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Eyeballing, measuring or both.


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#1 SSJR

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Posted 12 November 2005 - 08:56 AM

Eyeballing, measuring or both? which is the best way to focus 35mm being that focus is so much more critical than 16mm?
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#2 David Mullen ASC

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Posted 12 November 2005 - 11:33 AM

Eyeballing, measuring or both? which is the best way to focus 35mm being that focus is so much more critical than 16mm?


You do everything, including if you have a zoom on, zooming in and getting eye focus marks. But primarily on a Hollywood set, it's tape measurements, with an eye focus as a double-check if there's time, and a lot of good estimation on the part of the focus puller when actors don't hit their marks.

Keeping things in focus is a major, constant, neverending struggle. Try shooting 35mm anamorphic!

In prep, you would make sure that the lens "tapes out" -- i.e. the tape measurements and the eye focusing match each other so you can trust the tape measurement.
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#3 Stephen Williams

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Posted 12 November 2005 - 01:35 PM

Eyeballing, measuring or both? which is the best way to focus 35mm being that focus is so much more critical than 16mm?


Hi,

Assuming the lenses are collimated and the flange focal length is correct, then the focus marks are more accurate when using wide angle lenses. The optical viewfinder is probably no wider than F2.8 so it may not be possible to see perfect focus with a wide lens, wide open.

Stephen
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#4 SSJR

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Posted 12 November 2005 - 06:37 PM

Hi,

Assuming the lenses are collimated and the flange focal length is correct, then the focus marks are more accurate when using wide angle lenses. The optical viewfinder is probably no wider than F2.8 so it may not be possible to see perfect focus with a wide lens, wide open.

Stephen


Thanks guys!

Okay so shoot a film test to make sure measurements are accurate?
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#5 David Mullen ASC

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Posted 12 November 2005 - 09:47 PM

You shouldn't need to shoot a test -- just compare your eye focus to the tape measurement and see if they agree. Unless your camera has such a poor viewfinder that you can't judge focus.

You'd shoot a test more to check the sharpness of the lens at different f-stops, etc.
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#6 SSJR

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Posted 13 November 2005 - 11:03 AM

You shouldn't need to shoot a test -- just compare your eye focus to the tape measurement and see if they agree. Unless your camera has such a poor viewfinder that you can't judge focus.

You'd shoot a test more to check the sharpness of the lens at different f-stops, etc.



Thanks David You have been extremely helpful .
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#7 Stephen Williams

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Posted 13 November 2005 - 03:36 PM

You shouldn't need to shoot a test -- just compare your eye focus to the tape measurement and see if they agree. Unless your camera has such a poor viewfinder that you can't judge focus.



David,

If eye focus match the markings on the lens then the ground glass is OK. The flange Focal length could be wrong and the image is not sharp on the negative.

Stephen
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#8 Erdwolf_TVL

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Posted 16 November 2005 - 02:43 PM

Eyeballing, measuring or both? which is the best way to focus 35mm being that focus is so much more critical than 16mm?


Why do you say that focussing is more critical in 35mm than 16mm?
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#9 Michael Collier

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Posted 16 November 2005 - 03:09 PM

35 mm resolves more resolution. the picture is softer to begin with on 16, so a little (key word a little) out of focus doesnt register as much. also since the 35mm has a larger target area, depth of feild is crushed. That property alone tends to exagerate a soft focused lens
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#10 Stephen Williams

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Posted 16 November 2005 - 03:48 PM

35 mm resolves more resolution. the picture is softer to begin with on 16, so a little (key word a little) out of focus doesnt register as much. also since the 35mm has a larger target area, depth of feild is crushed. That property alone tends to exagerate a soft focused lens


Hi,

Well flange focal length is actually more critical with 16mm as the lenses are wider.
The Depth of field is not the same as depth of focus. A 20mm lens focused at 20 feet @ 2.8 will have more depth of field than a 100mm lens focused at 20 feet @2.8. However the 100mm lens will have 5x the depth of focus!

With all due respect, the film will resolve the same whatever the format.

Stephen
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#11 David Mullen ASC

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Posted 16 November 2005 - 03:53 PM

35mm has on average less depth of field because the focal lengths on average are longer than with 16mm. Combine that with the fact that 35mm is often used for theatrical print projection, where the image will be seen on a 50' screen or larger, and focusing becomes very critical.
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