I've put a 25mm extension tube and 1.4x converter on my 100mm macro. Fully extended, this should theoretically give me an enlargement of 1.38(ext tube) * 1.4 (converter) = 3.92. Using the Canon ML-3 (GN11) ring flash with my EOS3 at f16, All photographs are significantly underexposed with the subject about 6-8cm away from the front of the lens. Now, I'm wondering: - Is the TTL metering of my EOS3 completely fooled by this setup (25mm + converter) - Is the effective f-stop (works out to f47) just too small for this flash to light up a subject 6-8cm away? - Does the distance from lens front/flash source to film plane (about 27.5cm) play a significant role?
I know the standard GN=f*d formula, but I don't know if this takes into account the macro situation where your film plane could be significantly further back from the flash than the object is in front.
Has anybody here tried something simular?
Regards Thys
At short distances GN don't work the same way; with ring flashes even less so. GN calculations are predicated on the flash being effectively a point source. I have a ML-14, but with that unit and 100ISO, it starts tending to underexposure at less than f/16 effective at 1:1 without compensation on my EOS3. At 1:1 I generally can only shoot at an effective f/22 with full power on both sides of the flash. Also, Canon's TTL seems a lot more erratic (ie, less dependable) than I've seen on my Nikon system. Unfortunately, this is one area where Canon is still behind Nikon :-(.
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