Originally posted by Lowell Goudge
I have posted this chart several times,
The *istD is the most accurate metering body I have, the K7 is next and the K10D with stock screen is the worst. The K20 uses the same screen.
The issue with a TC and exposure will also be a function of the native aperture of the lens you put on, in terms of the number of stops of correction, but I usually use 1.3-1.7 stops for a 2x TC on an F2.8 lens
Hi Lowell,
This is interesting, and I would not have expected this, but I'm certainly not immune to making assumptions that aren't valid. . .
What is the vertical of the graph indicating? I'm assuming the luminance value of an image of a standard grey card with a consistent light source taken using stopped down metering.
So (if I actually understand this) this indicates that the metering systems are at least somewhat dependent on the max aperture (light gathering ability at widest aperture) of the lens, and that adding TC(s) would shift the baseline for the meter by decreasing the effective light gathering capability at max aperture. Greater magnification up to the limit of the AF system (for me since I use AF all the time, and consider the practical limit of the AF system to be @ f8) and the directly proportional light loss would result in an increasingly positive bias to the baseline of the metering system the longer I go (up to f8 effective at max aperture) assuming your cameras are typical.
I always thought that the metering system would essentially establish a linear baseline for all max apertures that at least fall in the range of AF capability -- since they're essentially mini computers and should be able to be tuned to offset variations in the analog sensors involved. Just shows (again) some of the difficulties involved in engineering a high performance DSLR. . .
Thanks for this insight. As you probably have guessed by now, I use TCs all the time, and this might explain some of the exposure anomalies that I've experienced, but just wrote off as being due to unusual lighting conditions in particular situations. Something else I'll have to take into consideration in the future. . .
Scott