Originally posted by warp New here so I appreciate some education on different Pentax lenses from anyone knowledgeable and patient. If someone can post a link where its already explained, I'll quietly go away and read.
I'll use different 50mm f1.4 lenses as examples on a K20D body to ask my dumb questions.
How does the A setting work? What is the difference between if I set the aperture ring to f2.8 vs. setting the aperture ring to A and then choosing f2.8 using the thumb wheel in Av mode?
Using the aperture ring setting can work in M mode and is sort of the same as using the A position and setting the aperture with the thumbwheel. There are only certain rare instances where it is ever useful. The camera
really wants to be able to control the aperture electronically. When it can, you get an aperture readout in the viewfinder and top LCD, multi-segment metering, aperture information recorded in the data attached to the image, P-TTL flash, all modes working, thumbwheel control of aperture, all half and third stop settings and probably other stuff (maybe better SR?. The aperture ring is useful to override P-TTL flash, with macro accessories and older film bodies.
Quote: K and M lenses. When I pop one on, rotate the aperture ring to f2.8, focus and press the AE-L or the green button, what is the camera showing me in the Viewfinder? What will the camera do if I just release the shutter? What exactly is stopping-down and who's doing it? And why?
You will see "F--" on the displays with these lenses. In all modes except M, the camera will ignore your aperture ring setting and use the maximum aperture the lens has, f1.4 in your example. In M mode, pressing the green button makes the camera move the lens's aperture to the setting on the aperture ring (called stopping down), meter the light and set the shutter speed to an appropriate value. The lens is stopped down again when you press the shutter and take the photo.
Quote: How does the camera meter through these lenses without opening the blades? How does it work in Av mode as some members were discussing it?
The camera has no ability to control the aperture on these lenses. Therefore, you can set the lens to allow the aperture ring to fully control the aperture setting. We take advantage of its normal behavior with a K or M lens - shooting wide open in modes like Av. Sometimes it's useful to use different techniques depending on the lens and your purpose. Screw mount lenses can allow more flexibility than a basic K-mount lens.