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Old May 10, 2005, 2:43 PM   #1
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I have noticed that the FEC doesn't really do much to correct underexposure unless I up the ISO to at least 200 and even better at 400 or 800. This is taking pics of people at about 8 feet, 125 sec and F/8. I kicked FEC up +1 2/3 with little change in exposure at lower ISO's. Is this normal? At 400 I can get a halfway decent exposure at +1 but why should this happen even using my 580EX?

I do notice that the exposure is better when metering off a lighter surface. I would have expected the oposite but maybe I am wrong..

H
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Old May 10, 2005, 6:52 PM   #2
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There are a couple things that may be factoring in here. First, with a speed of 1/125 you won't be getting much ambient light. If you drop the shutter speed down to 1/60 or even 1/30 you will get more ambient light in the exposure.

But, if your primary target(s) is underexposing it may be based upon your metering mode. Have you tried center-weighted mode as opposed to evaluative? I'll give you an example - when I was taking pictures at my niece's confirmation in a church - the first couple pictures I took of the alter area were extremely underexposed - when I changed to center weighted the exposure was better. Why? My guess is that on either side of the alter were candles and the metering was picking up that candle light and getting a bit screwed up - so when I exposed for just the center area life was good.

Give both these suggestions a try - lower your shutter speed and switch to center weighted metering and see if that helps.
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Old May 10, 2005, 11:49 PM   #3
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It just seems to me that the 580EX should have PLENTY of spare power to adequately light up a portrait at 8 feet at almost any ISO, aperture and speed! I will give center weight a shot though.

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Old May 11, 2005, 11:11 AM   #4
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Are you boosting the FEC on camera or on the flash?

If you boost it on the flash, you will get more output, but like JohnG said, the camera's meter will still set what the flash exposure should be. Post some examples to get better advice.
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Old May 12, 2005, 11:22 AM   #5
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bigboyhf wrote:
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It just seems to me that the 580EX should have PLENTY of spare power to adequately light up a portrait at 8 feet at almost any ISO, aperture and speed!
I agree (within the shutter speeds that synch w/o HS set) and just this Sunday experienced the same surprising gross underexposure. I had a Sto-Fen diffuser on the 580EX for the first time and, man, what a shock. After the first dark pic (manual mode 1/60, F8 at about 10 feet), I switched to Program mode and had the same result. Checked the flash andFEC was 0. Checked the 20D andFEC was 0! Newly charged batteries in the flash. The pictures were nearly unusable even with the RAW Shooter Essentials set to +2 stops (the maximum) exp comp.

Yesterday, I did some test shots without the Stofen diffuser. Better, but still got a left-biased histogram. What's the deal here? On my Digital Rebel, I normally have +2/3 FEC set on that camera (I have the Wasia firmware) to get a normal exposure with flash.

Anyone else find surprising results with the 20D and 580EX. These two devices are both ETTL II. I wonder if there is a connection.
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Old May 12, 2005, 12:02 PM   #6
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I don't quite understand why +1 2/3 FEC is needed to get a decent flash exposure as indicated by histogram. I will try to get some samples posted once I figure out how to post a histogram...??

H
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Old May 12, 2005, 12:29 PM   #7
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Ok... I sat down and took my idiot hat off :? I had a circular polarizer on my lens! After removing it, I was able touse +1 FEC for a perfectly centered histogram at all ISO's with the on camera flash. I guess the polarizer makes a difference. Duh...

I have to seeif the results are the same with the 580EX now.

H :O
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Old May 12, 2005, 12:30 PM   #8
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:| There are just so many things to remember!!! I would be taking by Ginko, but I forgot where I put it...
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Old May 12, 2005, 6:20 PM   #9
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bigboyhf wrote:
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Ok... I sat down and took my idiot hat off :? I had a circular polarizer on my lens! After removing it, I was able to use +1 FEC for a perfectly centered histogram at all ISO's with the on camera flash. I guess the polarizer makes a difference. Duh...
As long as the flash is powerful enough, it shouldn't matter. It is TTL metering after all. Whatever light gets through the polarizer and lens is measured by the camera, and the camera should correct the flash output for that.

It would only matter if the flash output needs to be adjusted to more than the flash can output. But since it works with the internal flash but not the 580EX (which I take it is more powerful than the built-in flash) I don't understand it yet.


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Old May 12, 2005, 7:09 PM   #10
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I did notice that when I start going smaller than F8, I have to start bumping up the ISO with shutter speeds of 125 or faster. It will get a decent exposure but one that will have to be tweaked post processing. I can live with that. I'm not sure why the polarizer made such a big difference, but it did...

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