Originally posted by Graham V Thanks both, very useful info.
I'm keeping my eye out for a Vivitar 19mm (budget constraints)
Am I correct in thinking I can select aperture on dslr, use DOF preview and retain aperture when put on film body?
Yes and no!
1. Old m42 lenses can be used in step-down mode, and there are some K lenses that work that way too. That means that there is no pin controlling the apperture blades, so that the controll is fully manual. You would tipically focus on a subject first, close the apperture to the desired value, do the metering and snap the picture.
2. Old K (K and M) lenses have the apperture ring and the lever controlling the blades. Those lenses have another pin which mecanically tells the body about the maximum opening and the setting on the apperture ring. New bodies (read all pentax DSLR) have no such coupling so there is no way the body can say what the position apperture is in. The body does the metereing with wide-open blades and the blades are closed in the instant the picture is taken. However it is possible to do the metering in M (manual) mode thus effectively setting the apperture in desired position.
3. A, F and FA lenses have "A" setting (automatic) setting thus allowing the body to set the apperture. Effectively it is same as having the apperture closed completely and then the rod opening the apperture is set to apropriate position. The camera know electronically the maxium and minimum available apperture.
4. FAJ and DA lenses do not have any setting at all, so they are effectively stuch in "A" setting. In such cases the old bodies can not tell the apperture capabilities of the lens and can not use the apperture at all.
You can find some more information about the K mount here:
Welcome to Bojidar Dimitrov's Pentax K-Mount Page
Now that said, there are still some "workarounds"
You can fix the apperture rod (by adding a piece of plastic or something like that) to wide open position, or actually any other position you like. However if you want to change the setting you still have to remove the lens, adjust the "stuck" position of the rod and put the lens back. Although it may be usefull for studio work it is just way too much job for nothing. Another option is to get a newer film body where you can adjust the apperture on the body.
There are still some filme bodies out there that you can get fairly cheap (MZ-6 / MZ-L MZ-7 or *ist). So basically with these bodies you would have the settings similar to those you have on a DSLR and you will still be shooting film in FF. Moreover you can use all features of your DA lenses as well (appart from possible heavy vigneting at the edges).