40D and the MT-24EX Flash Metering Problem

Started Sep 14, 2008 | Discussions thread
JimH Forum Pro • Posts: 12,911
That's good information.

I think that it might be useful to do some controlled tests indoors.

I may set something up to try this.

My idea is that I could mount the MP-E to a tripod using its tripod ring. I'd set up a fixed "scene" and then do testing with both the MT-24EX and the 580 on a bracket to put it in a similar attitude to one of the heads of the MT--24.

Then, because the lens itself would be fixed, I could also swap between the 40D and the 20D bodies for both the MT-24 and the 580 tests.

I'd try both evaluative and average modes for the ETTL for all four of those variations, giving me eight total scenarios:

MT-24 - 40D - Evaluative
MT-24 - 40D - Average
MT-24 - 20D - Evaluative
MT-24 - 20D - Average
580EX - 40D - Evaluative
580EX - 40D - Average
580EX - 20D - Evaluative
580EX - 20D - Average

And I could also test with two scenes. One with an "open" background (the kind of scene that seems to trigger the problem) and another with a close-in background (which always seems to work OK anyhow).

That would add eight more shots to the testing, but I could implement this by using a movable background piece made of nothing more than a piece of neutral gray paper.

With the main subject being so close-in, it should be fairly easy for me to control any reflections that would end up being powerful enough to alter the results between the 580 and the MT-24. I'll just keep it all well away from anything reflective.

I have been meaning to try this for a while, but until now, I wasn't sure anyone else was having this same problem. I thought it might just be an adjustment or connection to the new main board in my 40D that was to blame, so I was considering sending it back in for a fourth time to get that fixed.

I suppose we should test with other lenses and at different distances too.

Have you had the same problems when using other macro lenses? I have not tested that very much, but I have used the MT-24 on my EF-S 60 to shoot some close-ups that are not macros.

But the thing is that in those shots, there was not nearly as much open space around the subject. And it's that open space that seems to fool the flash metering when using the MT-24 with the 40D. So those shots were not a good test of all of this.

The very thing that Evaluative ETTL is supposed to handle for us (shooting a nearby subject against a distant or non-reflective background) doesn't seem to work correctly with the 40D when using the MT-24 for close macros.

But I should test this on more distant subjects with the EF-S 60 (to which the MT-24 mounts). That will help nail down whether it's a distance issue or purely a problem between the 40D and the MT-24.

I'll see if I can get some time to set up a test scene and do some controlled testing.

Also, I have read about people having problems with the 40D's flash metering with the 580EX II. In those cases, it seems as though the problem may be due to a contact integrity issue, and some people have corrected this by disassembling the 580 EX II themselves, or sending it in to Canon for repairs.

In those cases, what people noticed was that the flash would switch (on its own) from ETTL mode to TTL mode, and then emit full-power flashes until they reset the camera and flash by turning the flash off and pulling the batteries from the camera or some such thing.

I have not noticed the MT-24 switching into TTL mode.

But if other macro shooters are seeing the same thing as I've been seeing when shooting with the MT-24 on the MP-E with the 40D, then it cannot be only my camera that's doing it. And I don't think what we're seeing is necessarily related to the ETTL/TTL problems reported by 40D/580EXII users.

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Jim H.

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