What do the *istD really do with K and M lenses?

Started Sep 28, 2003 | Discussions thread
erwinv Regular Member • Posts: 329
Re: What do the *istD really do with K and M lenses?

After reading several explanations I think it comes down to this:

Non A lenses can only be used after setting the proper pentax function
When this is done:

In Av: Camera meters, but does not attempt to stop down lenses.

This results in corretly exposed pictures, and is good for things like mirror lenses or T-mount attached scopes which do not have a diaphragm.

It is also good for lenses with diaphragms placed on adapters, like M42 takumar lenses, 645 and 67 lenses. These lenses will operate on working aperture f.i. if you set f16 then you always have f16, also when focussing

However in case of K and M lenses the diaphragm will always be pushed open when mounting the lens, so the only way to use these in Av is wide open. (Disregarding tricks like not mounting the lens completely)

In M: Camera does not meter with non A lenses, but it does stop down the diaphragm when taking a picture.
Use a handheld meter to get correct exposure.

(Or the sunny f16 rule, or the histogram, or another A lens, or wide open Av mode for metering with calculated compensations back in manual, etc..)


Roland Karlsson wrote:

I have now read several explanations what the *istD
does with K-lenses. Maybe I am dense, but I don't think
they match. Even people that have the camera have described
the functionalit differently IMHO. So ... what is really the
functionality? And please ... no guesses or speculations ...
there are enough of those already ... the facts please.

1. The camera do not stop down the lens, i.e. it
always take pictures with max aperture. Is this
the case?

2. The camera do stop down the lens, but the
picture is faulty exposed at all but max aperture.
Is this the case?

3. The camera meassure the exposure (just before
taking the picture) with the lens stopped down.
Thus you get correct exposure but cannot
predict the exposure time. Is this the case?

4. Like 1, but if you losen the lens somewhat it will
stop down. And then - by a miracle - it also exposures
correctly. Looks like an April fools joke to me. Is
this the case?

5. Something totally different. What?

6. Like any of 1-5, but in manual mode you get no exposure
meassurement at all. Is this the case?


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