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Canon XL and GL Series DV Camcorders
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Old September 30th, 2004, 12:49 PM   #1
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knee? definition please

I don't mean the anatomical knee but the setting on the XL2 or any camera in general. I understand the Cinegamma and the black press/normal/stretch. But the knee setting has me puzzled.

thanks.
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Old September 30th, 2004, 01:02 PM   #2
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The knee circuit rolls off the exposure at the top end of the scale. It's designed to help prevent overexposure. On the DVX100A the knee setting lets you tell the camera what IRE level to kick in at (low=80, mid=90, high=100). I'm sure the XL2 has some similar setting.

A higher knee setting will make for more accurate image rendition at the expense of less latitude and greater potential for overexposure. A lower knee setting will give the most latitude and the most protection against overexposure, at the expense of (obviously) the signal being manipulated, so perhaps a less realistic image rendition.
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Old September 30th, 2004, 02:05 PM   #3
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Knee setting is only needed when high scene contrast is expected. Otherwise it unecessary compromises image quality
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Old September 30th, 2004, 03:29 PM   #4
 
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If you've ever played with the "Curves" adjustment layer in Photoshop, you'll understand the principle with "knee". In summary terms, if one was to plot input color vs. output color, the ideal would be a 45 degree diagonal line, indicating 1:1 conversion. As this line varies from perfectly diagonal, a more vertical line results on more image conrast, a more horizontal line results in less image contrast. Rather than adjusting the entire curve, the XL2 provides adjustment in the toe and the knee of the curve. A more vertical line above the knee produces more contrast in the highlight area of the image, a more vertical line in the black(press) produces more contrast in the shadows. Contrast is gained by sacrificing overall range(aka latitude)nothing is free.
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Old September 30th, 2004, 06:26 PM   #5
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What exactly is lost in image quality when adjusting the Knee or Blacks manually? Is it just related to contrast? Or is there other factors as well? Because seems to me, you'd want to get the lowest contrast possible in order to get the best lattitude possible and correct the image to your liking in post. Clipped highlights are visual information gone forever, but it's easy to add contrast in post and fine tune detail rendition.
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Old September 30th, 2004, 06:52 PM   #6
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Another way of stating what bill said is that a high knee setting on the xl2 causes compression of the highlights, and an increase in contrast in the midtones. A low knee does the opposite. The same holds true for Press and stretch (toe) in the black portion of the curve.

<<a more vertical line in the black(press) produces more contrast in the shadows.>>>

I think you mean stretch here, bill.

David....all adjustments to a tonal curve mean compression of some tonal areas and corresponding expansion of others. The more severe the adjustment, the more likely gaps will develop within the tonal range causing posterization. This is the primary benefit of making these adjustments in camera while you are working with 12bit tonal depth. Once you enter the post production environment, the 8 bits plus 5:1 compression can seriously limit what you can do without posterization occuring.

Also, one purpose of the knee is to aid in the transition of the highlight range...otherwise you would get more clipping (adam wilt has a very nice illustration of this on his site). By compressing the highlights slightly, the transition into that "clipped" tone is more gentle...a low knee... or a flat curve...would cause an abrupt on/off effect once it clips. Cinegamma and low kneesettings are best used when you have complete control over your lighting, otherwise clipping will be of the "wow look at that" variety.

Barry
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Old September 30th, 2004, 07:10 PM   #7
 
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yep, thanx Barry
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Old October 2nd, 2004, 11:41 AM   #8
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What hasn't been mentioned by the excellent explanations, above, is that the whole reason for this circuit is to emulate part of the characteristic exposure curves of filmstock.

Below, is a link to the sensitivity curves of Kodak's 500T filmstock.

http://www.kodak.com/US/plugins/acro...es/f0268ac.pdf

If you think of the response curve as a person's lower leg from knee at top-right, to toe at lower-left, you'll get where the terms came from.

More importantly, this characteristic response curve of film chemistry is the single most important element responsible for the "film look" that some videographers so avidly seek, hence the presence of a knee circuit in modern video cameras.
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