Ikonta: film-plane and checking focus.

[tunalegs]

This is indeed seen in camera's using (single element) Meniscus lenses. These lenses can not be corrected for a flat filmplane so a curved filmplane is needed to still keep some sharpness towards the corners.

That is one thing, but I'm talking about another. The film arches against the pressure plate or the back of the camera (in the case of a box camera) the plate (or back) pushes against the film which causes it to flatten in the center - indeed it will be pushed flat up to just about the very edge of the film. This is how some companies accomplished getting the film flat without a risk of scratching the emulsion, the force of the film curl against the plate is gentler than the force of a pressure plate against the film, particularly in an era when different films had different thicknesses. Often the pressure plate itself would have raised edges to hold it off the film gate, but some cameras used what appear to be film rails to do the same thing - but the result is the same, there is some "free space" but it's not large enough for the film to actually move around freely in, because the film's curl will push it against the plate. If film had no curl, it indeed may "float" in this area, but the curl is what keeps it from doing so in actuality.

 

[hanskerensky]

. If film had no curl, it indeed may "float" in this area, but the curl is what keeps it from doing so in actuality.

Yes, you are quite right in your observations.

And let us not forget that the film 70 years ago was much more curly then it is today.

 

[papaki]

The film is kept flat on these by tension between the take up spool and the roll and the pressure plate. There is no need for the film to rest between the guards as in others.

 

[micromoogman]

This just confirms my problems of adjusting these Super Ikontas, M-Ikontas, correctly in my collimator. I should have checked the accurate focusing before disassemble the lens. I went adjusting with the rollers as film plane but pictures are not as sharp as it used to be. Also tried the outer rails but no luck there either. I now have a Mess Ikonta with focusing problems at hand and need to do this again...

 

[hanskerensky]

This just confirms my problems of adjusting these Super Ikontas, M-Ikontas, correctly in my collimator. I should have checked the accurate focusing before disassemble the lens. I went adjusting with the rollers as film plane but pictures are not as sharp as it used to be. Also tried the outer rails but no luck there either. I now have a Mess Ikonta with focusing problems at hand and need to do this again...

As said before , I always use my auto-collimator on a film loaded in the camera. Not, as usual, on a mirror against the filmgate, as I observed that there were deviations showing on images taken with the cameras. No doubt due to the working of the pressure plate and the tension of the film.

 

[micromoogman]

Ok, thank you Hans. Do you mean that the film itself reflecting the light? Must try that...

 

[micromoogman]

Hah, it DID WORK! I will never use a mirror again, this is revolutionary... Thanks a lot Hans!

 

[hanskerensky]

Hah, it DID WORK! I will never use a mirror again, this is revolutionary... Thanks a lot Hans!

That a trick that I learned from a retired camera service man 🙂

However you will observe that there can be deviations between the film frames (curling, slack, etc.) so you have to measure more then one frame and take the average deviation to correct.