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.
San Antonio, TX
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.
When I prep, I basically rely
on and ask the rental house "did you guys >check test
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.
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
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
If it's too deep, lenses will never get to infinity.
All the best,
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.
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
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
My two cents.
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
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
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).
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,
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