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class="Paragraph" Testing Flange Focal Distance During Prep

class="Paragraph" Published : 4th May 2004

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I understand that during a camera prep it is important to go through every element, but one thing has me troubled and I do not have the right answer to ease mind after prep. This is the "flange focal depth." I realize its importance for if off by millimetres, it can position your lens at an incorrect distance from the film plane, causing your image out of focus on every shot and probably create a totally unusable image with longer lenses and lower f-stop.

When I prep, I basically rely on and ask the rental house "did you guys check test the "flange-focal-depth?" I usually am answered with "yep, it all checked out" or "no... we have it checked every so often."

I guess, that this is one of the only elements I know is testable but I do not test. I would love to hear from the rest of you guys on this.

Zach Nasits
1st AC
San Antonio, TX


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You simply need a pocket collimator or depth gauge. And it better not be off by millimetres. A few micrometers is enough to throw everything out of focus.

Mitch Gross
NYC DP


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>When I prep, I basically rely on and ask the rental house "did you guys >check test the "flange-focal-depth?"

I would recommend creating a check list of questions for a rental house before an AC starts prepping.

I once received a camera with the timing out. We shot an involved effects sequence with very ugly results.

Marc Dobiecki
www.dobiecki.com


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Any reputable rental house will do a flange depth test as a routine part of their gear check.

A zoom lens is a quick & dirty way to check if flange depth is correct. Zoom in, crack focus by eye then zoom out slowly. Focus should hold. If not the flange depth may be out. If you suspect the zoom lens may be dodgy, ask for another lens & see if you get the same result. One other possibility - the flange depth may be correct but the ground glass collimation may be out. It is unlikely that the flange depth will change during the course of a film, even a long one - 12 weeks or more.

Perhaps I have been lucky. I have never had a problem with the flange depth of any camera I have used.

Another post suggested a pocket collimator & a depth test kit. These items are really only useful in the hands of an experienced technician. Let's face it, it's not up to the camera assistant to rectify that kind of fault in a camera.

One other possibility - the flange depth may be correct but the ground glass collimation may be out. Eye & tape focus don't match. Depending on the error in collimation, the wide lenses will show the tape & eye focus discrepancy more than the longer lenses. The wider the lens, the greater the error, the longer the focal length, the closer you will be to the mark.

If the flange depth is too shallow, the lenses will focus through infinity.

If it's too deep, lenses will never get to infinity.

All the best,

Angelo Sartore
1st. AC
Melbourne
AUSTRALIA


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Angelo Sartore wrote :

>If you suspect the zoom lens may be dodgy, ask for another lens & see >if you get the same result. One other possibility - the flange depth may >be correct but the ground glass collimation may be out.

Thanks for the input regarding the wide angle and zoom lense tests. I am going to study it a bit more.

Zach Nasits


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A solid methods for verifying flange depth is simply comparing you focus distance to the focus markings on the lens. A good practice is to setup the camera at a focus distance of 4 feet from a chart, then focus by eye. Does the lens show 4 feet? if it doesn't it's ether the lens of the flange... How to know? check another lens and another focal length. In short, go through all the lenses in your set and do the same test. if all lenses are OK then you're fine. If one or two lenses are off the you should probably reject them. If ALL lenses are off the with the same deviation the flange is probably off...

BTW, if you go out with the flange depth off, it's actually the wide angle lenses that's soft and the longer lenses will probably stay sharp anyway. the reason is that with a longer lens the focus puller set's the focus by his measurements but the camera man will see it's soft and most likely comment on that so you can get it right (by going off your marks and working by eye). With a wide lens it will be difficult to notice the softness by eye and the picture will probably be soft...

My two cents.

Oren Arad
Tel Aviv


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Oren Arad wrote :

>A solid methods for verifying flange depth is simply comparing you >focus distance to the focus markings on the lens. A good practice is to >setup the camera at a focus distance of 4 feet from a chart, then focus >by eye.

Infinity is also important -- a lens that's mounted at the wrong distance will either not focus to infinity (too far from the film plane) or focus past infinity (too close).

Note that there are some telephotos designed to focus past infinity to compensate for thermal issues.

Jeff "infinity is too far away, man" Kreines


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Jeff Kreines wrote :

>Infinity is also important -- a lens that's mounted at the wrong distance >will either not focus to infinity (too far from the film plane) or focus past >infinity (too close).

That's right!

A Close focus test (like from =<4') is a good way to test those shorter lenses without popping your eye balls out of their sockets, and as mentioned before, those lenses are more critical to check then the longer ones. But in any case an Infinity check is advised too (if you have the room for that,
that is).

It should be said, though, that those "practical tests" are no replacement for your friendly rental place tech okaying the flange depth. In legal terms there is reasonable doubt! (I watch Law & Order...) You could theoretically have the ground glass in the wrong place rendering ALL of your 'focus by eye' tests useless and dangerous. You could by miraculous circumstances have two compensating deviations (an off lens by x and an off camera by -x) so 4 feet will be perfect while infinity will be off. And more weird stuff.

Make sure that a gage has measured the flange depth and the lenses have mounted a collimator in the last couple of years (or whatever time frame you're comfortable with) before you check out.

Cheers

Oren Arad
Tel Aviv