Focus accuracy - what's acceptable

[Bob McCarthy]

I removed the bowed Beatte and we installed a new linhoff ground glass w/o fresnel (for the moment).

I decided to test the new rig.

I have a test setup using books with clear text each set up about 3 inches deeper than the previous. I focus carefully on the center book and shoot a polaroid (55).

The test was with a 150 f5.6 at about 10 feet away to target. Lens wide open, close in giving a fairly shallow dof.

it appears the camers is about 6 inches back focusing. What is acceptable?

Bob

[Bruce Watson]

What you put in sharp focus on the ground glass should be in sharp focus on the film. Nothing else is acceptable, at least to me.

[Kevin Crisp]

Anything other than dead on accuracy is not acceptable. There are two many other variables that can effect sharp focus, why start off with a set up that wouldn't let you find it even if everything else went right.

[David A. Goldfarb]

A 6" discrepancy at 10 feet is definitely not acceptable.

If the Beattie had a fresnel on the lens side (it may have been a one-piece fresnel/focus screen unit), this could create such an error. What camera is it?

[Bob McCarthy]

A 6" discrepancy at 10 feet is definitely not acceptable.

What camera is it?

Technika 2000

Would the fact that I shot wide open impact your opinion?

bob

[Bruce Watson]

Technika 2000

Would the fact that I shot wide open impact your opinion?

bob
Not in the least.

[Bob McCarthy]

If the Beattie had a fresnel on the lens side (it may have been a one-piece fresnel/focus screen unit), this could create such an error.

It was the Beattie one piece that was bowed. That was the reason I replaced the GG with a Linhof glass GG.

I used the old mounting position, hoping that it was the original linhof setup. The adjusting screws were still painted over.

Oh well. F22 and be there!!

bob

[Mike Kovacs]

A 5% focusing error of 6" at 10' translates into a huge focusing error close up.

The fact that it is back focusing suggests to me that the image forming part of your ground glass is too close to the lens. Is your camera is designed to have the ground portion of the glass set further away from the lens, e.g. if there was a fresnel in there?

If you know your error accurately, you can determine roughly how much shimming to add to bring the screen to the proper position. If it formerly had a fresnel, you need to accurately measure the thickness of the fresnel and shim the screen that amount. Then keep testing and adjust if necessary.

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.

[David A. Goldfarb]

It was the Beattie one piece that was bowed. That was the reason I replaced the GG with a Linhof glass GG.

I used the old mounting position, hoping that it was the original linhof setup. The adjusting screws were still painted over.

Oh well. F22 and be there!!

bob

It might be that your back was recalibrated for the Beattie, and then the screws were painted over. On the current Technika backs, if I'm not mistaken, the groundglass goes between the fresnel, if you have one, and the lens, with the matte side of the groundglass facing the lens. It's probably best to send it to Marflex and have it recalibrated.

[Michael S. Briggs]

Using the equations for focusing a lens of focal length f=150 mm, an object distance of 10 ft (3048 mm) corresponds to an image distance of 157.76 mm; for an object distance of 10 ft 6 in (3200.4 mm), the image distance is 157.38 mm. So the discrepancy is focus position is only 0.38 mm. (I'm not 100% sure which direction you meant by back focusing -- the magnitude of the focus error is about the same, 0.43 mm, if you meant the other direction.)

Depending on how sharp the screen image is, the brightness of the scene illumination, how well your darkcloth blocks out ambient light, the power of your loupe, your experience, etc., there are limits to how accuractely one can determine the position of best focus. Are you confident that you focused to 0.4 mm accuracy? You could test for a systematic error from the camera versus a random offset by the photographer by repeating the experiment and seeing whether you get the same result. You could try shining a bright light on the book that you are focusing on while focusing to improve your abilty to focus.

[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.info/forum/showthread.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.info/forum/showthread.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.info/forum/showthread.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.

[KenM]

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

I made this (http://www.acanadianplace.com/images/depth-gauge.jpg). Very inexpensive to make, and it can be used to check both your GG (depending on the construction of your camera back), and your film holders.

I compared the GG on my Master Tech. (after I made an adjustment) to a brand-new Master Tech 2000, and I found that my back was possibly more accurate - the difference was 0.001", and mine was closer to the published spec of 0.197" (without film). I know my back is accurately postioned now....

[Brian Ellis]

I'd suggest that you send the camera and screen to Marflex or S.K. Grimes and let one of them install it for you. I've had one or the other of them install most of my screens. It costs a little (about $50 plus shipping IIRC) but it's worth it to me in peace of mind.

[robc]

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

From your answer above I take it that your figure of 6 inches is approximate since you had no way of measuring that 6 inches was accurate.
Given what others have said about that equating to only .4 of a mm on focus rail and the fact your figure is an approximation, I'd say your results are not conclusive.

My guess is that your GG is very close to being in the correct position since it appears to have never been altered.

As I have already said, it is more important that your GG depth is the same as your film holder with film in it depth rather being equal to a standard depth.

There is a lot information at this site in articles about film holders.

The decision is simple, either you are going to check it yourself or you get a professional to check it for you. But of course there are professional professionals and not so professional professionals which has to be weighed up against being able to measure using a depth micrometer accurately which is not as simple as it might seem.
Been there and done that and it took several attempts to get it right.

If you get a Linhof fresnel then place it on outside of GG under clips. If it is placed on inside of glass it alters focus distance and therefore requires adjustment of screws which may not have enough travel for that since the back was never designed to have fresnel on inside.

[KenM]

I compared the GG on my Master Tech. (after I made an adjustment) to a brand-new Master Tech 2000, and I found that my back was possibly more accurate - the difference was 0.001", and mine was closer to the published spec of 0.197" (without film). I know my back is accurately postioned now....

Heh, I'm mixing GG position and film holder depths in the statement above.

The depth of the GG should be 0.190", and the depth of an empty film holder should be 0.197". Film is about 0.007" thick, so when inserted into a holder, that would raise the depth to 0.190", which is where the GG should sit.

Sorry for any confusion that statement might have raised.

[KenM]

As I have already said, it is more important that your GG depth is the same as your film holder with film in it depth rather being equal to a standard depth.
Ths is certainly a valid statement. However, how many of us actually use a single holder? :D

There's very little chance that every holder you have will be identical. I've checked all of my film holders, and I've discarded the ones that deviated from the standard by more than a 'little' - the remaining holders were very close to the published standard. You could always do a quick test of all your holders using some sort of measuring gauge (like the one I mentioned) and determine the mean depth, and then set your GG to that depth. The standard indicates that the normal depth of a 4x5 film holder is 0.197", +/- 0.007".

I guess what I'm trying to say is that the holders were manufactured to a standard, and if they're off by too much, they're either defective or worn out, and shouldn't be used.

[robc]

well if you had read the thread which I gave the link to, where all this has been discussed before, you might have seen that the suggestion was to take the mean across all your film holders and that using only one make of the same vintage was advisable.

[Bob McCarthy]

I'd suggest that you send the camera and screen to Marflex or S.K. Grimes and let one of them install it for you. I've had one or the other of them install most of my screens. It costs a little (about $50 plus shipping IIRC) but it's worth it to me in peace of mind.

I spoke to Martin today. The back will be shipped out next week.

Thanks all. A lot of knowledge in this thread.

bob

[JW Dewdney]

Granted.
I'm coming in on this thread pretty late. But I have to say I'm a little bit horrified by some of the responses here. There are way too many assumptions being made here. I'd hate to see someone going to unneccessary cost and expense based on those responses. Here is what I'd say about the problems with assessing the situation;

1. Only a single trial was made. The experiment doesn't, therefore, rule out any environmental or procedural factors. Do it ten times. Log the results and figure out the mean result first. Possible influencing factors may be tilt of camera angle (vertically) causing the film to move, jarring the camera during the exposure (possible), shutter vibration, failure to lock down the standards and general film plane and subsequent movement, improperly functioning hardware causing focus shift during lockdown, film movement due to improper seating, film movement due to temperature, etc. etc. etc...

2. A big one. This experiment is valid ONLY for the original poster's particular Polaroid holder (which maybe defective, dirty, or have some other problem).

3. The experiment is valid only for the batch of Type 55 in the original poster's specific holder. If he is shooting film in holders - this would provide a much better means of determining the focussing accuracy if that is the OP's interest.

And many more.

Given the fact that there were so few controls to the procedure, I'd say the result in terms of focus accuracy is encouraging. There may be absolutely NOTHING wrong with the camera setup - it's really impossible to tell without further testing. If, in further investigations, you were able to determine that the mean error was absolutely consistent under given conditions - then I'd say don't worry about the ground glass. You can shim it if you're absolutely neurotic about it. But I thin a better solution would be to figure out what angle you need to turn your focussing knob through to correct it and then apply the same (counter) rotation when you shoot. Again - apply this technique and then test the result. Your technique is every bit as critical to you success as your tools are. If not more so.

Just my 2 cents.

[Jay W]

By back focussing, I assume you mean that the image is in focus behind the object you wanted in focus.

Here's a mental exercize. If you shim the GG out, you would move the standards closer together (than previously) to get the image in focus. The new image would be focus would be further behind compared to the first go-around. Correct me if I'm wrong, but it seems the GG needs to be closer to the lens rather than shimmed out.

Jay Wenner

[James E Galvin]

I've found that I can flex the back standard by pressing the loup too hard against the GG. I can see the flex (on a Calumet CC400), so it might easily be .4 mm. If the back is flexed forward, then the lens is adjusted forward for focus. When taking the pic, the back unflexes, the lens is now too far forward, and the pic is focused nearer than desired.

[Bob McCarthy]

In the end, since the camera was purchased used, I elected to send it off to the Linhof repair facility (Marflex). That way I'm starting with a good base line. If it was the polaroid holder then I will know that for sure.

I decided not to do more multiple tests as P/N 55 is $85 a box (20) locally. It would tell me no more than I already knew - I likely have a problem.

The Linhof screen is a ground "glass" not ground 'plastic" like the beattie.

I do like the idea of having a depth instrument to check film holders, etc.

Bob