Originally posted by boriscleto That page would be handy but for one confusing item.
They appear to use the term "Auto-aperture" incorrectly (first column on the left), because they show the M and K series as lacking auto-aperture. Those two series do indeed have auto-aperture. Auto-aperture means that if you set the aperture to f 5.6, for example, the iris stays wide open while the light meter measures the light intensity coming through the lens, but gives you the meter reading as if the iris were closed down to f 5.6. The iris closes down to f 5.6 only when you trip the shutter. (
http://www.photographytips.com/page.cfm/1589) Early thread-mount Pentax lenses were not auto-aperture, but later SMC Takumar thread mount lenses had a switch on the barrel to choose auto- or manual aperture.
However, the M and K series do not have the electrical contacts on the lens mount to allow the use of "shutter-priority" auto-exposure mode on cameras equipped with that mode. In this mode of auto-exposure, the user sets a desired shutter speed and the auto-exposure system on the camera measures the light, then tells the lens which f stop to close the iris down to when the shutter is released. The creators of that web page should have titled the left hand column "Shutter-Priority Capable" or something similar.
Having said that, to use an M or K series lens on a K-3, you have to set the exposure mode to Manual, manually set the aperture on the lens, manually set the ISO, then meter by using the Green Button. Pressing that button stops down the lens before metering the light, then sets the shutter speed for the correct exposure. After this process you trip the shutter. This mimics the process called "stop-down metering" that you would have to use when a manual-aperture lens is mounted on a film camera. You focus with the iris wide open, then close the iris to the desired f-stop, then meter the light to see whether your combination of f-stop and shutter speed needs more or less light.
Last edited by pete-tarmigan; 03-05-2014 at 01:31 PM.
Reason: added link to definition