Repair - infinity focus precision calibration - custom mount ring shims

 There are numerous lens with native mount or adapted mount that has infinity focus misalignment. One of the common reason – not very precise thickness of the mount ring (or adapter ring). In this article I’m showing how to calibrate infinity focus when focus ring is “over passing” real infinity focusing distance (you can focus to infinity, then ring still can be rotated further).

Physical reason of infinity focus “over passing” in most lens is – optical core is too close to camera when focus ring is set to “infinity”. Fortunately it is relatively easy problem to fix – you just need to move optical core from the camera a bit (usually to about 0.05-0.3mm).

Lens factories are using metal shim plates for that, positioned between lens frame and lens mount ring. Thickness of shim plate is different, and engineers can add few of them to make lens core distance to camera sensor perfect.

Some inexpensive lens may be not properly calibrated on factory, because it takes extra time and manufacturing cost of calibrating shims. If your lens has this issue of “over passing” infinity focus – you can make custom shim plates from available materials, and calibrate your lens.

I’m using very inexpensive 7Artisans 50mm F1.8 lens as an example in this article. My particular lens is “over passing” infinity, which is annoying. I’d like lens to be focuset to infinity when I rotate focus ring to maximum, without checking if focus is correct.

First step is to unscrew three bolts on mount ring, and take the ring out.

Now I’m using quite old technique of creating “blueprint” of the mount ring inner shape. Take sheet of paper and wrap mount ring inside. Then use pencil to draw sliding diagonal lines on the top. As a result the contours of mount ring will appear.

Then remove mount ring, and use pen to draw the contours more visible.

Cut out the circle. In particular case I’m making shim shape a bit wider in the areas of bolts.

Cut the inner area. Then measure its precision by aligning it on the inner side of the mount ring.

Now use caliper to measure thickness of the shim. In my case it’s 0.1mm thick.

If you need more precision, cut same shim from much thin material, e.g. from 0.01mm.

Then position shim between lens and mount ring and assemble it back. Attach lens to camera, set maximum open aperture (F1.8 in my case), focus to infinity by looking on screen focus point. Then check position of focus ring. With 0.1mm shim inserted my lens is still “over passing” infinity focus, so more shims needed. You can see on picture that infinity mark is to the “left” from focus indicator diamond mark.

This time it is easier, I just draw new shim using existing one, then cut it out.

I made two more shims, and attached inside lens, then tested on camera (three 0.1mm shims – 0.3mm thick adjustment in total). Now my 7Artisans 50mm F1.8 can not focus to infinity at all because lens core is too far. I need to remove one shim (so make it 0.2mm adjustment). After that I can focus to infinity and focus ring position is perfect – infinity mark is exactly above focus indicator diamond mark – that is where focusing ring stops.

Now I can focus to infinity without looking, by just rotating focus ting to very end – that’s much faster than before. It means my 7Artisans 50mm F1.8 lens is perfectly calibrated.

NOTE: I used paper as shim material. Alternative options can be: thick aluminum foil or aluminum sheets from metal soda cans. Just make sure the material has equal thickness (e.g. plastic bottles may have variable thickness of walls). Another point to consider – do not use very thick shims, because in this case mount bolts will not be secured to safe position.