Summarit 5cm F1.5: modify for using F1.5 at all distances

A discussion thread on Getdpi made the light bulb turn on for a problem that has frustrated me for a long time. Summarits are optimized for F2.8 used close-up, and F1.5 at distance. No shims, no stand-off rings, not a single luxury to adjust it.

So I took the lens apart and used a shim from a Jupiter-8 to increase the spacing between the front and rear groups by 0.2mm This increases the focal length of the lens from 51.1mm to something closer to the Leica standard 51.6mm. It is my belief that the 51.1mm focal length was a compromise of the design, done to deal with focus shift. 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.

The actual focal length of a complex lens can be set by adjusting the spacing between the groups. 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.

The Shim from a Jupiter-8 fits perfectly around the glass of the rear group. I used a 0.2mm shim, based on my experience with the 1950 Jupiter-3 and used the Force. 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.

At F4, focus with RF then backed off ever-so-slightly as i do with a Sonnar.:

100% crop:

I was making this up as I worked on the lens, but will photograph the procedures using a Parts lens.
Step-by-Step, using my Parts Lens.

1) Remove three set screws closest to aperture ring, and one of the set screws over the "Latch Port".

2) Use screw driver in latch port. Rotate the optics module until the screwdriver drops into place. There is a small hole that must be latched to hold an inner sleeve in place. after latched, you can unscrew the front section.

3) Module unscrews and is out.

4) rear retaining ring out.

5) unscrew the front section, grasp rear and front ring.

6) Front section Out, must push rear out through the aperture. BE VERY CAREFUL!

7) rear out.

8. Shim in place, re-assemble lens in reverse order.

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