Canon 5D2 + 580 EXII

Started May 12, 2011 | Discussions thread
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Re: Canon 5D2 + 580 EXII
In reply to sacentre, May 13, 2011

sacentre wrote:

My test setup

Camera on a tripod, in M mode, ISO 100, Shutter speed 1/00th sec. Camera is 5 feet from a white wall with one of black plastic speakers on a stand as the subject to provide contrast for the AF. (My results have been the same when I used the wife's face as the subject. She got fed up with being dragged away from her tv soaps!).

The 580 EXII is mounted on camera, all camera flash settings set to default, flash exposure metering set to "Average", flash set to ETTL. No HSS.

Other than changing the camera to "evaluative", I don't see anything I'd change.

ETTL will set off a preflash to measure the light coming back into the lens and then set the output power accordingly. The difference here (bare/with modifier) is the amount of light or a reflection entering the lens. If the lens were "blocked" the output power would OVER EXPOSE the image and if now (with the modifier) a new reflection OR the position of the modifier is such that light from it was DIRECTLY entering the lens, the image would be under exposed.

I can't think of any Canon bare flash/lens combonations where the front of the flash head sticks out even to in front of the lens (normally the flash is BEHIND the lens when viewed from the side).

My guess is that light from the modifier is directly entering the lens and causing the under exposure (you DO have a lens hood on right?). You could try shielding the lens from the modifier or as BAK suggested, point the flash/modifier straight up or off to one side or other or behind you. (or otherwise get it away from the lens)

I have no experience with ETTL/Flash on a 5DC (classic) however use it on the 5DII all the time and get the same results you noticed without the modifier: consistant and proper exposures with 5D2+580EX2.

With the camera on a tripod and the flash attached to the camera via a OC3 cord you could hold the flash in your left hand and trigger the camera with your right hand while moving the flash+modifer around to see at what point the setup starts underexposing.

My EWAG is that when the front face of the modifier is forward of the front element of your lens you'll start getting under exposed images.

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