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ThomasATC May 6, 2005 12:04 PM

Hi everyone,

I have the Canon Eos 20D. I recently purchased the 580EX. Ioften shoot in AV mode with the center AF point only selected. While taking photographs in midday bright sunlight, I have been setting the flash compensationaround negative 1 to negative 2 as a starting point togently fill the shadows. Then I make adjustments from the results.

However, I read somewhere( not sure of the online source) that with the above equipment, the flash automatically reduces output for fill flash under such bright lighting conditions . If this is true, am I reducing the output from an already reduced setting when I start at -1 or so FEC? Although I am comfortable bracketing and fine tuning the final result, I am looking for a good starting point for these harsh lighting situations. Would I be better off starting with 0 compensation and working from there? I guess my main question is;Under certain lighting, does thiscamera with the above flash attempt to automatically take care of a function ( reducing flash by 1 to 2 EV) that would otherwise be handled manually with older equipment. Am I making more work for myself? I read the manual, but didn't find much if any information on fill flash.

In other lighting conditions, I am absolutelyamazed at the ETTL's ability to handle so many situations automatically. Any advice or comments greatly appreciated here. This forum is truly a wealth of information.

Thank you


NHL May 7, 2005 5:30 AM


The Canon's will do fill on all auto modes including Av, i.e. it will meter both the ambient light and flash and exposed for a 'balanced' fill:

Problem is the camera 'meter' to what it thinks is an 18% grey of a 'standard' Grey card - Your subject(s) is rarely this neutral hence the various exposure compensations. The other issue is the flash exposure compensation on the camera works the same way as its normal exposure compensation (ie this is not a separate/independent selection from ambient), so what you set for one affects the other - even if you perform an FEL in any of the Auto modes...

To vary the ratio between flash and ambient lighting flip the camera to Manual mode - The flash will still remain in automatic E-TTL mode when the camera is on manual and will exposed correctly -> but now you have control over the ambient by varying the aperture - Just pay attention to the: -2..-1..v..+1..+2 in the viewfinder which indicates where you are with respect to ambient light and you can keep this scale centered by varying both the shutter/aperture ;)

I would start @ 0 compensation and work from there :cool:
(unless you already know your subject(s) is brighter/darker than 18% grey)

BTW if you always want to overide your flash with a -1 to -2, set it on the flash head itself then the 'regular' camera exposure compensation will be used for the overall (the problem with this is don't forget to undo it otherwise the flash will always underexpose) :?

ThomasATC May 7, 2005 6:35 PM


Thank you for the very informativeexplanation! And the link too. :cool:

You certainly know your cameras! Amazing how the camera and flash compare the ambient light and flash ratios for a balanced fill flash. Takes all the fun away from guessing! But like you said, not many scenes are pefectly neutral.

I like the technique for Manual mode making adjustments on the ambient light making aperature adjustments.

I will try some more shooting starting at 0 compensation. Thanks again, I really appreciate your taking the time to help !



bigboyhf May 9, 2005 4:11 PM

The 580EX does automatically reduce exposure compensation for fill. I don't think that there is a documented amount, but it is probably somewhere around -1. You can further decrease it if you want, but I would experiment to find out what suits your taste in fill lighting.

This is the most comprehensive Flash FAQ I have seen for Canon speedlights. Pay attention to the ETTL-II notes for the 580EX.


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