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Old Jul 6, 2005, 8:00 PM   #1
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I am using a Sigma EF-500 DG flash with a Canon rebel XT. When the subject is far away (but still well within the flash range) in TTL mode I consitently get the flash firing at what seems to be too dim a setting.

sigma.zip file has two pictures, both shot with ISO100 in raw mode:

IMG_0984 -- shot in program auto mode on the Canon and TTL mode on the Sigma. When I open in CS 2, the (digital) exposureis 2.45, meaning that I effectively get a much higher ISO setting. IMG_0984_crop in the zip file shows a small portion of the picture (I needed to crop to keep the file size down).

IMG_0987 -- shot inmanualmode on the Canon andMh (max power)mode on the Sigma. When I open in CS 2, the (digital) exposureis -0.15 (i.e.very little correction).IMG_0987_crop in the zip file shows a small portion of the picture. IMG_0987_small is the whole picture but scaled to a very small picture to keep the file size down.

The _crop versions show that the manual version (0987) is much clearer than the auto version. Both versions use the f5.6 and 1/60th second shutter time.

is there any way to get results like IMG_0987 wihtout going to full manual? If not, is there a way I can estimate correct f stop / ISO combination at full flash power?
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Old Jul 7, 2005, 10:43 AM   #2
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The reason you're getting too dim of a setting in Auto is because there's an overwhelming 'white' content in your picture
-> The camera is calibrated for neutral grey hence it 'thinks' the image will be too bright and decrease the flash output accordingly...

Use the FEL (*) i.e. Flash Exposure Lock in auto on an area that you believe is more leaning toward neutral grey, like the blue wall, and the camera/flash will expose correctly
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Old Jul 7, 2005, 11:13 AM   #3
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Thanks; that seems to help. I guess the flash is not as smart as CS2 -- CS2 seems to evaluate the scene better and know the correct exposure as it compensates for it by automatically picking a high exposure adjustment. (Of course, the high exposure adjustment means that the picture is noisy -- you need to get it right at the begining!) In fact, when I shoot raw + JPEG I noticed that theCanon JPEGs usually have a much poorer (digital) exposure setting thenCS2 does. So I suppose that shooting raw not only gives you more "control" but even if you do things automatically in the PC the PC software seems to beat thecamera software ...

BTW, where are the brains for the flash exposure? In the flash or the camera? In other words, could I possibly get better results if I had not been cheap when I bought the Sigma flash instead of getting a Canon flash?
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Old Jul 7, 2005, 3:34 PM   #4
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elinzer1 wrote:
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BTW, where are the brains for the flash exposure? In the flash or the camera? In other words, could I possibly get better results if I had not been cheap when I bought the Sigma flash instead of getting a Canon flash?
The brain/metering is in the camera - the flash is just like a lens, it is told by the camera what to do
http://eosseries.ifrance.com/eosseri...ork_fonct.html

-> for example put an ND filter in front of the lens (the flash won't know this) - The camera will still see through this darker filter and 'command' the flash head to output the higher and 'correct' amount...

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