Backfocus ?

GrAfFiT · 02-25-2006

Hello,
I have a problem with my GSN. I noticed on my test roll that there is a general backfocus on all the photos. The in-focus zone is always a few dozens centimeters behind the intended zone.
I opened the back and put a square of tracing paper properly placed exactly in the film plane to see what's being recorded. I set a target and focused on it using the home-made matte screen. I checked the rangefinder and it effectively didn't match. The image was correct on the screen with the focus at 1m instead of 1.5m given by the rangefinder.
I dismantled the top plate to see what could be adjusted but it seems that there was not enough margin for such a large correction. I didn't understand very well how the horizontal RF adjustment screw (the one under the hot shoe) worked. It seems that it is some kind of sinus of the angle of the screw. The amplitude wasn't large enough here. I discovered while doing this that the target was in fact exactely at 1.5m according to my ruler.
I checked the pressure plate, it's working. So are the lenses displaced ? What should I do now ?

The whole rangefinder part works. With a target at 1.5m, it will converge with a distance of 1.5m on the focus ring. But the image on the film plane is blurry. I must set the focus ring at 1m to get a sharp image. What's wrong ?

Kim Coxon · 02-25-2006

Hi The most likely cause would seem to be that either the whole lens or at least some of the elements are displaced or not seated correctly. If the focus is out right across the range, it would point to the lens unit being out. If the amount it varies is out, I would suspect one group is out.

Kim

Quote:

Originally Posted by GrAfFiT

Hello,
I have a problem with my GSN. I noticed on my test roll that there is a general backfocus on all the photos. The in-focus zone is always a few dozens centimeters behind the intended zone.
I opened the back and put a square of tracing paper properly placed exactly in the film plane to see what's being recorded. I set a target and focused on it using the home-made matte screen. I checked the rangefinder and it effectively didn't match. The image was correct on the screen with the focus at 1m instead of 1.5m given by the rangefinder.
I dismantled the top plate to see what could be adjusted but it seems that there was not enough margin for such a large correction. I didn't understand very well how the horizontal RF adjustment screw (the one under the hot shoe) worked. It seems that it is some kind of sinus of the angle of the screw. The amplitude wasn't large enough here. I discovered while doing this that the target was in fact exactely at 1.5m according to my ruler.
I checked the pressure plate, it's working. So are the lenses displaced ? What should I do now ?

The whole rangefinder part works. With a target at 1.5m, it will converge with a distance of 1.5m on the focus ring. But the image on the film plane is blurry. I must set the focus ring at 1m to get a sharp image. What's wrong ?