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I recently purchased a Speedlite 600EX-RT for use with my 5D Mark III. I'm shooting with a 17-40 f4/L lens. Particularly when using the flash in the standard forward position, I get extremely severe underexposure when shooting in P mode with the flash in either High Speed Sync or First Curtain release with E-TTL2 on in evaluative mode.

There are perhaps one or two fringe areas of the image that are exposed (less than 3% of the total image) and none of them are at all close to the chosen focal point. Many images haven't even had any portion of the image properly exposed. I'm using Center Weighted Average Metering for my metering setting.

Any idea what could be causing the issue? Does the metering setting for the camera and the flash have to match for proper results? Is there simply an issue with the communication between the flash and camera body? I'm really not sure what is going on and why I'm getting such poor results with the flash. I notice it also seem to do much better when using the flash in a bump flash configuration.

Edit: As mentioned below, I do realize there is a flash exposure compensation adjustment in the flash that I can make to boost the exposure, but I'm trying to figure out why it is misbehaving rather than simply trying to work around the problem. I'm specifically looking for any additional insight in to what could be making it underexpose by multiple stops.

JPEG sample without any correction done RAW File (CR2) as Shot

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What types of subjects are you photographing and could you post a sample image? – Steven Cunningham Feb 15 at 7:39
I agree with the above: without examples, it's hard to pinpoint the issue. Try a shot in evaluative metering as well as center-weighted metering for exposure, too. Maybe e-ttl2 metering on the flash and cwa metering on the body are fighting each other for correct exposure? – TroyR Feb 15 at 9:48
I have a limited subset of images to work from so far, but I will try to get a couple of them up. My initial review (only looked at them in camera so far) seemed to be fairly consistently off, but it is still disconcerting that it is off so severely (multiple full stops, most of the image is almost black). – AJ Henderson Feb 15 at 13:53
1  
A few example images would be very helpful. Also, are you using a 14-40mm L lens, or the 17-40mm L lens? If Canon is making a 14-40mm f/4 L-series lens, that would be pretty hot news. I am pretty sure you mean the 17-40....just wanted to make sure. – jrista♦ Feb 16 at 1:18
@jrista yes 17 to 40mm. Sorry, not sure why I was thinking 14. – AJ Henderson Feb 18 at 17:32
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3 Answers

up vote 1 down vote

If you are off by a consistent amount on different images of similar settings, have you considered exposure compensation? I think there is a secondary setting for the flash compensation as well if I am not mistaken.

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There is a flash exposure compensation setting and it is off by a consistent amount, so this can work as a fix, but with it being so severely off, I'm more worried about why it isn't working correctly for such high end gear. – AJ Henderson Feb 15 at 13:54
Well, without examples it is hard to judge. Some people will calll a difference of 2/3 of a stop severe, or 1/3 of a stop. Alternatively it might be 2 stops out... - the flash is definitely firing? It is also fully charged? (On my EX430 II, if I snap too many images I might shoot an image without flash which then is too dark.) – DetlevCM Feb 16 at 13:53
I'm getting some of the images posted now. The flash is definitely firing and I'm using the Canon CP-E4 extended battery and not doing rapid bursts. So I'm fairly positive that it is fully charged. (Full charge takes less than half a second with the CP-E4.) It's not 1/3 or 2/3 of a step off, it is more like multiple full steps off. The image is nearly black. – AJ Henderson Feb 18 at 17:47
Well, the example is maybe one stop off (at a guess) but good to know you checked those points. It means more points to eliminate. Having said that, I have a suspicion... – DetlevCM Feb 19 at 10:59
up vote 1 down vote

One possibility is that you started firing off shorts before the flash was ready. It has happened to me before and makes the images severely underexposed. It happens if the battery power is low or if you have been firing off a lot of flashes and the flash needs time to recycle.

You said that you were not able to reproduce the issue? Did you happen to put new batteries on them? Did you use the flash with the same intensity as before?

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This wasn't the problem unfortunately. I had new batteries in, was not firing rapidly and also had the CP-6 battery pack in use so my cycle times for full charge were under 1/3 second. – AJ Henderson Mar 20 at 19:46
up vote 1 down vote

The image seems to be quite underexposed indeed. Even the brightest spots seem not to be bright enough to me, so there must be something else. The flash has enough power to cover the distances in the picture, power should not be the problem. Broken mechanical parts (aperture blades, metering sensor, etc.) could maybe cause the problem, but that would be weird and seems not very likely to me.

With just one picture and the given information, I think it is a software problem in or between the camera and the flash. When the amount of underexposure is constant, you could just add an amount of 'overexposure', when the amount of underexposure differs at different pictures I should go to the store and say the product does not work properly.

Newest firmware of the flash and camera is installed?

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Yeah, I was leaning towards something like this. I found some other people posting about similar issues. I haven't been able to reproduce it since then though and I'm hoping that the April firmware update for the camera body may not have the problem if it is just some weird edge case. – AJ Henderson Mar 20 at 21:08

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