Originally Posted by Doug
....I have a Pentax M42 lens adaptor for mine, but of course the lenses are used in pre-set diaphragm mode.
ruben said:
I do not understand, could you explain ?
The Pen F has a mechanism that actuates the diaphagm for Pen F lenses, so that you focus with the lens wide open for brighter viewing and more focusing ease, and when the shutter release is pressed, just before the shutter actuates the mechanism stops the lens down to the opening set on the lens aperture ring. This automatic diaphragm control is typical for modern SLR cameras of course, and Pentax does the same.
But the Pen F and Pentax M42 mechanisms are not the same, not compatible. And the adaptor that allows the M42 lens to fit the Pen F body doesn't "translate" that difference. So, no automatic diaphragm control. The lens diaphragm must be operated separately before pressing the shutter button. Fortunately, most M42 lenses have a manual/auto switch on the lens. So you can switch to Auto, for wide-open viewing and focusing, then slide the switch to Manual to stop it down to the taking aperture before pressing the shutter release. In this way, it's called "pre-set" because you had set the aperture previously, then operated a simple control to make it go to that set aperture as needed.
Before full-automatic diaphragms were nearly universal, there was semi-auto, where the lens would stop down automatically with the shutter release, but had to be "re-cocked" to the full open position afterward. And more primitive than that was a type called Pre-Set that had two aperture rings, one of which moved smoothly and without clicks between wide open and the aperture set on the other ring. This made it easy to stop the lens down to that pre-set aperture before releasing the shutter. Yet more primitive even than that (for SLR use) was the "manual" aperture lens with no automatic mechanism and only the single aperture ring, just like any Leica or Voigtlander rangefinder lens.
I'm sure you already know and understand most of the above, just mentioned in the interests of clarity in answer to your question!
The essentially pre-set mode of use is a typical drawback of adapting "foreign" lenses to a camera. It's an extra step that one must remember, and if forgotten is likely to result in overexposure due to shooting wide open when not intending to. I am so error-prone in this that I often will just set the lens to a fairly wide aperture and leave it there, accepting slightly dimmer viewfinder and poorer focusing.