Zorki 6 adjusting focus help needed

[brankop]

Hello all,

I need help with adjusting my Zorki 6. Here is what I measured:

1) lens (Industar 50) flange to focus distance (@infinity): 28.733 mm

2) camera mount to film distance: 29.007 mm, 28.815 mm, 28.893 mm, 29.065mm. These are points measured next to the screw, starting with the top left, clockwise facing camera.

3) measured actual focus (photographing target) when lens is set to 100 cm: 99 cm

4) measured actual focus (photographing target) when lens is set to 500 cm: 420 cm

5) measured thickness of CN400BW film (base + emulsion): 0.145 mm

6) maximum allowed banding of the film (Maizenberg): 0.05 mm

a) So if I add: 1) + 5) + 6) = 28.928 mm. That is what camera mount to film distance should be. Correct?

b) Based on 3) and 4), lens is further then it should be from the film.

Please advise how to adjust this camera.

 

[brankop]

Correction:

a) So if I add: 1) + 5) + 6) = 28.928 mm. That is what camera mount to plate (instead to film) distance should be. Correct?

 

[mburns]

http://jay.fedka.com/index_files/Page433.htm

In the absence of film, the flange to plate distance should be 28.8 mm; the characteristics of the film are not relevant then because the plate moves back in the presence of film.

 

[KnipsOmat]

http://jay.fedka.com/index_files/Page433.htm

In the absence of film, the flange to plate distance should be 28.8 mm; the characteristics of the film are not relevant then because the plate moves back in the presence of film.

Correct. While taking photographs, film thickness does not effect focus, as the film base is lying behind the emulsion. Emulsion side is facing the lens, and distance to lens is fixed by the film gate of the camera. You want a sharp image on emulsion, as this is what records the light.

 

[brankop]

RE: film thickness

Understood.



So the pressure plate in my camera should be at 28.783 mm from the surface of the lens mount to match the lens.

Therefore, the four points are then 224 um, 32 um, 111 um and 282 um too far from the image plane.

I removed the lens mount and I couldn't find any shims, so I cannot do any adjustment.

Is it possible that the pressure plate in not sitting properly? How to check that?

 

[Livesteamer]

How are you measuring this. This needs to be carefully done. A micrometer from the lens flange to the film rails would be a workable choice. Remember it's easy to sand off metal to reduce thicknesses but do so slowly with patience. Good Luck. Joe

 

[brankop]

Thanks for the answer. I do not have a depth micrometer, but I have an metallurgical microscope with long working distance objective and height scale (5um granularity). When you said to send off metal which part should I send off: film rails or lens mount?

 

[KnipsOmat]

So the pressure plate in my camera should be at 28.783 mm from the surface of the lens mount to match the lens.

Not quite. The flange-film distance of the lens is 28.8 mm, so the front side (emulsion) of your film should be at that distance from the lens mount flange surface.

As worked out above, film thickness is not part of this equation. Front side, position defined by camera's film gate, should be at 28.8 mm. Back side of film can vary with thickness, film can have different thicknesses, and camera would still be in focus, as emulsion is on front side.

However, the pressure plate does not rest in flange-film register distance in an unloaded camera. The distance to the pressure plate is slightly longer than flange-film register.

If you look at the film gate from the back side, you will see two blank "rails" to either side of the opening. The inner ones are where the film rests on, and this defines the 28.8mm distance to lens mount.

But the outer ones are slightly higher, further away from the lens flange. You can see or feel this, if you look closely. The film is guided by these outer rails, as it runs between them and hence cannot move upwards or downwards. The pressure plate in an unloaded camera is resting on these higher, outer rails, it does not touch the inner rails, which define the distance of film front side.

So the distance from lens flange to film is not the same as the distance from lens flange to pressure plate in an unloaded camera. Distance to pressure plate is slightly longer than flange-film distance. Unless you know exactly the difference in height between inner and outer rails, there is no point in measuring against the pressure plate of an unloaded camera.

BTW, this is not Zorki-specific, but the case in most cameras.

If you want to measure flange-film distance, get yourself a small glass plate which is wide enough to cover both inner rails, but narrow enough to fit in between both outer rails. This will ensure your glass plate is in correct film distance (if you get a ground glass plate, you can use it for optically checking the focus).

If you did not account for this when checking focus, your measurements (like 420 cm for 5m) might have been wrong for this reason. Anything resting on the outer rails will not give you correct focus. But anyway, don't take the distance scales on a lens too serious, Frequently, they are anything but exact.

 

[Livesteamer]

I am not sure I would trust a microscope to do this kind of measurement. A depth gauge is also not the first choice. An outside micrometer would work or, if you have a friend in a machine shop, get him to measure it. A coordinate measuring machine works well.

If you do remove metal to adjust the distance, do it between the lens mount and the body. Do not file or sand the outer lens mount or the film rails.

Remember the old rule, measure twice, cut once. Joe

 

[maddoc]

At first makes sure to measure the correct flange-film distance, using a ground-glass positioned on the inner rails. If that distance is 28.8 mm at all points (measure four points to make sure the flange is not tilted) then check the lens for correct distance.

It would be best to check with a second lens first before messing with the flange film distance of the camera because otherwise you end up with the camera fitted to your possibly out-of specs lens and any other lens won`t focus correctly with this specific body.

 

[brankop]

Hello all,

Thanks for this recommendation. I measured (using depth micrometer) flange to film distance (metal plate positioned on the inner rails) at four points. It is between 28.829mm - 28.851mm.

How to remove this excess of 50 um from the back of the lens mount?

 

[KnipsOmat]

How to remove this excess of 50 um from the back of the lens mount?

If, and only if, you're absolutely certain your measurement is correct: I'd sand off the back of the lens mount flange (the side facing the camera body), as this is the easiest part to sand off material evenly. Also, in case you've done something wrong, it's the easiest part to replace or re-shim to correct distance.

But remember what Joe said:

Remember the old rule, measure twice, cut once.