Strange Behavioure with 5D & 580 flash

[rama2407]

I may be missing something but I cant explain this.

Set my 5D to AV at F4, ISO 200. The camera meters 1/20th of second. If I then turn on the 580EX flash the camera now meters at F4 but at 1/40th second.

It seems to do this at a variety of settings ie higher ISOs and different F numbers. It gives half the exposue when the flash is turned on.


Can anyone explain this.

Phil

[Jim G]

Set it up on a tripod against a scene with obvious, static lighting - a white wall or somesuch perhaps. See if it still does it then... As far as I understand it Av mode doesn't factor in flash whatsoever when it figures out the metering... I'd guess that it's just the metering shifting as the camera slightly moves and the scene slightly changes. It'd be easier to do with partial or spot metering probably (as you could point it at a slightly differently lit part of the scene) or if there's any kind of changing light.

Someone more knowledgeable may be able to provide a better suggestion or maybe even an explanation :)

[tiha]

That behavior is feature called NEVEC (Negative Evaluative Exposure Compensation). In short, as soon the flash is switched on, camera (if it is in Tv or Av mode) will change shutter speed. Compensation amount is -1/2 to -1 EV, depending on light conditions and ISO value. More detail at: http://photography-on-the.net/forum/showthread.php?t=138907 or http://eosdoc.com/manuals/?q=NEVEC

[rama2407]

Thanks tiha

Yes that seems to explain it. I must say though I am really dissapointed that canon can do someting so stupid. So now when I am photographing a fast moving wedding in AV mode I now have even more to think about. I thought I could get a good result by metering in AV and using fill in flash with -2/3 FEC. Now I have to also dial in +1 stop exposure compensation to undo the stupid NEVEC. I thought canon was supposed to make thigs easier for photographers not more difficult.

Phil

[PacAce]

Thanks tiha

Yes that seems to explain it. I must say though I am really dissapointed that canon can do someting so stupid. So now when I am photographing a fast moving wedding in AV mode I now have even more to think about. I thought I could get a good result by metering in AV and using fill in flash with -2/3 FEC. Now I have to also dial in +1 stop exposure compensation to undo the stupid NEVEC. I thought canon was supposed to make thigs easier for photographers not more difficult.

Phil

For your average photographer, Canon is trying to make it easier for the photographer. If the camera is in Av mode and the setting for a proper exposure is, say, 1/20 @ f/4, then wouldn't adding even a little bit of flash cause that picture to become overexposed if the setting was not adjusted to compensate for the flash?

[zacker]

For your average photographer, Canon is trying to make it easier for the photographer. If the camera is in Av mode and the setting for a proper exposure is, say, 1/20 @ f/4, then wouldn't adding even a little bit of flash cause that picture to become overexposed if the setting was not adjusted to compensate for the flash?


bravo!!!

You beat me too it!
lol :lol:

[Curtis N]

If the camera is in Av mode and the setting for a proper exposure is, say, 1/20 @ f/4, then wouldn't adding even a little bit of flash cause that picture to become overexposed if the setting was not adjusted to compensate for the flash?Only if you were metering on the subject, or if the subject is in the same light as the background.

If the background is brighter than the subject, then average or evaluative metering would create an underexposed subject, even without NEVEC. Flash could be used to properly illuminate the subject without any adjustment of the ambient exposure.

I understand the reason for NEVEC. What I don't understand is why a) it's completely undocumented, and b) there is no way to disable it. The fact that NEVEC behaves differently depending on the ambient light level makes it all the more unpredictable.

[PacAce]

Only if you were metering on the subject, or if the subject is in the same light as the background.

If the background is brighter than the subject, then average or evaluative metering would create an underexposed subject, even without NEVEC. Flash could be used to properly illuminate the subject without any adjustment of the ambient exposure.

I understand the reason for NEVEC. What I don't understand is why a) it's completely undocumented, and b) there is no way to disable it. The fact that NEVEC behaves differently depending on the ambient light level makes it all the more unpredictable.

Curtis, I wasn't arguing for (or against) NEVEC. I was simply responding to Phil's statement, "I thought canon was supposed to make thigs easier for photographers not more difficult.", citing an average photographer as an example.

[Curtis N]

I wasn't arguing for (or against) NEVEC.Yeah, I know. You just gave me a good opportunity for a rant. ;)

[rama2407]

For your average photographer, Canon is trying to make it easier for the photographer. If the camera is in Av mode and the setting for a proper exposure is, say, 1/20 @ f/4, then wouldn't adding even a little bit of flash cause that picture to become overexposed if the setting was not adjusted to compensate for the flash?

Yes I suppose you're right about that. Also it seems that this NEVEC only happens at lower EVs. So when the conditions are bright then a little flash can be used to lighten the shadows. Situation is not so bad as I first thought. Thanks for all the helpfull replies.


Phil