Flange distance when adapting the Canon 11-165mm PL S16 Zoom Lens to the Blackmagic Pocket 4k Camera

[Lucas Murray-Reynolds]

Hi all,

I recently did some minor tests with a handful of Super 16 zoom lenses on my BMPCC4K in the 2.6K S16 crop mode. I was finding that whilst sharping up at the longer focal lengths is no problem, when I pulled out to the wider end everything became a bit soft and I couldn't find the focus. At first I thought that it was just that the focus was so deep at the softness was part of the look of the lens but in hindsight I tested mostly at T5.6 so I'm starting to think the flange distance was just off. 

I have shims for my PL-MFT adapter but I didn't get the chance to try them out. Any lens specialists that know a bit more on this subject - from what I'm describing, does this just sound like a flange distance issue? And if so - how do you know if the lens needs to be closer or further away from the sensor? Obviously my shims will allow me to make minor extensions of the flange distance but I have no way of bring the back element closer.

If you've made it this far, thanks for taking the time to read! Any feedback is appreciated. 

Edited April 23, 2022 by Lucas Murray-Reynolds
to add tags and make the post easier to find

[Dom Jaeger]

If you’re finding this with multiple zooms it seems pretty likely that the flange depth is out, yes. It is most likely the adapter, but could also be the camera, or a combination of both. No guarantee that some of the zooms might be out too of course, or have other issues.

It can be hard to accurately check and set the flange depth without the right test equipment. You can try to gauge where the focus lines up at the wide end (at maximum aperture, after focussing at the long end). If the distance mark is longer than the actual distance then the lens is not reaching and the adapter (or camera mount) needs shims removed. If the marks are closer than the distance then you need to add shims. But judging the exact focus mark at the wide end of a zoom can be difficult. It might be easier with a prime lens (use a wide to medium focal length again at full aperture) but you need to know that the lens itself is very accurately set.

Personally I would recommend sending everything to a lens technician who has the right equipment to check the camera and adapter for correct flange depth and each zoom for correct back focus. It’s a fairly quick job for a good tech, and removes all doubt about what’s happening. It’s really the only way to ensure you’re getting the best images you can from your gear.

[Robert Hart]

Sometimes with PL-Mount adaptors, there is a sharp edge instead of a rounded edge or chamfered edge on the inner front diameter of the hole which accommodates the lens shoulder. This butts against a fillet radius in the front corner of the shoulder of the PL lens tail and the flange face. It holds the lens away from the flange face ever so slightly and sometimes makes the lugs on the mounting ring catch or barely overlap before snugging down. More likely you will need to shim your mount but the interference by the adaptor with the fillet radius would be the first thing I examine.

Edited April 24, 2022 by Robert Hart
error

[Robert Hart]

Further to the previous reply, the accompanying image illustrates where a clearance for the fillet radius should exist. 

If you are wondering what that object is propping up the PL-Mount, it is a plastic template of an adaptor for the Century Optics 16:9 anamorphic adaptor for Sony DSR PD150 camera to fit to a lens filter diameter of 52mm with 0.75mm pitch thread. I made made one in metal to fit Nikon AIS lenses and CP Ultra T* 16mm cinecamera prime lenses. 

Edited April 24, 2022 by Robert Hart
added text

[Lucas Murray-Reynolds]

 

4 hours ago, Dom Jaeger said:

If you’re finding this with multiple zooms it seems pretty likely that the flange depth is out, yes. It is most likely the adapter, but could also be the camera, or a combination of both. No guarantee that some of the zooms might be out too of course, or have other issues.

It can be hard to accurately check and set the flange depth without the right test equipment. You can try to gauge where the focus lines up at the wide end (at maximum aperture, after focussing at the long end). If the distance mark is longer than the actual distance then the lens is not reaching and the adapter (or camera mount) needs shims removed. If the marks are closer than the distance then you need to add shims. But judging the exact focus mark at the wide end of a zoom can be difficult. It might be easier with a prime lens (use a wide to medium focal length again at full aperture) but you need to know that the lens itself is very accurately set.

Personally I would recommend sending everything to a lens technician who has the right equipment to check the camera and adapter for correct flange depth and each zoom for correct back focus. It’s a fairly quick job for a good tech, and removes all doubt about what’s happening. It’s really the only way to ensure you’re getting the best images you can from your gear.

Thanks for the detailed reply Dom! I had a similar experience not too long ago when trying to adapt an old Kinoptic S16 lens to this camera body - the result was that the body had to be sent back to Blackmagic for testing and it came back that the sensor had been incorrectly positioned so they sent me a new body. My adapter came from MTF Services here in the UK and I payed a pretty penny for it after speaking to them in length about the issue so it has my confidence. As you say, it's very difficult to get to the bottom of without a technician and the appropriate equipment.

The thing the got me was that at the wide end it appeared that everything was a bit soft and that I couldn't find any sharp spot, whereas with the Kinoptic 9.8mm I tried I could clearly see that the sensor was too far back as the focus was macro. By the sounds of what you're saying, for the purpose of this shoot my only option will to be experimenting with the shims I have. I hope to one day own one of these lenses and get them properly collimated together but until the bank account allows for it il have to wait.

Many thanks for the response! Always eager to learn more about these topics.

[Lucas Murray-Reynolds]

Thanks for the response too, Robert! I've examined my adapter's mount and it does indeed have the chamfered edge you're describing, so I think I'm good on that front. My doubt are with the lens as after having to get a replacement body from Blackmagic because of previous similar issues everything seems to be lining up as it should with lenses I own. Until I own this lens however, it sounds like there's little I can do to perfect the setup.

[Dom Jaeger]

Ok so it sounds like the camera should be good since it went back to BM already, and you trust the adapter. So is it just the 11-165 that you feel is soft at the wide end? You mentioned testing a handful of zooms so did the others seem to work alright? And other primes seem to line up OK?

If so it’s possible then that the zoom has a fault. The fact you can’t seem to find sharp focus anywhere at the wide end suggests more than just a back-focus or flange depth error. Usually at close to minimum focus you should be able to find a sharp zone and 11mm isn’t particularly wide for S16. Try zooming from different directions, in case it seems sharper when you come from one direction.

Again, you may have to save up for a tech to assess it. Canon S16 zooms are pretty reliable in my experience, but they are getting a bit long in the tooth now. There could be some play in the zoom cams (which the different direction test might indicate), which control the variator and compensator groups in the zoom mechanism. They need to be perfectly in sync to produce a sharp image. If the variator is lagging in one direction the image will appear soft.