Blackmagic Forum

I bought this lens for the original Blackmagic URSA 12K. I wanted a parfocal useful zoom that's not heavy or long. This PL mount zoom fit the bill: Venus Optics Laowa Ranger S35 17-50mm T2.9 Cine Lens.

Unfortunately, to make it parfocal, you have to add or reduce shims -- which is a pain because you have to keep guessing, unscrew the lens and try it multiple times until you get it right (when the focus holds at all zoom points).

I'm hoping that someone out there has this lens and can tell me how many shims they ended up using. That would help a lot.

I'll do a review for the lens when I get it working.

Thank you!

https://www.bhphotovideo.com/c/product/ ... _50mm.html

I think you'd have to get that feedback from someone who has the exact same model of Ursa 12K as you; achieving parfocality (is that a word?!) might require different shimming depending on whether you have the OLPF model or the original, for example.

I'm afraid you'll be on your own. Even the mounts by BM have a bit of tolerance, and it's true that there are differences between models with and without OLPF. I had to shim our UMP 4.6K after adding an OLPF, and it was not identical with another sample.

If the witness marks on that lens are somewhat precise, you may try to get it into the ballpark first. Use the widest setting, 17mm, and check if it's short or long. If too short, like not reaching infinity, but getting closer than indicated near focus, take some out. If it's long, like focusing beyond infinity, add some.

After that, you should need only minor corrections to get it parfocal, if that lens is any good.

Here is a video for you to watch. This should help you.

Micro intolerances are pretty common, otherwise lens shims wouldn’t exist. What works for someone else could easily be +/-.002 for your setup. I’d start with a star at 9, 10, and 11 feet. Zoom in at 10 feet and slowly pull out to see which way the focus shifts. If the focus gets further away remove a shim, if it gets closer then add.

Good Luck

Thanks for the info, everyone. It’s not fun to do the shims, but not that hard.