Originally posted by skid2964 ...
I just took some photos with the lens cap on, 15 seconds, not near as many as I saw in the first photos I took when the camera first arrived. Could it have something to do with not being used for a long time?
Perhaps. If the istD has the Noise Reduction option like my K1x0Ds have then I'd suspect the NR kicked in at 15 seconds and reduced the noise. Pentax is vague as to when NR goes into operation, I can tell on my camera it IS on at 1/2 second and my be on at faster shutter speeds.
As "alohadave" mentioned temperature is a factor. Long exposures, repeated exposures, and ambient temperature affect noise.
Noise is very subjective, depends on the subject matter, exposure, etc and is hard to quantify.
You could post a simple example with the exposure details and those with a better eye then I could tell if it is normal.