I just made some more test shots with the *istD. I shot some outside, some inside with built in flash and some with the external flash mounted on the camera. I also reset the camera to its' default settings. Guess I better shoot some more with external flash with cord hooked up and see what happens. The exposures were all right on target. Now, I wonder if the problem could be with the external flash cord I am using. Hmmmmm. Do you think this would cause an underexposure problem?
From my tests on my *istD, I concluded that the meter is correct but that the ISO sensitivity is overestimated by 0.5. Of course: --this is not necessarily the case with all *istD: many underexpose like mine, some others are correct, some others are worse and need repair --there are always situations which lure any meter, and in which you have to use your own brain, with the help of the histogram (after one test shot). My camera seems to love the low end of the histogram. This is not a problem if the histogram is not clearly cut at the left end: it must show a oblic line at this point, not a vertical. The oblic line is the large majority of cases. However, my practice is now to apply a +0.5 exposure compensation with an eye on the possible burned highlights. For comparison, I had no underexposure problems with my Olympus C2000Z and E10, and my Canon G3. The minoltas A1 and A2 of friends were about 0.5 overexposing. I have no experience of other reflexes.
I've found that the *istD underexposes ( as it should metering 18% grey ) light subjects, or with light backgrounds. A white wall using TTL ( AF500ftz ) behind any subject it always turns grey! You need to manually adjust 1 stop over. With dark subjects, overexposing with out blowing out much. If your used to the metering from a PS digi cam the *istD takes a bit of getting used to.
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