Focusing error

[thechrisproject]

In some shots where I shoot wide open (f/5.6 on a 135 lens) I've noticed that the plane of focus on the negative/scanned image is consistently in front of where it was when I set the focus on the ground glass. Here is an example:

http://farm4.static.flickr.com/3443/...a7b4a8b3_o.jpg

I'm fairly certain that when I composed and focused on the ground glass, the plane of focus encompassed the window frame and the mask on it. I wasn't really doing any adjustments if I remember correctly. When I scanned the shot, it certainly looks like the plane of focus is about 1-2 feet in front of where I wanted it to be. I have another shot where the subject was about 30 feet away and the plane of focus is a few feet in front of it.

What are my possible culprits and how do I find out for certain?

It's likely to be user error, but which kind? Bumping the camera forward? Loose rear standard? Or could it be something with my holders or the way I put film in, or a mechanical misalignment of some sort? Something with the lens?

Usually I stop down a little so this isn't much of a problem, but it's certainly annoying. If I have to do some rounds of lens chart tests and suchlike, I certainly will.

[Bob Salomon]

1 ground glass position
2 lens -they aren't made to be used wide open
3 film holder

So first have the gg checked, is in installed properly? Ground side down? Is it the proper position?
Then try shooting at the apertures the lens was made to perform optimally at.
Are you out of focus with all holders or just one holder?

[Vaughn]

It seems like the left side of the window frame is sharper than the right side. Perhaps a little front and/or rear swing was used?

Since the plane of focus shifts forward, the distance to the lens and the film is slightly longer than the distance between the lens and the GG.

So it is a matter of type of camera and film back you have. If the GG had a fresnel lens on the inside of the GG (rare to have them there) and the back was designed for it, then if the fresnel lens went missing, then the GG would end up just a little closer to the lens -- and would create the situation you are having.

I once had questions concerning my 8x10. I took off the back and measured down from the front side (towards the lens) to the GG. I then inserted a film holder, pulled the darkside and measured down from the front of the holder to the plane of the film in the holder. I did not use a ruler or any such fancy measuring devices -- just a piece of paper I marked the distance and then compared it. I was lucky, the distances were the same.

Vaughn

[Robert A. Zeichner]

If I had to bet on the cause, I would say your ground glass is not properly aligned. Of course it would help to know exactly what camera you are using. I think it safe to assume you are shooting 4x5. The film holder depth error spec for 4x5 is +/- .007". It is common for holders to exhibit errors within that range, but if you gg is for example, shallow of 0/0 by say .008" and your film holder is deep by .007" (within spec for the film holder), the total error will add to 0.015" and you will now be out of spec. If you are using a Linhof Technika, and the ground glass hasn't been messed with, it is likely to be just about perfect as the pads on which the glass sits are very precisely adjusted at the factory. On the other hand, if using a camera with a plastic gg as a friend of mine once had, where the gg actually warped, focus will always be off. Wooden cameras can be surprisingly well adjusted where gg alignment is concerned, but many are not. Tell us a little more about what you have and maybe this will better point to the source.

[Bob Salomon]

"If you are using a Linhof Technika, and the ground glass hasn't been messed with, it is likely to be just about perfect as the pads on which the glass sits are very precisely adjusted at the factory."

As long as there hadn't been DIY adjustments over the life of the camera. or they took the pads off (which we see quite a bit of).

[Robert A. Zeichner]

"If you are using a Linhof Technika, and the ground glass hasn't been messed with, it is likely to be just about perfect as the pads on which the glass sits are very precisely adjusted at the factory."

As long as there hadn't been DIY adjustments over the life of the camera. or they took the pads off (which we see quite a bit of).

Just like I said, if it hasn't been messed with.

[rdenney]

2 lens -they aren't made to be used wide open

Bob, this statement confuses me mightily and maybe you can clear it up. It seems to me that we focus the lens wide open, and then we stop it down and perhaps check focus. In all my experience, I've never seen a focus shift when stopping down, except when using just the rear cell of a convertible lens.

But if the guy is using the lens at the aperture at which he focused it, there can be no focus shift because there is no change in aperture. I just can't make your statement square in my mind.

Rick "who focuses lenses wide open" Denney

[Nathan Potter]

Well I notice what Vaughn has noticed - right side of window is sharper than the left side. I'd go through a critical focus check of the system.

First make sure the front standard is exactly parallel to the rear standard. You need to use vernier calipers or any solid length rod or bar about the focal length of the lens you're using and rack the bellows in until the rod just touches one of the corners of the front and rear standards. Check the other four corners by noting that the rod just touches those four corners with less than a hairs width gap. Note that the rod needs to be exactly orthogonal to the standards. If you've got swing and tilt engravings on the standards note the positions when you've adjusted for planarity.

Next make sure your lens board sits flat against the front standard and is not warped at the lens mount opening.

Next set up a finely divided yard stick centered about 6 ft in front of the camera and disposed at an angle off the optical axis so that you can see the divisions in the GG. Position it near the center of the view and focus with the lens at maximum aperture on 15 inch mark using a high quality loupe. LOCK down the standards and recheck focus critically. I generally use a 210mm lens on 4X5 for this so the magnification is roughly 1:10.

Load your best film holder with film and with great care. DON'T disturb standards, camera or tripod. Take the image with lens wide open and develop the film. Polaroid is nice but do you want to check the 545 back or your best film holder? Develop film. After you've exposed and removed the holder check the focus again to confirm that you didn't move the setup.

Inspect film using a good loupe and determine if the sharpest spot is at the same 15 inch location. If not note the new location in, hopefully some fraction of an inch difference and record it.
Now you have a delta in the DOFld and using the appropriate formulas this can be converted to an equivalent DOF error probably in a few to several mils.

Also before you start this test and as mentioned above by others make sure that the ground side of the GG faces the lens and rests unobstructed on the flat surface of the rear standard, or as Bob points out small pads or flats found on some standards such as Linhof. If using Quick or Readyloads or 545 backs make sure the pressure plate is adequately spring loaded and sits exactly in the proper plane. It would also be wise to check that the film sits relatively flat against the backplane (plenum) of the holder once the dark slide is removed. By all means waste a sheet to confirm this with all your holders.

All this is a PITA but its like a detective story so have fun.

Nate Potter, Austin TX.

[thechrisproject]

Lots of great responses, thanks all.

The camera is a Chamonix 045n-1. The lens is a Nikkor 135 f/5.6. The ground glass is as it was installed at the factory, which I'm fairly certain is the correct orientation.

I don't focus with a particularly powerful loupe. I don't particularly want to go through too much nitpicking if the problem is just user error and poor technique. Although it's probably a good idea to go through the nitpicking anyway, for the experience.

If you're familiar with the Chamonix you'll know it's pretty easy to have the standards not totally parallel. It's pretty free and loose. I do have vernier calipers though, so I could check it when doing testing. If that's the cause I can deal with it.

[ic-racer]

Shim your ground glass back (away from the lens). You can use trial and error, or a quick calculation. Did you focus on the window and the frame on the wall is in focus? If so your error can be estimated by the 1/p + 1/x = 1/focal length equation.
So if you focused at 5000mm and the film showed focus at 4500mm from the camera, the ground glass is off by around 1.3 mm (if you are using a 150mm lens)