Any internal refraction of a ground glass?

[wp_cheng]

I cut a ground glass from the focusing screen of a medium format

camera. I put it on the film rail of an M3 which was recently

calibrated by a technician, I found the following when focusing at a

newspaper which is exactly one metre from the film plane:

 

- Rangefinder read as one metre

 

- sharpest image formed on ground glass at slightly below one metre

 

I did this on a tripod with a 50mm Summicron and a 50mm Summilux with

the same result. The same happened with another M4. I start to

wonder whether any internal refraction of the ground glass would

cause this. If yes, then how was this corrected when we are focusing

with a medium format camera.

 

Somebody may tell me not to bother about this and leave it to the

technician, results on photo are the only thing important. I know

that, but I'm just curious about this.

[roger c]

Do you have the ground glass the right way up (ground face on the rails)? If it's ground on both sides then anything could happen.

[jean_christophe_barnoud1]

I understand that the sharp image on the ground glass is of a subject in front of the newspaper. If so, apart from camera calibration, two issues may arise :

 

1. You set the ground glass against the outer film rails. In 35mm cameras the film moves without friction in the film channel. These cameras have two pairs of rails, the film rests against the inner rails, the pressure plate against the outer ones. In such a design the distance bewtween lens flange plane and inner rails plane is less than camera register (flange to film ideal distance) and distance between flange and outer film rails is more. The film rests in between the inner rails plane and the pressure plate (same plane as the outer rails) with the emulsion not really flat but on average just very slightly behind the inner rails. Difference between the two rail planes is near 0.2mm which is slightly more than the film expected thickness. If you set your ground glass against the internal rails, the focus should be (very) slightly behind the newspaper. To do an exact test you may need a very thin (single digit hundredth of mm) shim between the inner rails and the ground glass.

 

2. (Less probable as it should mostly compensate for the absence of a shim) the ground glass is very coarse : the depth of the etching is significant, the mechanical reference plane is the outermost lens side part of the etching and the best focus plane is an intermediate within the etching depth (further from the lens).

[charles barcellona www.bl]

Here we go again.

 

Film in a Leica, or any other 35mm camera for that matter, does not lay in a flat plane. If looked along the length of the film, in the film gate, with the back closed, the film starts out slightly off back (toward the lens), then touches the pressure plate at about 1/3 the way across, then bows away toward the lens again in the middle, then back to touch the pressure plate, and then slightly toward the lens again at the opposite rail. Its sort of a wavy M shape.

 

Manufacturers choose some spot behind the inner rails as the focusing point for the camera. Roughly, this is very close to the position of the outer rail minus film thickness, or in otherwords, if you layed film on the pressure plate when the back was closed, the FRONT of the film is where its supposed to focus (more or less). This is more or less, since every manufacturer thinks that the "standard film thickness" is slightly different (it is), and that the amount of wavy M is slightly different (it is, but only a little). They also figure their lens designs into this I'm sure.

 

Whats this all mean? Its a guesstimate. For use with a ground glass, you'll need to shim it for use on the inner rails. How much shim? I have no idea.

 

A long time ago, I developed one method for aligning Nikon F and F2 bodies. At the time, Nikon (EPOI) Garden City was doing just terrible service and would send anything out the door if it made an image. During my investigation, it was found that .0025 inches of shim worked as good as anything - but that was on a Nikon F/F2 body.

[wp_cheng]

Thanks for all your response.

 

Roger, I placed the ground glass correctly.

 

Jean & Charles, you are absolutely right. I sticked 3M adhesive tape on fringe of the ground glass as the shim, the ground glass is now focusing correctly at both infinity and one meter.

 

Once again, thanks very much.