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Light reflection and mid-gray balance


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#1 Ben Barrett

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Posted 28 March 2007 - 06:48 PM

This is probably a stupid question really, so my appologies in advance:
Attached is a frame from a video shot with a Cannon XL2. The subject (and even the scene in general) is a bit grayer - not really darker, just grayer - than would you would normally expect for a well-light shot. There is a strong light coming in from behind him. Is this grayness caused by the camera balancing the shot to mid-gray (although the stong light isn't actually visible in the frame) or is there some other reason involving the light?

Thanks for your time,
Ben

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Edited by Ben Barrett, 28 March 2007 - 06:48 PM.

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#2 Michael Nash

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Posted 28 March 2007 - 07:21 PM

Looks like your lens was getting flared by the backlight. "Gray" has nothing to do with it -- light hitting the lens or filter is fogging the blacks, which lowers overall contrast and desaturates color. In this case the flare is called "veiling glare," which simply means the flare is washing out the whole image, not just giving a small localized flare or spot.

When there is a strong backlight, even one that is out of frame, it can flare the lens. In situations like this you have to protect the front element of the lens from stray light, with a flag or "eyebrow" near the lens, just out of frame.
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#3 isaac_klotz

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Posted 29 March 2007 - 01:46 AM

heavily backlit images are more contrasty and less saturated. your subject has very little frontal light (key and fill) which is the type of light which would give you vibrant colors. the white wall is white and the black railings are black, so the image has the appearance of being properly exposed. however, your subject is underexposed by a couple of stops, causing this look (which you describe as a grey look).

how do you like it? i think it can be beautiful in certain situations. sometimes an eye light will help.
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#4 Ben Barrett

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Posted 29 March 2007 - 06:54 AM

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Edited by Ben Barrett, 29 March 2007 - 06:55 AM.

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#5 Ben Barrett

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Posted 29 March 2007 - 06:54 AM

Looks like your lens was getting flared by the backlight. "Gray" has nothing to do with it -- light hitting the lens or filter is fogging the blacks, which lowers overall contrast and desaturates color. In this case the flare is called "veiling glare," which simply means the flare is washing out the whole image, not just giving a small localized flare or spot.

When there is a strong backlight, even one that is out of frame, it can flare the lens. In situations like this you have to protect the front element of the lens from stray light, with a flag or "eyebrow" near the lens, just out of frame.

Thanks, that makes sense now. So I should have used a top flag to block out any stray light and that would have removed the flare problem? Would a polarising filter have made any difference?
The subject is being lit by reflected light only, would lighting them have reduced or removed the problem, or should I not try to compete with natural sun light?

Thanks for your time,
Ben
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#6 Ben Barrett

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Posted 29 March 2007 - 07:07 AM

the white wall is white and the black railings are black, so the image has the appearance of being properly exposed. however, your subject is underexposed by a couple of stops, causing this look (which you describe as a grey look).

how do you like it? i think it can be beautiful in certain situations. sometimes an eye light will help.

Okay, so this can also be controlled by opening the iris? Although wouldn't that cause the other colours there - such as the red in the background - to become overly bright?
Hmm, I'm not really a fan of it - well certainly not here; there are probably some situations where reducing contrast can look good, although generally unless the mood of the scene permits it I prefer good colours in the pictures - just my opinion though.

Ben
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#7 David Mullen ASC

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Posted 29 March 2007 - 07:39 AM

Okay, so this can also be controlled by opening the iris? Although wouldn't that cause the other colours there - such as the red in the background - to become overly bright?


Trouble is that you're getting a veiled flare from the bright wall in the frame and opening up the iris would make it worse -- basically you need to add more light to the subject so you can close down the iris and darken the wall, or use an ND grad on that side of the frame.
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#8 Michael Nash

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Posted 29 March 2007 - 05:25 PM

Would a polarising filter have made any difference?


A polarizer would not have helped in this situation. Polarizers only eliminate light rays across one axis, but the flare looks like it's coming from a larger source with multi-direction light rays (was there a window in the stairwell above frame?)

Sometimes filters can actually make this kind of flare worse, as stray light hits both the filter and the front element of the lens.
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#9 Ben Barrett

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Posted 29 March 2007 - 06:39 PM

Trouble is that you're getting a veiled flare from the bright wall in the frame and opening up the iris would make it worse -- basically you need to add more light to the subject so you can close down the iris and darken the wall, or use an ND grad on that side of the frame.

Okay, so the white out on the wall is causing the veiling flare. Is it possible to have a whited out area from a strong light behind the subject while avoiding veiling flare? I noticed Barry Ackroyd did this in United 93 and The Wind That Shakes The Barley. How is that possible, surely problem can't be exclusive to video?

Many thanks for your time,
Ben
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#10 Michael Nash

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Posted 29 March 2007 - 08:35 PM

Different lenses have different flare characteristics. Higher-end 35mm optics are made to hold up well to this type of flare, although you still have to protect the lens for the best image. Motion picture cameras also usually use matte boxes (sometimes with hard mattes) to control flare as much as possible.

http://cgi.ebay.com/...1QQcmdZViewItem

There are matte boxes made specially for smaller video cameras as well. With a zoom lens it's harder to make practical use of hard mattes, since they're made for specific focal lengths (or a very small range).
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#11 Ben Barrett

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Posted 05 April 2007 - 04:38 AM

Different lenses have different flare characteristics. Higher-end 35mm optics are made to hold up well to this type of flare, although you still have to protect the lens for the best image. Motion picture cameras also usually use matte boxes (sometimes with hard mattes) to control flare as much as possible.

http://cgi.ebay.com/...1QQcmdZViewItem

There are matte boxes made specially for smaller video cameras as well. With a zoom lens it's harder to make practical use of hard mattes, since they're made for specific focal lengths (or a very small range).

Thanks Michael. I had a matte box for the XL2 but didn't use it for this shot (duh!) I see now though. I should have angled the french flag to cut out stray light causing veiling glare. Admittedly though I never heard of thi stype of flare before.

Thanks for your help,
Ben
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