The Summarit has an interesting design compromise to deal with focus shift. Focus shift occurs as a lens is stopped down, due to spherical aberration. Focus shift basically shifts the actual focus of the lens towards infinity as the lens is stopped down, toward the image plane as the lens is opened up.
The Summarit is a 51.1mm focal length lens, shorter than the Leica 51.6mm standard. The optics move 1:1 with the RF cam, an indexed cam is not used to correct the error. The lens is factory set for infinity to be in proper focus at F1.5. Close focus is acheived at F2.8, the focus shift pushes the lens in agreement with the RF. At F1.5, the 51.1mm lens front-focuses because the actual focal length is shorter than what the RF is calibrated for. Clear as mud? Me. to, so I changed the actual focal length of the lens to agree with the RF.
The actual focal length of a complex lens can be set by adjusting the spacing between the groups. Basically, think of it as a zoom lens. The rear groups are held in by a single retaining ring. The fit is tight, and the front section needs to be unscrewed to push them through. Be careful of the aperture blades- open them fully, be careful to keep them in place. If anyone wants to try this, I'll photograph this up.
The Shim from a Jupiter-8 fits perfectly around the glass of the rear group. I used a 0.2mm shim, do not think I could hit it better. SO: Rear groups moved out 0.2mm, lens re-assembled, focus is quite good at F1.5 from close-up to infinity.
Infinity at F1.5,
Bokeh looks good, F1.5.
fence Slats and brick Walls are good for testing the corner-to-corner performance of the reassembled lens.
If an element is out of alignment or in backwards, this usually gives a clue.
Summarit Swirlies are still swirling.
I am looking forward to trying this lens on the M9 Monochrome. The Summarit is great at taming highlights and preserving shadow.
The modified lens back-focuses when stopped down, but at least is consistent at all distances.