Ground Glass focusing errors

[Dick Clark]

The last few Q&A forums have had discussions on groung glass/frenel screen focus ing accuracy, with the comments indicating that this is a VERY tricky matter. N ow I wonder about a couple of things:

1) Why haven't film holders been standardized to the point where, for cut sheet film holders at least, we'd have a uniform one-size-fits-all exact degree of to lerance in terms of how they seat in the camera? This way camera makers could m ake their designs more precise. We have a standard film size, standard ISO meas urements, etc. From what I've read, I wonder if you can even trust the focus of a new camera straight out of the box!

2) So let's say that I'm way off base and there are legitimate reasons for the disparity of holder dimensions, my dumb question is what is the proper way to te st if your focus is "on?" I ask because the camera I have came with the corners cut out of the GG (as if the corners of the picture are somehow unimportant - c hecking for vingetting? I don't think it's quite that simple). My idea was to replace the GG glass, but now I'm wondering if this will only cause more problem s.

One test I thought of is to use my 210mm and focus it at about 2' from a newspap er hung on the wall and shoot at F5.6. If everything is sharp,my focus is OK (s houldn't be that much DoF at 5.6). Any Ideas?

THX in advance

Todd

[Glenn Kroeger]

1. There are ISO standards for film holders... but whether any particular holder meets those standards is a different questions. Generally, good quality, relatively new holders do well, but there are bad apples. Older wooden holders are more problematic. I have calibrated my groundglass to a Fuji Quickload holder and find that that position also matches my Grafmatic backs very well.

2. Corners cut from the groundglass do allow you to check vignetting, although if you are setting a bellows lens shade, it's usually easier to do from the front of the camera.

2.5 You can't just focus on one plane like your newspaper, since you won't be able to tell if that is the BEST plane of focus. There are two common approches. One is to build a stepped chart. This is very well documented in Robert Zeichner's article on the subject in the Nov/Dec 1996 issue of View Camera magazine. Briefly summarized, 5 crisp dollar bills, mounted on foamcore, are staggered about 1/2 inch apart. You focus on the middle bill. Not only should the middle bill be sharpest, but the one in front should be in slightly worse focus than the one in back given the asymmetry of the depth of field. The other approach is to focus on one point of a yardstick or tape measure laid diagonally across the optical axis. Again, the point of focus should be sharpest, with sharpness decreasing faster toward the camera than away. A 210mm wide open should work well for either test.

[Michael S. Briggs]

I have tested three 4x5 cameras of three brands for focus and all three were accurate. I have used both photographic tests and measurements with a depth micrometer. The best photographic test is to photograph several objects at varying distances, focusing on the middle one. The previous poster explained a good way to do this (5 dollar bills). You probably should stop down about two or three stops. Wide open the lens might make a fuzzy image outside the center, which would make identifying correctness of focus more difficult. The cleverest approach is to calculate the spacing of the objects based upon the expected depth-of-field at the taking aperture.

I don't know why so many people have reported problems. Older equipement, either camera or holders? Particular brands?

[Doug Paramore]

Todd: One of the fun areas of large format photography is nitpicking and discussing items to the point of exhaustion. That is kinda what we are doing here. I have been using large format cameras since 1957 and have simply found that most cameras focus close enough unless someone has done something major to the film plane or put a Frenel screen in wrong. There is not only a depth of field in front of the lens, there is a depth of focus at the film plane, which increases at smaller apertures. Try this: focus sharply on an object a mid- distance with a good loupe, then gently lift the ground glass by a corner. You will find you have to move the ground glass quite a bit before the image goes to pot. There is more variance in where the film lies due to buckling and sag than one might imagine. Film is not perfectly flat. Still, if we do our part the image is still good. I have found after all these years that usually if an image is out of focus, it is me and not the camera,that is at fault. I have had many more pictures out of focus because I didn't lock down after a movement or didn't return to center after shooting something earlier that required movements than I ever had due to camera faults. Just take your camera out and shoot pictures and enjoy it. Good Shooting, Doug.

[Glenn Kroeger]

I should have added that I have tested 3 modern cameras, Toyo 45A, Arca-Swiss 45FC and Wista VX, and found that in all cases (assuming the fresnel was installed correctly) that the gg was very accurately positioned for good film holders.

But I would gently disagree with Michael. You really should use the lens wide open (if it is a reasonable good lens) since depth of field is the real enemy in evaluating accurate focus. I have used a 120 Apo- Symmar wide open, and the clarity of the best focus is very good.

[Sean Billy Bob Boy yates]

I'm hopping on the Doug bandwagon.

Look at all those images by the greats with wooden cameras, no fresnels or loupes and uncoated lenses. Are they not the standard by which we judge ourselves? Certainly todays lenses, films, etc. are capable of higher resolution, and likewise, as well made as those handcrafted cameras were, aren't our cameras manufactured to tighter standards and with greater precision and consistency?

Let your final prints or chromes be the judge. Do you have consistant complaints about their sharpness or are you worried because there's more to it than you thought?

[Sergio Ortega]

Todd,

I agree with the poster's suggestion that the ground glass' position should be m easured to determine whether it and the film plane are in correct registration. It's really pretty easy to do if you have one of those micrometer devices, with the protruding pin used to measure the depth of holes. You can pick one up at Home Depot for about $20-$30.

This measurement should be done with any large format camera, as well as all you r film holders. Of all the LF cameras I've used, I only had to slightly shim th e ground glass on a couple of them to bring them into alignment with the film pl ane measurement. Like anything else, LF camera backs can be incorrectly machine d, particulalry older wooden field cameras.

During one of my more anal periods, I also laboriously measured all of my 4x5 an d 8x10 holders (over fifty of the damned things!) to determine whether they were all the same, and properly aligning the film plane with the ground glass. I di scovered that several of the holders were faulty. They were apparently warped, deformed and not perfectly flat, and were not yielding the same measurements in the center and all four corners. This could lead to unsharp images in the corne rs/edges of the film. I had to throw away a couple of holders since there is no way of adjusting these things.

You can never be too anal when working with large format! Good luck, Sergio.

[Robert A. Zeichner]

Todd, you raise a very good question. One of the things I feel is misunderstood about film plane/gg alignment is that the holder is really a very clever design and more often than not isn't the villain many make it out to be. When I was servicing professional motion picture equipment for a living, one of the challenges we faced as a rental house was to maintain an inventory of cameras and lenses that could be freely interchanged and always provide consistently sharp results. The camera manufacturers provided us with a "spec" for flange focal depth and the lens manufacturers provided us with a "spec" for back focus. Unfortunately each spec by itself proved meaningful only if the other device was perfect. If we followed each manufacturers recommendation, we would have to create "Synergistic" combinations. "Hey, George, don't send Arri S 678443 out with Angenieux 10x12 #458997632. Everyone complains that there footage is soft!" Can you see the problems this could create? What we ended up doing was halving the tolerance of each. This allowed us to measure each lens and camera independently without having to match deep cameras with deep lenses, etc. What does this have to do with film holders? Well, it's really quite similar. The film holder (4x5 for the sake of illustration) has a spec of 0.197" +/- 0.007". This means that if the ground glass is in the perfect plane, holders that vary in depth from 0.190" to 0.204" will render satisfactory results. Given that these holders are made of molded plastic and aluminum, that's not bad! Some plastics can be very dimensionally stable. Delrin is a good example of this. It's also very expensive. It might be ideal for small investment castings, but it's probably not too practical for film holders. Would you want to pay $100 for a 4x5 holder? Holders have no adjustments. They are what they are and so all of the leeway for error must be reserved for the holder and NOT the gg! How most gg's get out of alignment, I believe is through dinking around with the damn things. Install a Fresnel between gg and lens and screw up gg focus. Remove a Fresnel from a camera designed with one and screw up gg focus. I'm not saying these things can't be done, just that most people haven't the equipment to make these changes and insure proper re-alignment! One contributor mentioned measuring depth with a $30 caliper. Don't think so. It's not just the wrong measuring device, but doing this accurately requires some precision fixtures and a knowledge of the materials involved and how interpret the results. Have you considered where the surface of the film really lies? Do you think it's flat against the septum? You might be surprised that this isn't always the case. The holder system is less than a perfect one, but if the ground glass is in the perfect location, the variations amongst the holders will, in the vast majority of instances yield satisfactory results. If you want to see a rather impressive example of just how serious one manufacturer takes the business of gg alignment, take a LOOK at the incredibly elegant system Linhof employs for the fine adjustment of the ground glass position. Just DON'T turn anything!

[Kevin Kemner]

A couple issues back in Photo Techniques (www.phototechmag.com) there was an article on comparing the alignment of the ground glass and the film holder plane. It was a fairly elegent solution requiring a squarish stick, a toothpick and an alligator clip. You should be able to determine if your camera system needs adjusting or not in about 30 minutes. It works very well.

Good luck Todd.

[Robert A. Zeichner]

I'm going to take exception to that last post. There is nothing elegant about making depth measurements with toothpicks, rulers and clamps. This method measures NOTHING. How can you possibly measure a tolerance in the range of 0.014" with toothpicks? Worse yet, if you dilude yourself into thinking your ground glass is misaligned and procede to make "adjustments" based on this crude system, you will SURELY wind up with the gg in the wrong plane. Test with film at wide apertures using a stepped target!