Lens Focus is Off, Thoughts?

[bayernfan]

I was just testing a few of my Leica lenses on my Ricoh GXR M-Mount unit and came across something rather strange with my 35/2.8 Summaron M-mount.

I set up a tape measure to the wall (exactly 2m), focused with lens on an M2 and the lens focus scale read exactly 2m. I did the same test with a 50mm summicron, a 40mm m-rokkor, all read precisely 2m.

I then mounted each lens on the digital GXR to confirm that the lenses were in fact focusing correctly at 2m. The 50mm summicron and 40mm m-rokkor were dead on to focus at the 2m mark indicated on their scale. The Summaron, however, attained focus when the lens scale read 1.5m.

Any thoughts on what, precisely, is occurring?

 

[Sarcophilus Harrisii]

I was just testing a few of my Leica lenses on my Ricoh GXR M-Mount unit and came across something rather strange with my 35/2.8 Summaron M-mount.

I set up a tape measure to the wall (exactly 2m), focused with lens on an M2 and the lens focus scale read exactly 2m. I did the same test with a 50mm summicron, a 40mm m-rokkor, all read precisely 2m.

I then mounted each lens on the digital GXR to confirm that the lenses were in fact focusing correctly at 2m. The 50mm summicron and 40mm m-rokkor were dead on to focus at the 2m mark indicated on their scale. The Summaron, however, attained focus when the lens scale read 1.5m.

Any thoughts on what, precisely, is occurring?

How did you focus with the lens on the M2? Ground glass over the film rails, for example? Or were you using the rangefinder?
Cheers
Brett

 

[bayernfan]

rangefinder. with every lens, when the rangefinder images are aligned, the lens focus scale reads 2m on the mark.

when re-focused from the same distance with the GXR in live view (lens at f/2.8), the Summaron focus scale is pointed right between 1.5 and 2m, let's call it 1.75m.

is this kind of discrepancy normal? it just seems odd that the other lenses i tested are bang on, or very very close to being an exact match to their indicated focus scale. maybe 0.25 m is not a big deal in the rangefinder world?

 

[Robert Lai]

Try the M2 with film in it. It will show you if the Summaron is front focusing. The helical and rangefinder cam of the Summaron are probably accurate, thus the RF indicates focus at the proper distance of 2m. However, the optical cells may need adjustment.

The GXR indicates that the focus is clearly 25cm closer to you than what the focusing ring distance scale indicates.
25 cm is 1 foot, which is a huge focus discrepancy, and unacceptable.

Don Goldberg is the person I use to correct lenses with these focus errors.

 

[bayernfan]

Try the M2 with film in it. It will show you if the Summaron is front focusing. The helical and rangefinder cam of the Summaron are probably accurate, thus the RF indicates focus at the proper distance of 2m. However, the optical cells may need adjustment.

The GXR indicates that the focus is clearly 25cm closer to you than what the focusing ring distance scale indicates.
25 cm is 1 foot, which is a huge focus discrepancy, and unacceptable.

Don Goldberg is the person I use to correct lenses with these focus errors.

Agreed, the helical/cam config seems accurate, but the elements/groups are off. I think a change in shim/spacer will be necessary. Already sent an email to DAG, will see what he has to say.