backfocus, video cameras vs film cameras.

[martin bendersky]

Hi, a friend of my (he's a focus puller) ask me an interesting question.
Why video cameras with a single chip and a PL mount have a back focus ring and film cameras don't.

I will love to hear some coments. Please don't tell me what the backfocus is, i allready know.

 

[Jacek Zakowicz]

reasons

reasons

Hi, a friend of my (he's a focus puller) ask me an interesting question.
Why video cameras with a single chip and a PL mount have a back focus ring and film cameras don't.

I will love to hear some coments. Please don't tell me what the backfocus is, i allready know.

First of all not all single chip cameras have a focus ring(D-21, Alexa?-not sure about this).
1. film cameras don't get as hot because of their relatively low power draw and relatively light duty cycles(long breaks between takes)
2. film cameras have meticulously machined steel film gates very precisely and securely installed. Digital cameras have a (hot) chip mounted on fiberglass PCB - lot less precision and stability. I'm not so sure they stray a lot anyway- it's a split between poorly maintained lenses and cameras IMO
3. film cameras can have the flange depth measured and verified with a depth gage(done so for many decades) and jury is still out on checking the FD on digital cameras... I for one made an instrument for it that works quite well
http://optitek.org/optimator.php -shameless plug- check it out.
What does the number 3 have to do with FD adjustment collar on electronic cameras?- the lack of standards for measuring and the consequent uncertainty motivated the designers to create field adjustable back focus for end users.:thumbup1:

 

[martin bendersky]

tks jacek. Great answer!

 

[John DeBoer]

Why video cameras with a single chip and a PL mount have a back focus ring and film cameras don't.

Confused by your question.....what video camera is single chip and has a PL mount? There are a few in developement, but none that I know of has hit the market. Unless you mean the internal backfocus adjustment on the F35.....which is really just a way to adjust the chip to be at the correct distance from the flange.

If you mean, why does a standard 3 chip, 2/3 inch B4 mount camera have a back focus adjustment on the lens, then its because sports and variety shows want the ability of the operator to adjust the backfocus in the field easily. Generally, video camera operators are adjusting focus by eye, not by the marks on the lens. If the lens holds focus from tight to wide, then they can zoom in, focus, and anything they pull out to will have the same distance in focus. If it doesn't, adjust the back focus till it does. The reason is the tolerances of each camera.....they are not solid blocks of metal like a film camera, so there is some change due to heat issues. Most "backfocus" issues with 2/3 cinematic lenses are due to witness marks on the lens moving, not the backfocus changing. Backfocus is not correct unless you can zoom all the way in, focus, and pull wide and maintain focus. The knob is just to easy for the crew to change, IMHO.

Interestingly enough, its really just a quick way to shim a lens. Fujinon has set a new standard by allowing the tech to "shim" the lens using a back focus ring on their new top of the line zooms....like the 75-400. This allows the tech to quickly set up the "shimming" of the lens without dealing with shims. We have had these lenses on several shows now, and the system works brilliant.

John DeBoer
SIM Video

 

[Jacek Zakowicz]

Cameras

Cameras

I think he mainly meant Redone- not much competition at this point...

 

[Thomas Wright]

I think most modern film cameras have a nifty construction technique, where, by opposing materials and mounting connections balance each other out when there are temperature changes. At least that's what I was led to believe, like the Panavision cameras having an outer and and inner shell with a rubber lining between which helps with noise issues as well as keeping the FFD solidly set.