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Filters, can they shift focus?


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#1 Allyn Laing

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Posted 13 June 2006 - 05:08 PM

I'm aware that filters degrade images, but do they put the focus out? and would it get progressively worse with more filters?

warm regards,

Allyn
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#2 Michael Collier

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Posted 13 June 2006 - 05:28 PM

Theoretically no, an optically pure glass plate will not induce a focus shift. You would note a small shift in sharpness, as every time you introduce more glass into the path of light, the sharpness is reduced. you may also note reduced contrast, especially if the glass is not well flagged/matte boxed. All these effects are relativley mild, and would be covered by the glass' filter effects (ei you would not see much reduced contrast from the glass itself in a frost or diffused filter, because lower contrast is part of the effect of the filter.)

As for focus, the only way it could possibly throw off focus is if the glas were to warp, which is mostly unlikley. (unless you happen to have a scene inside a kiln, though at that piont glass would be least of your concerns.)
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#3 David Mullen ASC

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Posted 13 June 2006 - 05:47 PM

I've heard that one should avoid glass filters in front of extremely telephoto lenses, just as one should avoid rear filter gels behind extremely wide-angle lenses.

Of course we're talking about professional filters here. Cheap consumer filters stacked together may start to degrade the sharpness.
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#4 Allyn Laing

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Posted 13 June 2006 - 06:17 PM

I've heard that one should avoid glass filters in front of extremely telephoto lenses, just as one should avoid rear filter gels behind extremely wide-angle lenses.

Of course we're talking about professional filters here. Cheap consumer filters stacked together may start to degrade the sharpness.


Can multiple amounts of filters shift focus of the image? say you don't have an 85nd6 combo, so you use an 85 and a ND6 then you also have a coulour enhancer, thats 3 elements all of which are tiffen filters?
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#5 David Mullen ASC

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Posted 13 June 2006 - 07:53 PM

No, only on a really telephoto lens or the long end of a 10:1 zoom maybe. Since the filters are in front of the lens, if there were a shift (which there shouldn't be) you'd adjust for it when you eye-focused anyway.

A focus-shift is a different phenomenon than softening due to too many glass elements. The problem with stacking three filters is more likely to be flare and double-reflections (ghosting), maybe slight softening. Like I said, focus-shifting might occur only on really long focal lengths.
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#6 Matthew Skala

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Posted 14 June 2006 - 10:03 AM

There are some filters like Classic Soft filters that use dimples in the glass for the soft effect. If you use these filters on a wide angle lens with a small f-stop like f8+ the dimples will start to come into focus and blur parts of your image. I have seen this happen, it sucks!
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#7 Allyn Laing

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Posted 14 June 2006 - 05:00 PM

Ok, this is what happened

I was shooting on an NPR with a 10-110 angineux, it started as an overcast day with not much light, we started shooting then the sky cracked open, I wanted to keep the exposure similiar for later in the day. I was using the 7218 corrected for daylight with a colour enhancer, then an 81ef to warm it up, then stopping it down from there with ND's. We measured for every shot, and it is noticeable with wides that it is out of focus? Can anybody give me some idea?


warm regards
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#8 Michael Nash

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Posted 15 June 2006 - 12:14 AM

Ok, this is what happened

I was shooting on an NPR with a 10-110 angineux, ... Allyn Laing


Did you use the same mount for the different lenses, or did you rotate the turret at any time when you changed lenses? I never trusted the NPR's turret 100%...
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#9 Allyn Laing

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Posted 15 June 2006 - 12:42 AM

Did you use the same mount for the different lenses, or did you rotate the turret at any time when you changed lenses? I never trusted the NPR's turret 100%...


well it's a film courses camera, and I checked at the beggining of the day to see weather the shutter was at 180 degrees and then I put it back do you think this could be it?

Allyn
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#10 Michael Nash

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Posted 15 June 2006 - 01:26 AM

well it's a film courses camera, and I checked at the beggining of the day to see weather the shutter was at 180 degrees and then I put it back do you think this could be it?


Shutter angle will not affect the focus of the lenses.

It's unusal that a wide shot is out of focus in 16mm. The increased depth of field tends to keep at least some part of the image in focus. If a wide-angle lens is out of focus, I would suspect either the lens or the mount is at fault. But it depends on all the details -- the more accurate information you can give us, the better we may be able to diagnose the problem.
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#11 David Mullen ASC

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Posted 15 June 2006 - 02:18 AM

So the only difference between the sharp wide shots and the soft wide shots were the number of filters in front of the lens -- the f-stop remained constant? At a wide-open aperture, any back-focus problems would be made worse, and too stopped down, then diffraction will lower sharpness.

Like Michael says, the problem with the NPR is that it has two points where the back-focus can be thrown off -- the lens mount itself and the turret that rotates between the C-mount and the bayonet mount. I had soft shots with that camera which caused me to always check that the turret was snugly in place.

So you had three or four glass filters total?
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#12 Allyn Laing

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Posted 15 June 2006 - 04:52 AM

So you had three or four glass filters total?


Yes at some points 4 in total, it feels like it was the turret may be the contributer
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#13 Hal Smith

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Posted 15 June 2006 - 06:51 AM

Definitely sounds like a back focus problem to me - Allyn were you focusing zoomed in, and then backing out for your wides and masters? I've screwed up with video cameras that were out of back focus in the past by not critically looking at my wide angle focus after zooming in to focus a shot and then backing out to frame a wide, etc.
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#14 David Mullen ASC

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Posted 15 June 2006 - 12:29 PM

Four filters in a mattebox is a bit excessive, although that never stopped Bill Fraker... My only concern would be reflection problems, but I'd also expect a little softening, which is fine in 35mm, but would be less than ideal in 16mm.

But there are many DP's who wouldn't hesitate to put four filters in a mattebox.
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#15 Michael Nash

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Posted 16 June 2006 - 03:37 PM

FWIW, I've noticed a focus shift while using a single Cokin resin filter on video zooms. So if any of the glass filters weren't optically perfect, multiple filters could throw things off a bit.

Isn't the Color Enhancer cut from a certain thickness of special glass for the right density, instead of laminated or coated like other filters? Could this have contributed to the problem?
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