Back focus and shimming mounts

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Ian Henderson

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Back focus and shimming mounts

PostSun Sep 16, 2018 9:04 pm

I have a 21-100 Zeiss Zoom showing what I think is major back focus issues. Focus distance scale is a 100% at 100mm but miles out at 21mm (so lens is far from par focal). Is this likely to be a camera or lens issue?

How would one tell if the lens needed to be adjusted or the lens mount itself shimmed? And can the EF mount on the UMP be shimmed? That’s what’s I’ve got on there at the moment.
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Denny Smith

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Re: Back focus and shimming mounts

PostSun Sep 16, 2018 9:42 pm

Is the Zeiss Zoom a EF Mount? Lens flange distance (back focus) needs adjusting or the lens needs to be re-collimated. If it is an EF mount, have the lens checked. If you are using a PL lens with the PL/EF adapter, loose the adapter and get rhe UM Pro PL mount, which can be shimmed to correct FFD.

The EF mount can be shimmed, not recommended as you will loose your EF contacts if you shim more than 0.010 of an inch.
Cheers.
Denny Smith
SHA Productions
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Ian Henderson

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Re: Back focus and shimming mounts

PostMon Sep 17, 2018 4:28 am

Hi Denny

It's an EF lens and EF camera mount at the moment. At some stage I'm going to pick up PL for both, but as I'm using also Contax primes it made sense to get the lens in EF for now, and the lens is properly supported so the EF is less of an issue.

Thanks for the advice - I've been in contact with the Zeiss agent, let's see what they say about the lens collimation then. It's miles out, at 8' (in focus at 100m) I need to focus down to 4'9" at 21mm, and at 4', the 21mm is at 3'1.5".

cheers, Ian
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Denny Smith

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Re: Back focus and shimming mounts

PostMon Sep 17, 2018 4:44 am

Good luck Ian. It is issues like this, that I do not use EF mount lenses. The EF mount is a still camera mount system that has more issues than Nikon or MFT.
Cheers
Denny Smith
SHA Productions
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Ian Henderson

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Re: Back focus and shimming mounts

PostMon Sep 17, 2018 4:59 am

Yeah, but even with PL surely you can't shim the lens mount to each lens - the mount itself has to be correct and the lenses correctly collimated otherwise the mount with correct for one lens and then infinity focus on all the prime lenses will be thrown out?
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Denny Smith

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Re: Back focus and shiming the camera mount

PostMon Sep 17, 2018 6:56 pm

Correct, shimming a mount is useually done to compensate for wear on the lens flange or PL mount, to reset it back to correct FFD. If a lens is correctly collimated for the PL standard, you are good to go.

EF mount is another story, it is not precise enough, and actuall Camera FFD May vary from one camera model to another. Also, some zooms need the back focus reset to get it parfocal. But again, a correct collimated Cine Zoom should be parfocal, if the Camera Mount FFD is correct.

One way to tell is to have two cameras, with the same mount set to the same distance on a kens test chart/target put the lens on camera A set what the witness marks lines up to, then put the same lens on camera B and check again. If both are set to correct FFD, witness marks will be the same, hosever with some less expensive lenses, the distance scale may not be completely accurate, and may not indicate the actual distance.

Another test is to rent a a Cine Prime, like a Zeiss Ultra Speed (if PL) or a CP2, and test the lens on the camera at say 6-feet, check the witness mark, it should be close to 6-feet, the backup to 10-15 feet to line up with the next witness mark. If the distance shown in the lens is less than the actual distance, the mount is shimmed too far out, remove the 0.010 shim and try again (lens will not focus to inf. If the distsnce indicated is further away, (normally the issue with EF lenses, so they focus to inf. on a variety of cameras) then add a thin shim, and recheck.

The most accurate way to test the camera mount FFD is with a collimator for the lens mount (EF or PL) put it on the camera instead of the lens, turn it and the camera on, and see if you get inf. Focus. Wooden Camera has a video with their MF/PL Pro adapter listing thst shows how to do this:
https://www.woodencamera.com/mft-mount- ... 233500.htm
Cheers
Denny Smith
SHA Productions
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Ian Henderson

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Re: Back focus and shimming mounts

PostThu Sep 20, 2018 4:30 pm

Don't ask me why I didn't do this two months ago, but I finally got around to testing all my lenses on the camera properly. Bit of an education. My Zeiss 21-100 isn't focusing to infinity either, and coupled with the non-parfocal performance, at this stage I'm trusting the lens, not the camera. All of my Contax Zeiss lenses focus past infinity as well, and my Sigma 18-35 is miles and miles past infinity. So I think it's safe to say that my EF mount is way off on the camera. Either way, it seems like this is going to be totally hit and miss because there seems to be huge variance from lens to lens, but at this stage my only concern is to get parfocal performance 100% on the Zeiss zoom lens.

I ordered a PL mount which arrived today, and the PL mount for the lens comes tomorrow. Is there any reason why I can't shim up the EF mount on the camera, since the PL mount comes with more shims? Denny you said it's only a concern to lose contact on the EF contacts? My Sigma is the only electronic lens I have - the rest are fully manual.
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Denny Smith

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Re: Back focus and shimming mounts

PostThu Sep 20, 2018 4:43 pm

Then, if you do not care about the electronic connection, try shimming it also. When adding shims, a little but goes a long... way. Start with thin 0.005 0r 0.010 first. I would give the WC site a look as it shows the basic principles of adding shims, and would try to rent a collimator like Ryan uses in his video, to get the EF mount set to 44mm as a starting point. If the lens is not focusing to inf., then, adding shims will make it worse. You need to remove a thin shim, if one is present, or if it is only one thicker shim, replace it with a thinner shim. You need to get rhe FFD to 44mm or slightly less to hit inf. focus correctly.
Cheers
Denny Smith
SHA Productions

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