted to remove the Fresnel, you could simply remove the shims as well to move the GG forward to the correct postion."

[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/ULF%20Back%20Measure/ULFMeasuring.html

[Chuck P.]

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

http://tonopahpictures.0catch.com/ULF%20Back%20Measure/ULFMeasuring.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