Leica 1.25x magnifier causing backfocus?

[happy]

I have a 1.25x magnifier that is pretty much always mounted on my M9-P. I was testing my 90 Summicron's focus around the house yesterday and found consistent backfocus.

On a whim, I tried testing again without the magnifier on. To my surprise, focus was perfect on practically every shot!

I then set up a target and shot three photos in a row with magnifier on, and three times with magnifier off (defocusing between each shot) and got the same results.

Have any of you experienced this? Could the optics of the magnifier be affecting the way the rangefinder works? It doesn't seem to affect my 50 Summilux, though I haven't tested that lens nearly as thoroughly with/without the magnifier.

 

[peterm1]

I regularly use a 1.25 magnifier (the Japan Exposures one) and have not found this. However I should add that I don't understand how a magnifier could affect back focus in the rangefinder since effectively all it is doing is enlarging the view. Does it also have inbuilt diopter adjustment? I suppose somehow that could have an effect in how the user interacts with the camera. The Leica viewfinder is designed to be sharp at a distance of (I think) 1-2 metres. If the diopter somehow screws with this maybe (????) it could cause the user to back focus when using the camera by somehow making the subject look out of focus when correctly focused. (But I am not sure if this is even possible).

 

[Dante_Stella]

It's probably changing the alignment of your eye to the RF, which does cause back and front focusing. Maybe your RF was actually off but appeared to be correct by virtue of your eye position without the mag.

D

 

[happy]

It's probably changing the alignment of your eye to the RF, which does cause back and front focusing. Maybe your RF was actually off but appeared to be correct by virtue of your eye position without the mag.

D

Ah yes, I am aware of this effect... I've noticed it affects vertical alignment too.

After some testing, I think I've confirmed that you're right about the eye alignment part. The magnifier causes a significant 'tunnel vision' and my tendency is to subconsciously center the frame lines in the visible window. This puts my eye closer to the left side of the eye cup at the distances I was testing at. If I very consciously 'center' my eye on the eye cup, the frame lines are not centered in the visible window, instead they are closer to the bottom right corner. But shooting like this, the backfocus is gone.

Because the 90 frame lines takes up so much less of the frame without the magnifier, I don't tend to do this without the magnifier.

Thanks! Mystery solved. I can now happily continue to use the magnifier and still get sharp focus.