Blackmagic Pocket Cinema Camera 4k back focusing issue

The place for questions about shooting with Blackmagic Cameras.
  • Author
  • Message
Offline

mao790615

  • Posts: 2
  • Joined: Thu Oct 03, 2019 1:31 pm
  • Real Name: Harry Mao

Blackmagic Pocket Cinema Camera 4k back focusing issue

PostThu Oct 03, 2019 1:38 pm

I have a Blackmagic Pocket Cinema Camera 4k and Metabone speedbooster ultra 0.7X and with canon 24-105mm lens.
I have problem back-focusing using BMPCC 4k, when I set my focus to an subject, without changing the our distance, I only change the zoom on the lens, but for some reason I lost focus during the process, meaning the focus point at 24mm is different at 105mm. Is there problem with the compatibility or other issue
Offline

Dune00z

  • Posts: 349
  • Joined: Thu Oct 18, 2018 3:40 pm
  • Real Name: Duane Eues

Re: Blackmagic Pocket Cinema Camera 4k back focusing issue

PostThu Oct 03, 2019 3:01 pm

The lens loses focus when you zoom? This is not according to my knowledge a back focus problem. This is a normal issue with zooms that are not parfocal. I have never used a Canon L 24-105 that WAS parfocal. The issue may be exacerbated by the adapter since it might need micro adjustments to get the focal plane to be exactly where it should be on the sensor. You will see in many documentaries or faux documentaries (The Office style) where this problem actually aides in the aesthetic.

True zooms that are parfocal tend to cost much more than the common varifocal zooms that are on the market, such as the 24-105 F4 L.

Generally, it is a good idea to use varifocal zooms more like variable primes and not to use them like industry parfocal zooms, such as those glued to studio broadcast cameras.


mao790615 wrote:I have a Blackmagic Pocket Cinema Camera 4k and Metabone speedbooster ultra 0.7X and with canon 24-105mm lens.
I have problem back-focusing using BMPCC 4k, when I set my focus to an subject, without changing the our distance, I only change the zoom on the lens, but for some reason I lost focus during the process, meaning the focus point at 24mm is different at 105mm. Is there problem with the compatibility or other issue
Offline

Denny Smith

  • Posts: 13062
  • Joined: Thu Aug 01, 2013 4:19 pm
  • Location: USA, Northern Calif.

Re: Blackmagic Pocket Cinema Camera 4k back focusing issue

PostThu Oct 03, 2019 4:28 pm

Additionally, cameras normally do not have a back focus adjustment. Some cameras with certain mounts, like PL mounts, can be shimmed to adjust inf. Focus. Back focus adjustments are found on Broadcast parfocal Zooms and some cine zooms. Still camera zooms are not real zoom lenses, but more of a variable focal length lens, and are not parfocal. The MFT mount in the Pocket 4/6K can not be shimmed, due to the lens contacts. Same goes for BMDs fixed EF Mount Cameras, like the Pocket 6K.

That said, Nikon’s new S Zooms for their Z cameras are parfocal on a Z6/7 Camera. Some Canon cameras can adjust the focus on the fly when zooming, to give the appearance the lens is parfocal, but this only works on the higher end Canon camera bodies.
Cheers
Denny Smith
SHA Productions
Offline

Kaku Ito

  • Posts: 29
  • Joined: Wed Apr 17, 2013 5:08 pm

Re: Blackmagic Pocket Cinema Camera 4k back focusing issue

PostSat Jul 24, 2021 9:55 pm

You can adjust back focus with your Speed Booster, go check out how to adjust the back focus on your Speed Booster. My Sigma zoom are working great with this adjustment.

On the other hand, I'm having problem with the back focus on the M. Zuiko zoom.

Does anyone know if there's shim sold for PCC4K?
Offline
User avatar

Mark Foster

  • Posts: 1081
  • Joined: Tue Oct 27, 2015 10:59 am
  • Location: austria - no kangaroos +g*

Re: Blackmagic Pocket Cinema Camera 4k back focusing issue

PostSun Jul 25, 2021 6:45 am

cMP 5.1 2x3,46/96GB/2x2TB 860pro/4x4TB HGST/SSD7101A 4x2TB 970evoplus/HP1344/BMD4k/Radeon VII
macOS 11.2.3

BMPCC 6k pro (7.3.1)
BMPCC 4k (7.3)+ MB speedbooster ultra 0.71 (3.60)
resolve studio 17.3.1
desktop video 12.1
intensity pro 4k
Online
User avatar

Kim Janson

  • Posts: 1522
  • Joined: Wed Jul 15, 2015 6:54 pm
  • Location: Finland

Re: Blackmagic Pocket Cinema Camera 4k back focusing issue

PostSun Jul 25, 2021 8:34 am

years back I did this



and after having electrical focus and zoom control, it was possible to do electrical parfocal correction



But without these corrections, the Canon 24-105 will not keep focus while zooming, no matter how much you adjust the speedbooster. It is not a parfocal lens.

Parfocal zooms tend to be very expensive, the photography lenses typically are not.
LeViteZer Smooths the movement, www.levitezer.com

Return to Cinematography

Who is online

Users browsing this forum: Majestic-12 [Bot] and 28 guests