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Critical focus


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#1 Paul Bruening

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Posted 12 January 2010 - 12:49 AM

When you measure for critical focus do you measure from the film plane or from the outermost element of the lens?
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#2 Mike Panczenko

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Posted 12 January 2010 - 01:46 AM

When you measure for critical focus do you measure from the film plane or from the outermost element of the lens?


In 35/16mm you would measure from the focal plane. In B4 style ENG video mounts- but not all the cine style mounts!!!- you would measure from the front element, or the green marking on the lens barrel.
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#3 Paul Bruening

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Posted 12 January 2010 - 01:52 AM

In 35/16mm you would measure from the focal plane. In B4 style ENG video mounts- but not all the cine style mounts!!!- you would measure from the front element, or the green marking on the lens barrel.


Thanks, Mike.

I just shimmed my Fries 35R3's lens board. The critical focus is perfectly on the money when measured from the film plane. I just had to make sure I wasn't goofing the reference point up.
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#4 Chris Keth

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Posted 12 January 2010 - 01:57 AM

Thanks, Mike.

I just shimmed my Fries 35R3's lens board. The critical focus is perfectly on the money when measured from the film plane. I just had to make sure I wasn't goofing the reference point up.


Sounds like you're all good. I've never measured focus from anywhere but the film plane. ENG lenses, as Mike noted, are measured from the end of the lens but I've never even thought about measuring focus with one of those.
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#5 Paul Bruening

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Posted 12 January 2010 - 02:02 AM

Sounds like you're all good. I've never measured focus from anywhere but the film plane. ENG lenses, as Mike noted, are measured from the end of the lens but I've never even thought about measuring focus with one of those.


Thanks, Chris,

I'll check infinity on all my lenses tomorrow when there's daylight. I'm going to go ahead and shoot a test roll for critical focus while I'm at it.
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#6 Tom Jensen

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Posted 12 January 2010 - 10:19 AM

In film you always measure from the plane but when you want critical focus you eyeball it on lenses 50mm and longer. If it's sharp to the eye, it's sharp on film. Non film, I have no idea.
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#7 John Sprung

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Posted 12 January 2010 - 01:54 PM

...when you want critical focus you eyeball it on lenses 50mm and longer. If it's sharp to the eye, it's sharp on film.


That works provided that the ground glass is correctly seated and the mirror shutter is in the right plane. That's almost always the case, but it's one of the things you test when you check out your package. Shoot diagonal newspaper tests to see exactly what's happening.




-- J.S.
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#8 Paul Bruening

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Posted 12 January 2010 - 05:24 PM

Darn! All the lenses over-focus. They go beyond infinity. At the infinity, end setting on the focus ring the image blurs quite a bit. Backing the ring towards the other end sharpens up infinity. Do I have too much FFD or too little?
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#9 John Sprung

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Posted 12 January 2010 - 07:51 PM

A lot of lenses do that. It's a good thing, that way you can roll thru critical focus at infinity. Otherwise, you'd be depending on the accuracy with which the hard stop was set. If it was set a little short, you'd be SOL.




-- J.S.
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#10 Paul Bruening

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Posted 12 January 2010 - 08:07 PM

A lot of lenses do that. It's a good thing, that way you can roll thru critical focus at infinity. Otherwise, you'd be depending on the accuracy with which the hard stop was set. If it was set a little short, you'd be SOL.




-- J.S.


Tim said pretty much the same thing, especially about Nikon lenses. He called it, "Overstroking." I've checked and all of these Nikons overstroke. All ten of them.
Matt Duclos collimated them all when he put the focus gears on them. I'm going to assume they're all set-up correctly.
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#11 Chris Keth

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Posted 13 January 2010 - 03:11 AM

Tim said pretty much the same thing, especially about Nikon lenses. He called it, "Overstroking." I've checked and all of these Nikons overstroke. All ten of them.
Matt Duclos collimated them all when he put the focus gears on them. I'm going to assume they're all set-up correctly.


It's also a good way to tell a green focus puller from a good one. The ones who are overstepping their experience will almost always assume infinity is where the barrel stops.
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#12 Jonathan Bowerbank

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Posted 28 January 2010 - 05:06 AM

Shooting ENG, just zoom in real quick and get critical focus by eye ;)
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#13 James Martin

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Posted 01 February 2010 - 08:41 AM

Regarding overstroking - I have been told that putting a significant amount of glass filters *CAN* (though not experienced myself) alter the focus slightly, hence the need to go beyond infinity. In my experience many cheaper lenses (including the RED zooms) are not even marked up at infinity right - I often found it to be sharpest a hair short.

For HDTV/B4 lenses, the general rule I have been taught is if the lens has a green ring around it (eg. Canon, Fuji), then that is what the scale on the lens is reading from. If it does not (Panavision Digital Primos, I assume Zeiss Digiprimes) then you go from focal plane.

Hope that clears some things up, but if I'm wrong let me know!
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#14 John Sprung

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Posted 01 February 2010 - 01:46 PM

Another thing you can try: Set the camera up locked off and focused by eye on something very near its close limit. Stretch your tape the other way around, with the dumb end on the subject, and see whether the focal plane or the front element lands closest to what the ring says.





-- J.S.
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