Re: EXTERNAL LINK: Owner Review of SLRMagic 50/0.95 posted in dpreview "Leica Talk"
Milesab's lens was front focusing, meaning the optics were too far from the image plane to agree with the rangefinder. If the lens was calibrated as free-standing, that means it "leans forward" and the RF image registers a closer focus distance than when the lens is supported by the photographer. As the lens is calibrated in this position, the focus will be correct. As the lens weight is balanced, the RF pickup is pushed backwards, towards infinity. This means the photographer will move the focus of the lens Closer for the RF to agree with it. It should then front-focus. This is one possible explanation. If the RF image is observed to move as the lens is handled on the camera, then it is the reason for focus error. It could be the weight of the lens causing it to bow down, or it could be slop in the helical. From memory of the misfocused image, it looked like 0.02mm shift in the RF calibration.
The lens is an 12-element (edited as per review below), all spherical surfaces. It is very long, meaning the light bundles travel through the lens at more shallow angles than they would in a "squat" lens like the Canon 50/0.95. This could be for two reasons: accommodate a digital sensor, and reduce spherical aberration/focus shift. BUT: this means the lens is long, and heavy and might not maintain good RF calibration. If it is a weight issue, not much can be done. If it is slop in the helical, then one with tighter tolerances is required.
There was an earlier 9 element version of this lens, shorter and lighter. This design should be revisited.
Just the laws of Physics. There will be a quiz at the end of the thread. Introducing aspherical surfaces to the 9 element design to reduce focus shift.