Originally posted by platinum i use a K10d + Zenitar 16mm f2.8 (fisheye): it is a M42 lens and so i must use an adapter ring (i have an original pentax ring).
i use "M" modality, i choose the diaphragm (for example f4), i push the "green button" and so i know the shutter speed (for example 1/125).
but i find problems for exposure: because i change aperture (before f4 and then f8 or another aperture) but the shutter speed is always the same!
where is my error?
Hmmmm. . . for M42 lenses with a
manual aperture mode (e.g. the auto/manual switch near the base of a Super or SMC Takumar) you can just put the camera into Av mode (as suggested above).
However, some M42 lenses are
auto-aperture only. You set the aperture using a ring, but the aperture stops down only when the pin at the base of the lens is depressed.
I
think the Zenitar M is auto-aperture only. There's no mechanism in a K mount body to depress the pin, so all exposures will be taken with the lens wide open. (You really don't want to do that.) In this case you will need to modify the lens to disable the auto-aperture mechanism.
The "manual mode + green button" technique is useful for K-mount lenses without an "A" aperture setting (such as the Zenitar K); it's not usually what you want for an m42 mount lens.