40D and the MT-24EX Flash Metering Problem

Started Sep 14, 2008 | Discussions thread
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John K Veteran Member • Posts: 7,283
40D and the MT-24EX Flash Metering Problem

Early this year I switched from using Canon’s Xti to the 40D and at the same time I was working on getting better diffusion from the MT-24EX. Oddly enough I was having exposure problems with the 40D that I never saw with the Xti: Scenes that the Xti would normally under expose by 1/3 of a stop the 40D would over expose by that same 1/3 of a stop or more. If I took six frames of a scene none of them would be exposed the same and they’d all be over exposed even though it’s normal for me to shoot with the MT-24EX set to -2/3 FEC.

Even shooting against something reflective would give me over exposures even though the camera should have under exposed. At first I thought the 40D was “hyper accurate” and that there was something wrong either with my technique or the diffuser materials that I had used on the twin flashes but as time passed I read more and more people make the same claim –the MT-24EX just doesn’t work well when attached to the 40D. The problem will manifest itself the most when you are shooting a subject when there is very little in the background for the flash to reflect off of –if the background is black in the final image then odds are the subject is completely blown (a problem that I never had with the Xti).

SteB, one of the posters at DP Review mentioned having the same exposure problem that I was seeing and that it seemed like the MT-24EX was operating in Average E-TTL mode instead of Evaluative. Sure enough switching the 40D to Average E-TTL mode seems to give the same exposures as Evaluative in my informal field testing. The work around that I eventually come up with is to make sure that the flash heads were pointed right at the scene I was shooting and to keep one flash head off centered from the top of the lens at the 11 O’clock position and place the other at 2 O’clock (relative to me). The angle of the heads depends on what you’re using for a diffuser –currently I’m using a pair of Sto-Fens over a piece of diffuser plastic that only blocks 2% of the light coming out of the flash. So I position the heads one click up from all the way down at life size and all the way down toward the lens for 2x to 5x shooting. The image included with this post is the result of an entire macro season of trial and error trying to solve the metering problem and get good light quality at the same time…

I’m still not getting consistent exposures, but it’s better than putting one of the flash heads at the very top of the MPE-65mm: It seems like one side of the subject gets thrown into too much shadow so the light meter over compensates for it –badly. If you see some insects with two sets of eyes and eight sets of wings it’s my fault cause the MT-24EX pumped out so much power it altered the critter’s genetics…

So here's what I need you to do: If you shoot macro with the 40D and the MT-24EX then please respond to this thread and tell me if you are getting consistent exposures or not. If you are then please tell me a little about your technique and the types of scenes you shoot. If enough people post then maybe we can get Canon to take a serious look at the 40D's light meter...

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My Blog: http://www.nocroppingzone.com
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Macro Tutorials: http://dalantech.deviantart.com/gallery/#Tutorials

Always minimal post processing and no cropping -unless you count the viewfinder...

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