Originally posted by blende8 This would be a work-around.
Either Pentax stores the exposure characteristics of every Pentax lens in a list, or it allows users to create such a characteristic on their own.
For this you have to measure the exposure for every aperture set on the lens.
The main reason why the behaviour is not linear is probably that the focussing screens transmission is not linear, but depends on the lens opening.
IMO it would be better if the users set this them self, as I doubt Pentax would like to spend time on it.
It would probably only take a few weeks before the community has updated the lens database on PF with this info on most lenses.
Originally posted by Scintilla I wouldn't think any comparison to wide open would be needed; but is it possible to do this fast enough not to increase shutter lag? How much time does the camera need in order to meter accurately in this situation?
It depends on how long it takes to stop down the aperture. Old manual lenses can be quite slow to stop down. But this would be a problem in using the coupler too.
Maybe this is a major reason for Pentax not doing anything about handling of manual lenses. If they try to automatize it, it might lead to a lot of problem when the camera try to capture the image before the aperture has stopped down completely.
The metering itself does not take much time. It can probably be done in a few hundreds of a second.
It would need to compare to wide open metering to know how much it has been stopped down, so it can calculate how much it should compensate for inlinearity of the metering.