December 8, 2011

How to Set the Back Focus on a Video Camera

One of my more embarrassing on-set moments happened when, years ago, I had been hired as a camera operator on a multi-camera shoot. Sometimes talking your way into a position can you get you into trouble, and I wasn't really qualified to be operating a full size camera, which I discovered when the backfocus was off. I didn't know how to adjust it, given at the time I'd only shot on Sony DV camcorders, and had to call on another camera operator to do my job for me. The horror! So here is a tutorial for setting back focus from The Complete Guide to ProHD Volume I, which applies to many videocameras (not just the JVC on which it's demonstrated). But first, what is back focus?

Back focus refers to focal flange length, which is the distance between the rear lens element and the camera's sensor. On cameras with interchangeable lenses, sometimes changing a lens will also change the focal flange length, and the discrepancy can cause an image that's sharp when zoomed in to be soft when zoomed out. If that's the case, you need to adjust the back focus. Here's how to do it on a JVC Pro camera, but the process is similar on many cameras (here's how to do it on a RED EPIC/SCARLET):

As interchangeable lens cameras become more and more prevalent, I thought this might be handy.

[via DV Info]

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9 Comments

Good post! Equally important is knowing how to calibrate your monitor with the PLUGE!

December 8, 2011 at 10:34AM, Edited September 4, 10:54AM

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Stephen

Not to mention with your colors bars, too.

December 8, 2011 at 10:40AM, Edited September 4, 10:54AM

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Stephen

Fantastic post koo, and great find. You probably saved me from an embarrassing moment.

December 8, 2011 at 10:48AM, Edited September 4, 10:54AM

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Yep, fell foul of this in the past when other folk have switched lenses and not re-adjusted the back focus, the moral is you should check bf every morning before a shoot, tied to a monitor, even with peaking up full the EVFs aren't always sharp enough to see the problem.

If you need the shot you need to check back focus.

On another point, don't assume that DSLR lenses are immune to similar problems. On a video lens proper, once you set back focus and front focus zoomed in, the focus should hold through the zoom to wide.

Not so on some SLR lenses, I actually find the older push pull zooms more reliable in this regard, my ancient 100-300 EF 5.6L holds focus fine, my 70-200 f2.8L needs checked whenever you zoom.

December 8, 2011 at 11:00AM, Edited September 4, 10:54AM

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Paul

SLR lenses can take design shortcuts because they aren't used continuously and they have the aide of autofocus; that's why many of them are varifocal.

December 8, 2011 at 2:13PM, Edited September 4, 10:54AM

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Angelo

With professional 2/3" zoom lenses you always need to have the backfocus marked for your specific camera. That way you can change lenses and see immediately if the backfocus is still where it should be.

Of course you might want to check the mark once in a while, but usually there's nothing to correct.
I think it's a bit insane to say you need to adjust the backfocus every time you change environments - the differences in backfocus through temperature changes are so small you wouldn't even be able to correct them with the - mostly rather clumsy - backfocus ring. Also you wouldn't be able to shoot with fixed back focus cameras, they would be off all the time, wouldn't they?

Believe me, I read and heard all this years ago, and I started checking the backfocus all the time - with the result that I always landed perfectly on the mark. I never experienced a situation where through temperature changes or vibration or anything the backfocus had changed so much that I needed to adjust it away from the mark.

When you put a new lens on the camera or use your lens on another camera the first thing you have to do is check the backfocus, though. Also when you rent a camera, first thing check the back focus!

December 8, 2011 at 4:49PM, Edited September 4, 10:54AM

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Heiko

Thanks for the info, keep it up koo!

December 8, 2011 at 2:01PM, Edited September 4, 10:54AM

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chris

For older pro-sumer fixed-lens camcorders (like the Sony HVR-V1U), you cannot adjust the back focus yourself but have to send it to Sony for 'repair' to the tune of $135/hour (1 hour minimum for diagnosis and quote).

Oy.

I've had a V1U with a back focus issue for a while now, which seems to be a recurring problem with these cameras (lots of posts about it on the net). I have the service manual but it's not really helpful (even if you're a semi-experienced tech) without Sony's tech notes or some experience fixing the camera.

Has anyone else been able to successfully fix back focus on a fixed-lens camcorder?

December 8, 2011 at 3:50PM, Edited September 4, 10:54AM

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Julio Alvarez

and EX1, 3 have auto backfocus facility...

December 11, 2011 at 10:44AM, Edited September 4, 10:54AM

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bru