The new diffuser that I built wasn't holding up well to the abuse I was putting it through -should have routered out a frame in the Sto-Fens so that I could have put the diffusion plastic on the inside instead of on the outside where it could get caught on things and pull off. So I removed the new diffuser and did some tests between it, my second set of Sto-Fens, and "no diffuser" (in quotes since I had some light diffusion and a 1/8 CTO hot glued directly to the flash heads).
I set the MPE-65mm to 1x and the 40D to F11, 1/250, and ISO 100 (my usual settings) and then took a shot in E-TTL mode and in manual to determine what power level I needed -it turned out to be 1/64 power with the Sto-Fen and the diffuser that I had built. I shot four images and opened each one of them in the Elements 7 RAW editor with the highlight warning enabled. The red you see in the yellow part of the Hot Wheels car is where the red channel is blown.
I switched to E-TTL mode and got a little better results and this verifies what JimH was saying about E-TTL mode having greater control over exposure (the camera can dial in power levels that I can't):
The slight difference in the red channel in the last two shots is due to a slight angle change -I wasn't using the camera on a tripod, just trying to place one of the AF points in the view finder over the same general area. Since there was little difference between the diffuser I built and the Sto-Fen plastic I'm leaving it on the MT-24EX for now. I'll probably build a diffuser that has a slightly larger surface area later on.
Although I didn't keep the images I did test the MT-24EX in manual mode all the way out to 5x with the Sto-Fens and this is what I got shooting that same car:
1x to 2x: 1/64 (at 2x the flash heads were closer to the subject so no change in power).
3x: 1/32
4x: 1/16
5x: 1/8
I sent an email to JimH asking how those figures translate into shutter speeds.
Interesting stuff - I only "realised" the other day when doing manual flash that ETTL must be capable of giving shorter flash durations than 1/64th available on my 430 Ex.
Brian V.
Thanks Brian. I'm probably going to shoot in manual mode for a while -I just have to remember to change the power level when I change the mag. It's an odd catch 22: E-TTL mode would give me more granularity with the flash power -but only when the light meter in the 40D works (which isn't often). Shooting in manual mode means always under exposing a little, but I'd rather do that than to over expose...
- I only "realised" the other day when doing manual flash that ETTL must be capable of giving shorter flash durations than 1/64th available on my 430 Ex.