On Dec 13, 2005, at 12:42 PM, Adam Maas wrote:
Godfrey DiGiorgi wrote:
I'm also not sure what you mean by "true Aperture Priority".
Since the lens is always stopped down to working aperture with
these lenses, the Av mode works for the normal range of
apertures, but is that more "true" than Av mode working with a
lens at wide-open aperture?
Aperture priority works at all apertures with M42 lenses, it only
works wide-open with K/M lenses. That's what I mean by true
aperture priority (Since it really isn't aperture priority if the
shutter doesn't vary with aperture).
lol ... I'm not certain that this is anything but a semantic bobble.
Aperture priority exposure automation simply means that the aperture
is fixed by the user and the metering system sets a shutter speed to
match it.
The limitation in use you are pointing to has nothing to do with
aperture priority operation, it simply points out that you cannot use
anything other than wide open aperture in Av mode with a K/M series
lens for mechanical/programming reasons. Much the same situation as
using a screw mount lens, with the exception that you have manual
control of the aperture; Pentax did not include a switch on K/M
lenses to defeat the auto-diaphragm mechanism.
The Manual exposure mode with AE-Lock button press is actually a semi-
automatic Aperture priority system. Set aperture, press button,
metering sets shutter speed. Change aperture, press button again,
metering sets shutter speed to match.
Godfrey
