PARFOCAL ZOOM LENS TEST

I'm wondering if you can help me.

I just got my URSA 12K camera back from Blackmagic. They installed the OLPF on my original URSA 12K. I'm now rigging it up and testing it for upcoming shoots.

I mounted a 17-50mm PL zoom lens on it. Today, I tried to achieve par focal with it, but I am no expert.

I created a video documenting the steps I took to achieve par focal with my camera with TEST FOOTAGE. I followed the manufacturers instructions and then they talked me through how to achieve fine tuning.

Do you think my zoom is par focal now? I'm basically just looking for opinions from you guys because this is my first time doing this, and I am guessing that many of you have done it before.

Thanks for the help!

It’s probably fine, but you can judge better if you’re shooting video after this. Depth of field increases as you zoom to a wide angle. You want to carefully focus on a subject a various distances using the wide angle and then zoom to the telephoto which will have a shallower depth of field. Do the subjects stay in focus?

In a real shoot even if you want the shot to go from wide to telephoto, focus first using the telephoto before you roll camera. With the lens then set to a wide angle, zoom into that subject. In post you can get a better idea how well your parfocal feature is performing.

NB: Infinity is actually quite far away. In your video, the building doesn’t seem to me to be far enough away. When I’ve done this I go somewhere that has a further horizon. Anywhere near infinity could be trees a couple of thousand feet away I should think.

If the air is clear enough to judge focus…

That’s a good point about maybe infinity not being far enough away. I hate this process. Adjusting the lens and then throwing it on the camera and repeating the process. I’m sure there are places that can do it for me, but I don’t know any around me and I don’t want to pay a lot.

You can do it with the camera on sticks, a precisely measured distance and a printed Siemens star or a test chart like this:
https://www.graphics.cornell.edu/~westi ... schart.pdf

And behind the lens, your distance is more critical at the wide end, test that first. Always open aperture, for sure.

So here’s an update…

I drove my camera and lens to a broadcast rental company. They charged me an hour to test out the lens using all of the proper gear. He said that I actually nailed making the lens parfocal — so that video I made actually worked. So I wasted some money, but felt good knowing that the lens is the best it can be.

Thanks for all of your help!

Enjoy!

Excellent!