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Back Focus Drift on HD lenses


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#1 Mark Wilson

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Posted 21 November 2005 - 01:19 AM

I'm not really up on the finer details of using HD cameras with prime lenses, but recently a client has been making noises about shooting a commercial on HD and seems to think he needs them.

I've heard that the "Panavized" Cine Altas suffer from focus drift as they heat up. Does this happen with Panavision's entire digital range of lenses, or is it just the zooms?

Also, is this a problem peculiar to Panavision equipemnt, or do other manufacturer's Digital primes behave the same way? I would have thought some sort of temperature compensation would have been possible.

Personally, I think the zoom lenses supplied as standard equipment with most HD cameras are pretty good, is it really worth the extra expense of renting primes, at least for a TV commercial?

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#2 David Mullen ASC

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Posted 21 November 2005 - 02:25 AM

The back-focus drift problem is sort of over-hyped.

The problem is the aluminum lens mount in the F900 (which is why Clairmont Cameras had theirs replaced with a steel mount). It's not particular to a Panavised F900 versus a regular F900.

I've shot four HD features using the Panavised F900, and four using regular F900's, and I don't see a difference in back-focus problems -- meaning that the degree of the problem is the same for both.

The solution is to just check it regularly. I check back-focus at every lens change, and if I keep the same zoom on all day, I check it at least twice, once in the morning and again after lunch. And if I'm concerned, I check it again. It's no longer a big issue with me.

Truth is that with regular checking, you never find out if you're having any back-focus problems because the act of checking it requires that you reset it.

It's a little harder to check it with prime lenses but it can be done with a star chart and tape measure. It goes faster if you have something like the portable collimator such as the "Sharp Max" made by Zeiss (I don't know if Panavision ever made one.)

Trouble you find with the wide-angle primes in HD is that the extra depth of field in HD makes you think the back-focus is off because a softly-lit face will look less sharp than the background sometimes, even if the face is actually in sharp-focus. This is because there is so much depth of field that the background details, which may have harder lines, will stand out compared to a softly-lit face with no hard lines, so it looks like the focus is in the distance when its really on the foreground face.

The non-Panavision HD zooms are pretty good if you avoid the ones that breathe horribly when you rack-focus.

The primes are necessary if you are doing steadicam work, because if you are using the Panavised F900, the only zoom that is light enough for steadicam is the old Canon 5.5-50mm, not any of their Digital Primo zooms. However, if you are not using a Panavised F900, you can probably find some good HD zooms that are light enough. The primes may be faster too, if you're trying to shoot wide-open for a shallower depth of field. I've shot entire features with just the 8-72mm Panavision Digital Primo zoom.

All of this is a good argument though for using a 20" or 24" HD monitor whenever possible to see focus problems better.

Edited by David Mullen, 21 November 2005 - 02:28 AM.

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#3 Laurent Andrieux

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Posted 21 November 2005 - 02:43 PM

BTW, David, does your assistant set focus by eye, at long focal length when using a zoom in HD, or usually use the tape measure, but for long focal lengths, as in film ?
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#4 David Mullen ASC

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Posted 21 November 2005 - 06:40 PM

Both, although when I'm working with a zoom, whether in 35mm or HD, I do a lot more zooming in and getting eye-focus marks for the AC than just relying on tape measurements.
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#5 Brad Grimmett

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Posted 22 November 2005 - 06:49 AM

The problem is the aluminum lens mount in the F900 (which is why Clairmont Cameras had theirs replaced with a steel mount). It's not particular to a Panavised F900 versus a regular F900.

David,
I don't think Clairmont's mount is steel. A few years ago at NAB Denny told me that it was some sort of NASA created metal that he refused to tell me the name of. They were keeping it super secret for some reason. It may indeed be steel, but that's certainly not what they're claiming at Clairmont. Sorry to get off the subject, it's just something I heard and am still curious about.
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#6 Mitch Gross

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Posted 22 November 2005 - 12:36 PM

Denny may have told you one thing, but in print ads in American Cinematographer Clairmont stated steel.
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#7 Brad Grimmett

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Posted 23 November 2005 - 06:26 AM

I hear ya Mitch. But it would make sense that if they were trying to keep a secret they wouldn't say what they were using in print ads. I guess it'll be a mystery to me, even though it may not be to others.
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#8 Max Jacoby

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Posted 23 November 2005 - 08:19 AM

The primes are necessary if you are doing steadicam work, because if you are using the Panavised F900, the only zoom that is light enough for steadicam is the old Canon 5.5-50mm, not any of their Digital Primo zooms.

On a feature I ACed on begining of the year the steadicam operator used the short Digital Primo Zoom on the F900.
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