50/1.5 C Sonnar optimized for f/1.5... perfect match for the HRF??

[awilder]

Experiece and measurement of the Konica Hexar RF (HRF) indicates a film channel approx. 0.04 mm deeper than the standard Leica M body despite having it's inner film rails located at the same distance from the lens flange as a Leica. As a result, there is a tendencey for Leica/ZM lenses to slightly front focus the image since there is more room for the film to sit farther back against the pressure plate located an extra 0.04 mm farther backwards in the HRF. Brief experience with the standard ZM 50/1.5 C Sonnar (optimized focus shift for f/2.8) was a distaster untill about f/4 regardless of whether at infinity or near focus due to the lens' tendency to front focus at large apertures, thus making the body's front focus tendency even worse. The same lens optimized for focus shift at f/1.5 however, may be a different story since the factory adjustment of the rf cam/ optical cell synchronization effectively adds some back focus to the lens thus possibly negating the slight front focus tendency of the HRF. I'll report on my results when I get my hands on the compensated C Sonnar.

 

[Sonnar Brian]

Sounds like a good J-3 would be a perfect match. They back-focus on a standard Leica.

 

[ferider]

The 1.5 adjustment will not help at infinity.

Roland.

 

[awilder]

I can't imagine why it wouldn't help at infinty. The sample f/2.8 optimized Sonnar focused much sharper on the foreground than long distance despite the lens being set at infinity. All Zeiss did with the f/1.5 optimized version is to move the optical cell slightly backwards to compensate for front focusing peripheral optical rays at larger apertures.

 

[kshapero]

My major in English Literature is not helping me at all understand this thread. Huh?

 

[ferider]

I can't imagine why it wouldn't help at infinty. The sample f/2.8 optimized Sonnar focused much sharper on the foreground than long distance despite the lens being set at infinity. All Zeiss did with the f/1.5 optimized version is to move the optical cell slightly backwards to compensate for front focusing peripheral optical rays at larger apertures.

I don't think that's what they do. I think they just rotate the RF cam by 180 degrees. Infinity collimation remains unchanged.

Roland.

 

[Roger Hicks]

I don't think that's what they do. I think they just rotate the RF cam by 180 degrees. Infinity collimation remains unchanged.

Roland.

Dear Roland,

Actually, 120, I believe. Otherwise my understanding is that you are absolutely right.

Cheers,

R.

 

[ferider]

Dear Roland,

Actually, 120, I believe. Otherwise my understanding is that you are absolutely right.

Cheers,

R.

Sorry - I keep doing the same mistake when writing it out (in my mind it's a third of full circle ... 😱 ) You are right of course.

awilder, check the rf cam of the lens and you will see what we mean. The
cam is split into three segments, and the lens has 120 (!) degrees focus
throw. Apparently the lens can be optimized for three different settings,
2.8, 1.5, and possibly a larger aperture (like the Canon 50/1.5's f5.6 ?).

Roland.

 

[awilder]

I'll know in a day or so. My point simply being that by resetting the rf cam, doesn't it effectively change the distance of the optical unit to the film via the focus selected by the rangefinder?

 

[Dante_Stella]

It will help a little bit. As far as I could tell, it was optical adjustment. It will still front-focus very slightly, but this is pretty much the story with every fast lens. Better that than the dread back-focus. On superspeed lenses, the focus point will always be shifting rearward when you stop down - and the trick is adjusting things to where it stays within the DOF of the lens.

DAG did an (unsolicited) adjust on my Hexar to reduce the flange distance by 0.05mm. All of my lenses seemed to work fine afterward. I'd almost bet that it's just removing bushings from beneath the mount. Since 0.05mm is bigger than the tolerance for RF cam on an M system (height from mount is +0.03/-0.00), you should get the rangefinder reset as well.

I never observed a problem any lenses with film on any body; the Hexar adjustment was done when DAG reset my M-Hexanons for use with the M8.

 

[awilder]

Thanks for the input guys, I'll report in a couple of days on my findings.

 

[Roger Hicks]

My point simply being that by resetting the rf cam, doesn't it effectively change the distance of the optical unit to the film via the focus selected by the rangefinder?

Not at all distances. The metered distance remains constant at infinity, because the lens is not moved, only the cam. As the lens is focused closer, the distance relayed to the rangefinder differs increasingly between the three cams, being optimized for the actual point of focus of the lens at a given aperture. At least, that is my understanding.

Cheers,

Roger

 

[awilder]

Thanks for the explanation Roger. That's consistent with what the other members have posted thus far.

 

[awilder]

Preliminary film results indicate that the f/1.5 optimzed version does indeed focus much more accurately on my HRF both near AND far showing slight front focus wide open and spot on focus by f/2.8. This applies to both minimum focus as well as infinity. The f/2.8 optimized version tended to front focus the subject near or far up to at least f/4.