"A" button on SMC 50/2 lens

[rahul_deshpande1]

There's an aperture setting labeled "A" on my SMC 50/2 lens just

after the f22 mark. I noticed I can move the aperture ring beyond

f22 to this "A" setting by pressing a little button next to the "A".

What does this do?

 

Another question about the same lens, there is a small yellow half

dome on the barrel. I'm not sure if its a button of some sort, maybe

a preview button? I can't seem to be able to press it or get it to

move. What is this for?

[straightarm]

1) its an auto aperture setting for cameras that have a program and or shutter speed priority mode.

 

You need to set the lens to this A for these modes to work.

 

Essentialy in this setting the aperture is set by the camera rather than on the lens itself.

 

2) this button is not a DOF preview; rather it's for alignment purposes when mounting the lens. You align this dome with the lens release button on the body. It's to facilitate lens mounting in dark conditions

[lon_williamson]

Just a quick addition: The "A" setting should not be used

on fully manual cameras (K1000, KM, KX, MX, etc). If your

camera supports aperature and/or shutter priority, then

the A setting is an option for you. Such cameras include

Super Program, ME-Super, Program Plus, LX, K2, ME, all ZX

cameras. Some newer low end Pentax bodies won't work _unless_

the lens is set on A. Search the internet for Boz's K mount

site for more details.