Focus accuracy - what's acceptable

[Michael S. Briggs]

......

I'm not quite sure how the professional repairman setup the screen on a LF camera. For 35mm and medium format cameras its possible to collimate to the film plane but I don't see how one can collimate a 4x5 in this way.

Your right, collimating through a lens wouldn't make sense for virtually all LF cameras since the position of the lens isn't fixed with respect to the back.

We went over the configuation of the back of Bob's camera on the previous thread, http://www.largeformatphotography.in...ad.php?t=17727. The screen rests against metal strips, the position of which are adjusted by turning screws. I'm not a professional repairman, but I'd do the adjustment by using a depth micrometer to measure the depth and adjusting the screws until it was correct. The depth should match the depth of filmholders, less the thickness of sheetfilm. One can easily make the measurement to sufficient accuracy.

[robc]

you must redo the test and each time use a different film holder, move the camera slightly and refocus. Then you will get a clue whether the error is consistent.

It could just be you or your loupe not being focussed properly.

Its only when you do these tests that you find out how really difficult it is to focus a view camera accurately!

[Bob McCarthy]

I did repeat the test twice. I was careful to have even lighting. I had plenty of light. The books I used were glossy white faced with good sharp text of various sized fonts. I was careful to insure the face of the books were parallel to the film plane. Everything was zeroed and locked down. The camera is fairly new and tight with no slop.

Fuji loupe, very carefully focused.

The new GG was installed with the ground (?) surface toward the lens, the smooth surface to the photographer. Just checking to make sure this wasn't the problem.

I used a 545 (metal) polaroid back.

I got the same result both times. I looked at the negatives by scanning them in at 2400 dpi.

So how to fix the problem. I plan to install a linhof fresnel. I was going to put it on the back, behind the GG with the two clips provided. But I have read discussion here that it can go in front of the GG!

I can buy a depth gauge and check all 4 corners to insure its at ??? depth ( I assume I just measure film holders with film). Someone kindly suggested how to build one but I just moved and my shop is still in boxes (and will be for some time). Or I can send it off to ??? and have it done professionally. Or is there a better (move convenient) way?

My appreciation for the advice being offered. Buying used off ebay is a a crap shoot. But, it appears the camera is otherwise sound.

What would you do if it were yours?

Bob

[robc]

I was careful to insure the face of the books were parallel to the film plane.

mistake. Book should be at 45 deg and focus on middle of page. Then you can see how far or behind focus is....

see thread http://www.largeformatphotography.in...ad.php?t=15166

is the polaroid depth the same depth as your normal film holder will be with film in it?

[Mik Wenger]

This is the procedure I use to check focus. It does not require exposing and developing film.

1) Remove the bellows.
2) Darken the room and focus the groundglass on a small and bright object with a loupe. A clear lightbulb is best because you can focus on the filament.
3) Insert a sacrificial sheet of film into a holder and insert the holder into the camera. Remove the darkslide.
4) You can now see the image of the lamp filament on the film surface. You can inspect this image with a loupe. Obviously you can't place the loupe on the film like you would on the ground glass because it would obstruct the light, so in most cases you'll reverse the loupe. Refocus the rear standard while looking at the film surface.
5) Note any differences in the focus position. Reiterate as many times until you get a consistent grasp on the focus error, if any.

I believe I can determine the focus to within +- 0.1 mm or better. The only caveat I'm aware of is that looking at the filament's image on the film surface can be a bit tricky because you're looking at it at an angle. It helps to choose the position of the filament within the format accordingly.

[Bob McCarthy]

mistake. Book should be at 45 deg and focus on middle of page. Then you can see how far or behind focus is....

see thread http://www.largeformatphotography.in...ad.php?t=15166

is the polaroid depth the same depth as your normal film holder will be with film in it?

I thought about using newsprint pinned to a wall at 45deg. I agree that would be more exacting, but in interest of getting a approximate idea of whether the camera focus was in the ball park, I set up the test as is. It is easy to get an accurate focus (precision of focus) when focusing a flat surface. I really didn't care if the amount of back focus was 4.5 inches or 6.5 inches. I just wanted a relative idea. I've tested autofocus lenses for back (front) focus you way you describe and the amount can be determined. Even then I put a small target parallel to the flim plane in the center of the newsprint to act as a target for the sensor. But then the camera/lens is doing the focusing. I wanted to "nail" my focus and thought my method would allow that with the greatest precision.

I don't know on the polaroid back. It certainly could be a concideration.

Bob

[Leonard Evens]

Let me elaborate on what Micahel said.

When you focus, it is as if there is a built in coc for how you view the ground glass which will depend on a variety of factors, one of which is the power of the focusing loupe. As a result you can't expect ever to focus exactly. Your error will tend to vary. It may vary randomly or if you always go about it in a certain way, it may be biased in one direction or the other. I found, for example, that, using a 3.6 X loupe, my typical focusing error was less than 0.7 mm and using a 7 X loupe is was less than about 0.3 mm. So focusing error can more than explain your being 0.4 mm off.

As I said, it is impossible to avoid focusing error, but you can reduce it signficantly as follows. Put a piece of tape on your focusing knob and mark a reference point off the knob to compare with. (You have to use the focusing knob rather than the rail because the errors are too small to measure along the rail, but the focusing knob by gearing magnifies things.) Focus repeatedly and each time mark on the tape the position of the reference mark. Examine how the positions vary and choose what appears to be the average position. If there is no bias present in the way you focus, you should be able to improve significantly the accuracy of your focus point. Generally, you can obtain a rough estimate of the error in the average by dividing the typical error in one measurement by the square root of the number of tries. So if your error is about 0.7 mm on one try, it should be less than 1/3 of that if you make 10 tries.

[Bob McCarthy]

I would focus on each side of focus going back and forth carefully until I felt I was (close to) on the money. When I think about it the difference between the point I focused on and where the camera focused was a fairly small turn on the wheel. The 1/4-1/2 mm a poster calculated seems correct.

I was thinking of buying a depth gauge and trying to adjust the glass myself. Is this a waste of money? Would you recommend that I send it off to ????. I hate to loose use of the camera, having just gotten it. But, oh well!.

bob

[Michael Gudzinowicz]

Bob,

Don't adjust the GG. Do a google.groups search for "Polaroid 545 focus" and you'll find that the holder is the most likely problem assuming your loupe is focused on the GG and you used aerial focusing of the page image. If the tests give a consistent 6" error (10'6" when focused at 10'), shim the Polaroid holder so it's 0.38 mm further back from the lens. If you intend to use other holders, check the GG back and holders with film vs the ANSI spec using a dial guage before altering the GG position (call Marflex before doing that).

[Robert A. Zeichner]

I think you are on the right track. The focus is way off. I do believe you should be doing your tests with film holders though. Measurement with depth gauges is quite involved as there are numerous variables and the fixtures you would need could be costly. It is possible to use a collimator to inspect gg/film plane coincidence as you would simply compare results with film holder and film in place vs. the gg. The lens would not be moved as another poster suggested. You might want to read my article on gg alignment for some ideas. Send me an email and I will send it to you.