Lens shimming question

I just bought a set of Nisi Athena primes and I’m trying to adjust the backfocus (which was off out of factory.) I’m using a SimMod PL to L adapter on the BMCC 6k.

On the 40mm lens, without shimming, infinity focus was hitting around the 20’ mark on the lens. I added a shim to the PL adapter and now infinity focus is perfect, minimum focus is perfect, HOWEVER the 3’ to 20’ range is way off.

At real world 7’ the lens is in focus while reading 5’ , etc.

Is this something that can be fixed with shimming more/less? I’m not very experienced in this area.

Thanks.

Lenses can be off too. Maybe you need to send it back.
What about the others, you wrote you bought a set? Try shimming the widest one first, it's the most critical.

More odd observations:

BMMC6k + Simmod PL adapter:
The 135mm was perfect in its lens markings.
The 18 and 40 were off toward the middle range.

I tested the lenses on my FX3 with a Simmod PL to E adapter (no shims).
The 135mm was perfect.
The 40mm was perfect.
The 18mm was off toward the middle range again.

I’m not quite sure where to adjust from here.

Accurate witness marks on lenses used as cinema lenses may not always be reliable depending upon the price range. In the good old days when focus distances were measured by a tape with marks on the floor for the actors to know where to stand (and the lens set accordingly) then accurate lens markings would be a time-saver for the camera operator. But your digital cameras come with focus assist tools that give you visual aids when the subject is on focus at a minimum (with some other cameras using autofocus with tracking), so I’m not sure how important the lens’s markings are for your shooting requirements.

Personally I rely on the focus assist tools the cameras provide even when setting up the shot distances to various spots before a performance, such as front of stage, centre stage, right and left of stage. Those distances also let me determine depth of field so I can adjust focus when needed as subjects move between spots.

If you do rely on focus assist. use the lowest sensitivity setting such as 25% or the thinnest focus assist (low rather than medium or high) that is useful to accurately set focus.

Rick,
It’s not a deal breaker for me, as long as min focus and infinity are accurate. I do think it’s possible to get them nearly perfect though with some shim adjustments. When I bought a set of Blazar Remus, they were way off in backfocus alignment. After some trial and error I got them very close to accurate.

Agree, if minimum and maximum focus are good, then the mid distances should be close. If infinity is good but minimum are not, it may just be a very small +/- adjustment to the total shim thickness to keep both looking good.

Years ago I had a set of Optars tuned up and I asked the tech to make sure the markings were accurate. He just laughed and said get a Russian tape measure, they made sure infinity was good and let the scales fall where they may.

There's no way to correct non-linearity in the scales solely with shims as they represent a single point of adjustment. Pick a point or split the difference.

BMMC6k + Simmod PL adapter:
...40 were off toward the middle range.

I tested the lenses on my FX3 with a Simmod PL to E adapter (no shims).
The 40mm was perfect.

Doesn't really seem possible that the 40mm could be perfect on the Sony yet has nonlinearity on the Blackmagic unless there is another issue.

Good Luck

Thanks. Good info. The e-mount being perfect w/ the 40mm is definitely throwing me off.

Update:

I bought a Nisi PL to L adapter and all 3 lenses are reading nearly perfect.