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Canon XH Series HDV Camcorders
Canon XH G1S / G1 (with SDI), Canon XH A1S / A1 (without SDI).


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Old July 31st, 2007, 01:57 PM   #1
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To use AGC or not for outdoor/wildlife

I have about three months of experience with the xh a1...focusing primarily on birds in the pacific northwest. shoot with AGC on all the time. however i saw a couple of threads relating to NEVER shooting with the AGC on. I see no grain, but that is not using a monitor but the LCD. I have used the AGC in lower light conditions, but primarily in overcast shady within tree canopies.

any comments?

bill
bothell, WA
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Old July 31st, 2007, 03:04 PM   #2
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The AGC will usually add a lot of noise that hurts the image quality... The only time I ever end up using that is when it is nearly pitch black with very little to no light...

Typically I just use the manual gain switch and set it to 0, +6, +12 db... If you have any natural light you should be able to adjust other camera settings accordingly...
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Old July 31st, 2007, 03:36 PM   #3
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Quote:
I have used the AGC in lower light conditions, but primarily in overcast shady within tree canopies.
Auto-gain is bad because the Auto Exposure (AE) system frequently overexposes, yeilding terrible noise. Since you haven't seen any "grain" (noise), you can know that even shady tree canopies on overcast days are bright enough for the XH A1 to stay away from the high gain settings.

Your location also highlights another reason that you haven't had trouble: shady subjects generally have very little dynamic range (i.e. contrast, lattitude, etc.), and are exactly the conditions under which the AE excels.

However, a sunlit subject against a dark background will prompt the AE to overexpose and blow highlights in the subject (even by two whole stops). For this and other reasons, I recommend shooting in manual only.

Another reason the auto-gain is bad is because the camera will many times choose to up the gain before it has even maximized the aperture. (E.g. +6db with f/2.2 when +3 with f/1.6 would have sufficed.)

Last edited by Daniel Browning; July 31st, 2007 at 03:39 PM. Reason: Add one more reason auto-gain is bad.
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Old July 31st, 2007, 03:44 PM   #4
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AGC is pure evil.
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Old July 31st, 2007, 03:54 PM   #5
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Originally Posted by Eric Weiss View Post
AGC is pure evil.
lol... agreed...
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Old July 31st, 2007, 04:03 PM   #6
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If there was a possibility to set limit for auto gain it would be useful in some situations (as you have noticed). But there isn't, so it's better not to use it in lower light. A1's auto gain doesn't work very wisely. If there was a limit and you could set it e.g. in +6db, it would be nice to use AGC sometimes. Sometimes I'm dreaming that my A1's AGC button had changed to OIS on/off button.. then I wake up ;)
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Old August 1st, 2007, 07:49 AM   #7
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AGC is, well automatic, and operates according to a program created by Canon to provide appropriate gain levels for what they consider "average" scene conditions, and that gain will be based on the scene lighting as evaluated by the auto exposure system in the camcorder, and the aperture, shutter, and presets.

But most scene composition and lighting conditions will NOT be "average" and thus the gain (and aperture and shutter as applicable to your shooting mode), selected by the automatic program may not be optimal for the actual scene, selected presets, and your artistic intent. Given a god monitor and the time to do manual settings (including adjusting presets) a reasonably skilled camcorder operator can always come up with an image that is at least equal to and usually better better than the automatic image.

However, if you do not have the time to make the manual settings, automatic exposure can often yield an image that is usable. Certainly more usable than one from a locked down manual setting that no longer fits changing scene lighting.

Does AGC cause noise? Its not quite that simple. Using gain does add noise to the image because with non-zero gain the CCD is no longer operating in its optimal range, and dark current variations below the threshold of visibility in an image with adequate light can become apparent in the image, especially in shadow areas. With AGC gain maybe added automatically as scene lighting drops, and thus noise level increases. Depending on image content, presets, and exposure parameters it may or may not be a problem. If AGC is the only exposure parameter you allow to change, gain may increase to excessive levels providing excessive image noise.

The key is your artistic intent. If using AGC (and various auto exposure modes for that matter) works for you and gives you images that meet your needs, you can safely ignore what other say about AGC being good or bad. After all, they judge it based on their needs, not yours. Just keep in mind AGC's limits so you can make informed decisions about using it when you move to different shooting situations.
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Old August 1st, 2007, 11:54 AM   #8
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I set AGC to manual and use 0 dB as much as possible
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Old August 1st, 2007, 12:19 PM   #9
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Quote:
Originally Posted by Eric Weiss View Post
AGC is pure evil.
lmao, you're my hero.

But yeah, maybe the reason you haven't seen any grain is because you've been lucky enough to be in enough light to where the auto gain. But as a precaution, unless you're in the dead of night or something, don't use it.

I know it seems like I'm repeating what everybody else is saying...but whatever.
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Old August 1st, 2007, 03:31 PM   #10
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thankyou don for a more definitive explanation, and to to daniel for explaining the aperature issues where the camera will choose a higher grain over opening up the aperature to F1.8? However the latter raises another issue i have tried to follow on this line. namely that an aperature setting between 4.6 and 5.2 creates the sharpest image. I do shoot AE most of the time, first going into manual if i have time, exposure lock, then set to AE.

i have had to shoot manual with high contrast..lighted bird with black background etc....but should i then just get rid of AGC mode and shoot the 0DB as much as i can?

my wife and i will be shooting humpback whales in two weeks in frederick sound, and typically there is grey skies and seas, and dark whales that appear unpredictably at a range of 30-100 meters bubble netting. i know i should shoot manual and estimate the distance as AF will not 'grab' a sea sky fusion of grey..but that AGC thing really has me still questioning the balance...a little more grain but compensate with sharpness and f stop?


thanks for all the great responses. bill
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Old August 1st, 2007, 03:44 PM   #11
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Originally Posted by William Boehm View Post
...an aperature setting between 4.6 and 5.2 creates the sharpest image. ... [That] AGC thing really has me still questioning the balance...a little more grain but compensate with sharpness and f stop?
I think it's a personal choice. I would rather shoot wide open and lose some resolution than go past 0db and introduce even a little noise. However, if some segments of the production must have a high gain, I'll often use the same gain for all the shoots to achieve a consistent look, even if it's as high as +6db.
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Old August 1st, 2007, 03:51 PM   #12
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Practice before the shoot if you can under similar lighting and subject conditions to evaluate performance of the options. That is the best way to be sure you get what you want at the actual shoot.

30 to 100 meters sounds pretty near optical infinity (well hyperfocal distance) for focus purposes, so setting focus to about 50 meters infinity may work on for all shots.
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