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Canon 20D with 420 flash
Old 08-15-2005, 08:03 PM   #1 (permalink)
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I'm having a problem with my flash. It seems like when I'm shooting several people at one time. (wide) that on the outer edges it seems to be real weak. Shooting in auto mode. Is thre a setting to get a full flash? tommymc
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Re: Canon 20D with 420 flash
Old 08-16-2005, 01:20 AM   #2 (permalink)
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Well, there are a couple of things that come to mind. First, make sure the zoom position on the flash head matches your lens. The 420 can cover a 24mm lens (35mm equiv). If the head isn't set to zoom automatically, its angle of coverage might not be enough for whatever lens you're using. That could be causing fall-off in the corners.

Secondly, you might be shooting at your largest aperture. Many lenses exhibit fall-off in the corners when wide open. Some even do it when closed down a stop. But I doubt this is the major contributor.

My best guess is that you're getting bit by the inverse-square law. You didn't mention what lens you're using. But if you're using what amounts to a 24mm equivalent lens (16mm or so on the 20D) and you're filling the frame from side to side, then whatever is on the edge of your photograph is about 1.4 times as far away from your camera as whatever is in the center of your photograph. That's enough distance to make the edge of your photograph 1 stop darker than the center, especially if the flash is providing most of the light for your exposure. If you're using a 35mm equivalent (~22mm) lens and photographing edge-to-edge, the fall-off from center to edge is still going to be about 1/2 a stop.

To see where the problem might be coming from, try the following:

First, make sure the flash is configured to set the zoom head according to the lens focal length and that you're not using a lens wider than 16mm (24mm equiv). If either one of these conditions are true, you've found at least part of the problem.

If everything above is ok and you're using a 24mm or wider lens then it's probably physics getting the best of you, especially in low light situations.

-Chip
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Re: Canon 20D with 420 flash
Old 08-16-2005, 08:04 AM   #3 (permalink)
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I don't think it's that complicated. It seems to me that the angle of coverage of your flash is narrower than the angle of view of your lens. More info on flahs brand and model and lens focal length would be great.
Jose
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Re: Canon 20D with 420 flash
Old 08-16-2005, 09:50 AM   #4 (permalink)
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If the shutter speed is below 200 then your flash will give you full power. If you go above 200 on high sync then you get less flash.

Another option is set your flash to slave, this will cause the flash to give you the widest flash regardless of what you have your lens set to.

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Re: Canon 20D with 420 flash
Old 08-16-2005, 12:27 PM   #5 (permalink)
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What are the variables..?

Lens, subject distance, settings on the flash, settings on the camera,..? Filter on lens?

JP
 
 
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