Canon f 0.95 focussing

[BillCB]

I have been using a Canon 7s with three lenses. The rangefinder is accurate with the 50 mm 1.4 and the 35 mm 2.0. The pix are sharp at full aperture, and looking at ground glass at the film plane with a 15x magnifier confirms the rangefinder's focussing.

However, when I mount the f0.95 it's a different story. The rangefinder becomes coincident at about 60 ft with a subject at infinity. Or, at a measured 6 ft the rangefinder wants me to focus at about 5.5 ft.

I have deduced that the distance scale on the lens IS accurate. If I focus at infinity for a far-distant subject, it is sharp. If I focus at 6 ft for a subject at 6 ft, it is sharp. This would imply that the rangefinder needs adjusting. But if I adjust it, it throws out the focus for the other two lenses.

What gives? The implication is that the cam on the 0.95 lens is "off" in some way. Can anyone help? As I say, the lens markings are right, the rangefinder on the other lenses is right - what gives?

Bill

 

[Sonnar Brian]

It sounds like the RF cam is off. If the RF focusses at 5.5ft when the actual distance is 6ft, the RF pickup of the camera is being pushed in farther than it should be. Same as the lens set to 60ft driving the RF of the camera to infinity, it is pushing it in too far. If the distance scale of the lens is accurate, it means the shim of the lens is good. I would have a pro do this job, it is an expensive lens.

 

[BillCB]

Thanks - your logic is right on! I'm going to take a few measurements with a micrometer to see if I can confirm.

I have dismantled the 1.4 lens for helicoid re-greasing with success so I may take look at the 0.95 too - without dismantling the optics, though!

 

[John Shriver]

Could also be that the RF roller is riding on the rear element instead of the little protruding cam. That would cause a serious error. It would happen if the RF coupling arm was hanging a bit low.

 

[Sonnar Brian]

The cam on the 50/.95 is fairly thick. If it did miss the cam, the error would probably be larger.