Help me understand a focus test.

campy51 · Sep 17, 2019

I decided to check the focus on my Mamiya 7 and Medalist II by focusing on an object about 4 feet away with the viewfinder and then checking with a ground glass on the film plane and both cameras matched, but when I did the same thing on my 500 C/M they didn't match. Now some of my handheld shots with the 500 weren't sharp but I contribute it to being my error since it takes me several attempts at determining if it's in focus and there may have been movement from my subject. Is there a way of accurately checking focus other than taking a test picture?

Pieter12 · Sep 17, 2019

The question I have is how do correct the problem once you have determined the viewfinder image focus doesn't correspond to what is captured on film? I guess it would vary by camera, maybe using shims? And where would something like that be available...

Kino · Sep 17, 2019

On the 500CM, the focus screen can be shimmed to match; the rangefinders can be tuned via a screw that displaces the pivoting mirror.

If shimming the focus screen is not effective, then you'll have to start looking at the reflex mirror as being a possible source of misalignment.

Just be sure the ground glass on the 500CM film aperture is sitting flush on the film rails and not possibly above due to roller height beyond the aperture.

cramej · Sep 17, 2019

campy51 said:
I decided to check the focus on my Mamiya 7 and Medalist II by focusing on an object about 4 feet away with the viewfinder and then checking with a ground glass on the film plane and both cameras matched, but when I did the same thing on my 500 C/M they didn't match. Now some of my handheld shots with the 500 weren't sharp but I contribute it to being my error since it takes me several attempts at determining if it's in focus and there may have been movement from my subject. Is there a way of accurately checking focus other than taking a test picture?

How are you getting a ground glass at the film plane on a 500cm? The film plane is inside the confines of the back. If you're setting it on the back of the body where the film back mounts, it is not in the same place as the film.

akphot · Sep 17, 2019

First you need to start at infinity. If both ground glass & reflex focus are good then check close focus.

Dan Fromm · Sep 17, 2019

OP, your test is basically a "go-no go" acceptance test. It relies on discerning the point of best focus on a piece of ground glass held in the camera's gate. This, as was pointed out in post #4 above, can be a little iffy. And it isn't as informative as I think you should like.

A more informative test would be with film. Put the camera on a tripod. Point it at 45 degrees from a suitable target a moderate distance away. Traditional targets are brick walls and picket fences. Mark the intended point of focus on the target. Focus carefully with the camera's focusing aid (RF in the case of your Medalist, viewfinder ground glass in the case of your 'blad). Shoot a sequence of shots one stop apart from wide open to fully stopped down. Process the film, examine the negatives at moderate magnification. You'll be able to see where the actual points of focus are and whether they shift as the lenses are stopped down.

campy51 · Sep 17, 2019

I took a couple of test shots with the 500 C/M with 2 different lenses but it may be awhile before I finish the roll. In the mean time I forgot I had 2 focus screens and used one of them as my ground glass at film plane and it was much better than the one I used before so it maybe fine. The test shots should tell me.

jim10219 · Sep 17, 2019

Sometimes I'll use the ground glass method. Actually, I usually use cheap, transparent tape as ground glass. Though, for something like the 500 C/M, where using the ground glass would be difficult to do, I take a photo of a ruler. I use a zero center ruler, though any ruler would do. Just make sure to mark it where you focus. Then lay it out in front of the camera with the ruler extending forward below the lens. When you develop the photo, the numbers on the ruler will indicate where the camera actually focused. So, if the camera focused on 26" on the photo and 25" on the viewfinder, then you know the focus is off. All you have to do now is set up the camera in the same setup as before, focus on 25" and then shim the viewfinder (or mirror or whatever is off) until 26" is now in focus. Then do another test run to confirm everything is fine.

The reason for using a ruler up close is because the focus up close will give a smaller depth of field, which will make it easier to test accuracy. However, depending on the camera, it might be wise to test a few distances. For instance, with my Speed Graphic rangefinder, I usually test something like 4 feet, 15 feet, and infinity. Though it has a ground glass and the Kalart rangefinder has multiple calibration points.

It's been a while since I messed with my old 500 C/M (and I sold it), but if I remember correctly, there's some foam that the mirror rests on when it's lowered. If that foam has deteriorated, then that could effect the focus. So you might want to check that before assuming it's the ground glass in the viewfinder.

Sirius Glass · Sep 17, 2019

Very simply the shims are off, the screen is not in place properly or quite likely the camera body is not square. Take it to a Hasselblad repair man and have the camera checked out including placing the body on the Hasselblad jig [only a Hasselblad repair man would have one] and have the body properly square up. Whatever it cost will be worth it, because all you focusing problems will be corrected at once and you will feel better because you will stop bashing your head against the wall.

Grim Tuesday · Sep 17, 2019

cramej said:
How are you getting a ground glass at the film plane on a 500cm? The film plane is inside the confines of the back. If you're setting it on the back of the body where the film back mounts, it is not in the same place as the film.

Don't ignore this important post. On most cameras you can stick a ground glass on the film gate on the body. On a 'blad, you need a ground glass back adapter.

RalphLambrecht · Sep 18, 2019

campy51 said:
I decided to check the focus on my Mamiya 7 and Medalist II by focusing on an object about 4 feet away with the viewfinder and then checking with a ground glass on the film plane and both cameras matched, but when I did the same thing on my 500 C/M they didn't match. Now some of my handheld shots with the 500 weren't sharp but I contribute it to being my error since it takes me several attempts at determining if it's in focus and there may have been movement from my subject. Is there a way of accurately checking focus other than taking a test picture?

no way,other than taking a test picture

campy51 · Sep 18, 2019

Grim Tuesday said:
Don't ignore this important post. On most cameras you can stick a ground glass on the film gate on the body. On a 'blad, you need a ground glass back adapter.

When I used my spare focus screen it fits in the area I think you are talking about which made me think it's ok.

itsdoable · Sep 18, 2019

campy51 said:
I took a couple of test shots with the 500 C/M with 2 different lenses but it may be awhile before I finish the roll. In the mean time I forgot I had 2 focus screens and used one of them as my ground glass at film plane and it was much better than the one I used before so it maybe fine. The test shots should tell me.

The focus screens for the Hasselblad 500cm are mounted in a frame with the focus plane offset from the foot - you cannot accurately place this in the film gate so the GG is in the same position as the film for focus tests. You will need a bare focus screen, and make sure you put then frosted side against the film rails.

campy51 said:
Grim Tuesday said:

Don't ignore this important post. On most cameras you can stick a ground glass on the film gate on the body. On a 'blad, you need a ground glass back adapter.

Click to expand...

When I used my spare focus screen it fits in the area I think you are talking about which made me think it's ok.

The film plans in inside the Hasselblad back, a few mm behind the back of the camera - if you put the GG on the back of the camera, you are not on the film plane. The Hasselblad backs do not allow you to look directly at the film plane as the insert is side loaded, so it's difficult to see the focused image there unless you have the GG adapter. You could use an old "C" back that has the rear port, or a Kiev NT-back, but in any case, you are not testing the actual back you are taking pictures with. You will have to use film to properly test it.

I use a GG to set the film plane, then do a final test with film. I have not had to re-shim a CM or later body yet, the only time I had issues with focus screen was because the after market focus screen's frame (which sets the GG position) was slightly off - and it only really became apparent with an F2 lens.

ic-racer · Sep 18, 2019

If to long, the camera can be shortened by carefully striking the plate with a plastic hammer. (Focal length is 71.40 mm).

The mirror level is adjusted by bending the mirror support (13121) on the left hand wall and/or the mirror catch lever (13356-1).

To adjust the screen position there are four special screws (21606) in the screen frame. Use key V-4704
for the adjustment. Each rotation of the screw alters the height by 0.35 mm

There would be three adjustments on a Hasselblad to make it focus correctly. Camera length, mirror exactly at 45 degrees and focus screen height.

jerrybro · Sep 18, 2019

IIRC there is a ground glass accessory for the SWC that you put in place of the film back to accurately focus the camera for a photograph. Using that same piece on a 500 would allow you to accurately compare the actual focus at the film plane to the image on the view screen.

Sirius Glass · Sep 18, 2019

jerrybro said:
IIRC there is a ground glass accessory for the SWC that you put in place of the film back to accurately focus the camera for a photograph. Using that same piece on a 500 would allow you to accurately compare the actual focus at the film plane to the image on the view screen.

I use that adapter and then put a prism or mirror to see the focus plane.

Help me understand a focus test.

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