Your Vivitar does not have contacts and requires that you be in M mode for correct stop-down metering. I do not have a K-30 and cannot comment about the conductive vs. non-conductive base and metering on that camera. I know that it makes no difference on my K10D as long as the lens is detected as lacking the A-contacts (F-- in the display).
Sooo...first part of lecture over, I would ask a few questions...
- When you press the green button, do you hear the stop-down mechanism?
- When you press the green button, do you see the viewfinder dim as the lens stops down for the meter measurement?
- After you press the green button, does the shutter speed in the viewfinder display indicate a reasonable exposure for the aperture setting on the lens? Compare with a known-good lens.
- Do you have your eye to the eyepiece when you press the green button?
If you don't hear the mechanism, there is something wrong with your camera or configuration or the camera "thinks" the lens is A-contact with ring in the "A" position. If you don't hear it with your Vivitar, try again with your Takumar (Bayonet) as a double check.
Likewise, if the aperture ring on your Vivitar is set for, say f/11, the viewfinder should get noticeably dim when the green button is pushed. If you hear the mechanism, but the viewfinder does not dim, there is a problem with the coupling. I would not expect this to result in underexposure, however.
That last point is a bit of a stretch, but with stop-down metering and smaller apertures, extraneous light through the eyepiece can be significant. On my camera, I have noticed up to two full stops underexposure when shooting on tripod in bright light. The solution is to shade the eyepiece with your fingertip when taking the meter reading.
One last consideration that is also a bit of a stretch is that it is rather easy to exceed the limits of the metering system in stop-down mode. It is hard to explain, but with the lens stopped down in dim light the amount of light actually reaching the sensor may be less than the low end of the range (0 EV(100) for the K-30)*. This is true regardless of your ISO setting. It is also likely that the meter is not fully linear as the light approaches the lower limits of its sensitivity. At least that is how it is on my camera. Do your testing using a well-lit subject.
Steve
*Here is a real world case:
- Room light is somewhat dim
- Desired taking aperture is f/8
- A sensitive hand-held meter (say a Gossen Luna Pro) indicates 1/32s at f/8 for ISO 1600
- When the lens is stopped down to f/8 (green button pushed), the light reaching the meter cells is only 0 EV(100) and the camera's meter is at the ragged edge of its linear sensitivity
This was common knowledge 40+ years ago when stopped down metering was the norm for many TTL metering SLRs. There was usually a table in the owner's manual that showed the useable range of aperture and shutter speed combinations at ASA(ISO) 100.
Originally posted by Ole 
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