In article
>In article <49i645$dfo@newsstand.cit.cornell.edu>,
>Bob Neuman
>>The problem is not with the optics (the 85mm f1.4 Nikkor is sharp on
>>both converters [when used with understanding of their limitations]),
>>but with the speed of this or any other lens faster than about f1.8-2.
>>- metering becomes inaccurate unless compensation is made.
>>Hope this helps.
>I guess I don't quite understand what is happening here. What kind of
>compensation is necessary at very wide apertures? Why? Is it only a
>problem for external metering, or does is affect the metering system
>and the film differently? A 1.4X TC would bring this back to an f/2
>lens, so if no special compensation is necessary for an f/2 lens, why
>is there trouble using it on a TC? (Or is the compensation already
>built in, and with the TC one should not be applying the
>compensation?)
Camera TTL metering at all apertures are in error with lenses faster
than about f2 with converters because the converter glass diameter
reduces the amount of light passing through the converter with lenses
open more than about f2 (an f1.4 on a 1.4x should show a loss of one
stop if all were well, but it will show a loss of 1 2/3 to 2 stops
[I forget the exact amount, and I am too lazy to go down stairs and
check it out]). The converter makes the camera meter think there is
an f2 (or so) lens out there (which becomes f2.8) when there is
actually an f1.4, f1.2, or maybe f1.8). Since metering is done with
the lens wide open, and the converter will not pass all of the
light of the fast lens wide open, there is a problem. Applying a fixed
compensation to the meter restores all to metering bliss (but, remember
to remove the compensation when the converter is removed!). Stop-down
metering would give correct exposures for all apertures, though it
would show little or no difference between about f2 and wider apertures.
Hope this helps.