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.
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Denny Smith
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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.
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Denny Smith
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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
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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.
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Denny Smith
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Ian Henderson

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Re: Back focus and shimming mounts

PostTue Sep 25, 2018 12:34 pm

An update... my PL mount with shims arrived so I could have a go at the EF mount. After a bit of trial and error I got it very close. Required 0.15 shim added to the default 0.50 which it comes with from the factory.

Then, since I had ordered PL amounts for both camera and lens I decided to switch over and went for help to the experts at Media Film Service, where they have the proper collimater and measuring tools. We did the mount and lens separately. The special mount measurement tool gets it right to within 0.01:

Image

Image

The lens had its mount swapped and was collimated separately and then paired up everything was spot on.

Image

Image

Big thanks to Martin at Media - happy to have my camera and lens performing 100%!
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Denny Smith

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Re: Back focus and shimming mounts

PostTue Sep 25, 2018 5:20 pm

Good, you made the right choice, going with PL, since you have a a manual lens, and that Zeiss Zoom deserves a PL mount. :roll:

Congratulations my friend. 8-)
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Denny Smith
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rick.lang

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Re: Back focus and shimming mounts

PostTue Sep 25, 2018 9:59 pm

¡Excellente!


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Alastair Traill

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Re: Back focus and shimming mounts

PostWed Sep 26, 2018 6:54 am

I have some experience in collimation problems as I have rebuilt lens mounts in the past and been greatly assisted by a simple formula that relates lens focal length ‘F’ to the distance of the object from the lens ‘D’ to an increase in flange depth ‘X’.

The formula is X= F squared divided by D.

It is important to note that F is squared. It is worth playing with this formula as it shows how critical the correct flange depth is for shorter focal lengths in particular. For example consider three lenses 100mm, 20mm and 10mm used on a mount that is 0.1 mm too long – admittedly a large error. In this case the 100 mm lens would not focus further than 100m, the 20 mm would not focus past 4m and the 10 mm lens would not focus past 1 m.
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rick.lang

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Back focus and shimming mounts

PostWed Sep 26, 2018 6:54 pm

Thanks Alastair for that formula. That’s very helpful when shimming a PL lens. The traditional method when you don’t have other optical tools is simply trial and error. But using your formula, it will tell you the shim correction needed! Much faster assuming you need to add a shim.

What if the shim is a negative number meaning you need to reduce the shim? How do you adjust the formula or procedure for that?


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Alastair Traill

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Re: Back focus and shimming mounts

PostThu Sep 27, 2018 11:47 am

Hi Rick,

The purpose of my post was to stress just how critical precise flange depth is for short focal length lenses.

The formula tells you how much too long your lens / camera combination is but if it
is too short focus will improve as the lens barrel is rotated towards infinity but focus will not be achieved. The formula does not help directly in this case however if a selection of shims of known thickness is available one could be selected and tested. If it is too thick and focus is achieved at a point on the focus scale the formula could be used to calculate X and thus indicate how much too thick your shim is. You would of course be relying on the accuracy of the focus scale. I have a series of ‘C’ mount lenses that I use on GoPros modified to take ‘CS’ mount lenses. The older lenses have clear markings for focus and aperture while the newest might just have ‘near’ or ‘far’ arrows i.e. no useful information. To collimate my GoPro 5 I needed two 0.005” shims to shim out the lens mount. Fortunately my reference lens showed that the flange depth was then correct but the rest of my lenses were too long. To convert ‘C’ mount lenses to ‘CS’ mount a 5mm long adapter is required. I used my lathe to shorten the adapters 0.0005” at a time. The shortened adapter was then fitted to a lens and collimation checked. If required another 0.0005” was removed. All rather tedious, however all lenses but one now focus to infinity when set to infinity. To make sure the adapters do not get removed I have added a lock screw to the customised adapter. The odd one out is a vari-focal IR corrected lens that does not have an infinity stop, I have no idea how to check if this lens is up optimally.

There are some great devices that assist in solving collimation problems quickly and effectively and I would strongly recommend using a professional service if possible.
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rick.lang

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Re: Back focus and shimming mounts

PostThu Sep 27, 2018 3:51 pm

Thanks, Alastair. I recently had an issue when I changed my mount for the B4 on the URSA Mini 4.6K camera. Since I first used the Fujinon lens on the camera about 2 1-2 years ago it has been flawlessly parfocal. The camera manual back then specified adding two 0.05 shims and all was good.

However sometime perhaps around a year ago there was a wording change that said to add two 0.05 shims to the existing 0.05 shim used with the 4.6K PL mount camera. So this time I thought I should RTFM and follow the directions given. Well after 30 minutes of trying to adjust the back focus but never getting close to being parfocal, Denny advised me to take out one 0.05 shim (so only a total of two shims) and in a minute or two the Fujinon was back to perfection.

With the convenience of your formula, I think from now on when a parfocal lens doesn’t behave properly, I’ll take out the thickest shim and then plug the values into the formula and it will calculate what thickness I need to add.


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Rick Lang

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