SMC-A on K10D exposure

I posted a while ago about A vs M lenses for K10D use. I found an SMC-A 28 2.8. Nice little lens, I like the

field of view. I does however overexpose by @1-1.5 stops when used on A or M w/green button. If I set a

little -ex comp. it works fine. Also weird is it doesn't seem to expose this way indoors, just outdoors. I

thought that by going with the a version I wouldn't have the exposure errors I've read about with some

older lenses. I have tried changing the custome setting for "use apeture ring" and the same thing occurs. I

did notice one thing. Operating the diaphram with my finger the diaphram reacts very very slightly slower

than a new 43mm. The difference is so slight I may be imagining it. Any ideas on the exposre or is it just

the way I will have to use it. Thanks

It's very likely that your aperture is a little stiff, as you have seen when operating with your finger. The diaphragm can't stop down quickly enough to give the correct exposure in bright light. When indoors, it doesn't have to stop down as much if at all.

 

You can test for this: Take a bunch of pictures indoors and out with the lens wide open (f/2.8). Are they all correctly exposed? Diaphragm!

 

I had this problem with a 35 2.8 and had to clean the aperture blades -- they were a little oily.

I read your comments as saying you are using the aperture ring on the lens to control the

f/stop setting. This is the wrong way to use these lenses. The K10D is designed to control

lenses with an "A" setting via the body and is perfectly accurate when doing so, unless the

iris mechanism is sticky.

 

You should use Pentax-A lenses set to the A position on the aperture ring and forget

about the green button stop-down metering. For Av mode, set the aperture with the

thumbwheel. The green button does nothing in Av mode. For Manual mode, set the

aperture and shutter speed with the thumb and forefinger wheels. The green button in M

mode will set the exposure in accordance with the program line selected.

 

I have two A-series lenses left (sold all my others) and both work perfectly with the K10D's

metering when used as designed.

 

Godfrey

Godfrey, thanks for the answers. All my first tests/trys were with the camera on A, lens on A

and "use apeture ring" set to OFF. In manaul I got the same too bright exposures. Green

button worked as it should but the final exposre was the same as teh A shots.

 

So I was using it as it should be used. The diaphram is fine. Same shot/exposure taken with

a 43 vs the 28A lens is very different. Also weird, if I take shot (or was it a preview shot) and

then go to info instead of it saying 125 @f8 as an example it will say very very low shuter

speeds, like maybe 1/6th? So that is weird but also no way is the image 6 stops off. More

tests today.

I suspect then that your A28 needs service. MIght have a sticky aperture, might have some

problems in the electrical connections from aperture ring to camera body. It should work

identically to the FA43 in terms of exposure. All of my A-series lenses did, and the two I have

left do as well.

 

Godfrey

Either yor lens or body is faulty. I have a A 28 F2.8 that works perfectly with my ist D and ist Ds.