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Eye Focus Through Glass

Published : 13th June 2004


What is a good rule of thumb for getting eye focus through filters?

I was using a long lens and ND 0.6 in front of the lens, I swung it out of the way to check by eye, the cameraman said "when you put that filter back, it changes the focus". I know that a behind-the-lens filter or a drop-in filter can change the back depth, and affect the focus, what about when its a "normal" filter ? Does this apply only in certain situations, certain focal lengths, certain filters, or is it bovine faeces ?

Rory Moles
1st AC London



I did the 'swing the matte box away to check eye focus' & on one occasion found a difference between matte box in & matte box out focus(sssss). I discovered the glass filter I had in the matte box, from a reputable filter company, had a subtle but distinct bend in it. So it was acting as a low power diopter. I have only ever seen 1 of those filters, an 81EF/Diopter1/8 combo!

Assassins everywhere!

Angelo Sartore
1st. AC
Melbourne
AUSTRALIA

ADOPT, ADAPT, INVENT, DESTROY !



>...I know that a behind-the-lens filter or a drop-in filter can change the >back depth, and affect the focus, what about when its a "normal" filter..

Uh…sorry, but this is basic camera assistant stuff. It's optics/physics. The rear filter has more effect on wide lenses and the front filter has more effect on long lenses. Read up on the theory of optical design. Those "flat" filter elements become part of the optical ray path and change the focus point(s).

Set all of your marks on that 400mm T2.8 with no filters then swing in the ND3, Pola, 81EF and hard grad and watch while you buzz the whole shot. Damn!

Also, as has been noted, the quality of that front filter becomes critical when using long lenses wide open. The Optimo 12:1 T2.8 or Primo 11:1 is a killer if all of your 6X6 filters aren't tested and OK'd prior to the shoot.

Rod Williams
Motion Picture First Camera Assistant
Petaluma, California
U.S.A.



>Set all of your marks on that 400mm T2.8 with no filters then swing in >the ND3, Pola, 81EF and hard grad and watch while you buzz the whole >shot. Damn!

What are your top tips for getting eye focus through that stack ?
Uncovered Maglite bubble ?

>Also, as has been noted, the quality of that front filter becomes critical >when using long lenses wide open. The Optimo 12:1 T2.8 or Primo 11:1 >is a killer if all of your 6X6 filters aren't tested and OK'd prior to the shoot.

And how do you test them ?

Rory Moles
1st AC
London



> What are your top tips for getting eye focus through that stack

It's really difficult. Sometimes when we have long lenses and I'm working with a DP who uses lots of filters, I have a focus star on the back of the slate. The 2nd used it for the operator or me to set eye focus through the lens. The open MagLite bulb is a good idea sometimes. What's tough is working with a DP who changes the filter pack at the last second. Like on car spots. I just let the DP know what's going on and give them the option.

>And how do you test them

Again, only if we have long, fast lenses I put each filter up and focus at infinity on the long end of the lens. I got caught once with an 85N6 that, when put on a 25-250 was unusable at the long end. Made everything just slightly off. Couldn't get anything into sharp focus. Been testing the filters ever since.

Rod Williams
Motion Picture First Camera Assistant
Petaluma, California
U.S.A.



Sometimes this helps: Release tilt tension to "0" and gently rock the camera vertically, this will unfreeze the ground glass pattern.

Ttougher on a gear head. Or, if you're locked off for moco etc, you can pivot a mirror in front of the lens if you calculate an equivalent focal distance.

Sam Wells



>I know that a behind-the-lens filter or a drop-in filter can change the >back depth, and affect the focus, what about when its a "normal" filter ?

I've had one instance where on a long lens a 4x4 filter made the picture soft . When we had a look at the filter, we found out that it had been polished (to remove scratches) and this had altered its characteristics, rendering it useless.

Whenever I check out filters now, I always get rid of any of those polished filters. They are fairly easy to find: there is usually no writing on the filter itself anymore (since it got removed during the polishing) and if you look through the filter and slowly rotate it, then from a certain moment on it will start to distort vertical lines.

Max Jacoby
2nd AC
Lux/UK



Sam wrote :

>Sometimes this helps: Release tilt tension to "0" and gently rock the >camera vertically, this will unfreeze the ground glass pattern.

WOW!!!

I never thought of that!!

I typically do this to be sure that the junk I see in the finder is in the viewing system and not spots on the Cyc...But to check focus...great idea!

Cheers

Jeff Barklage, s.o.c.
US based DP
www.barklage.com


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