I'm not sure what the exposure limit is for the 645D, doesn't seem to appear in the manual. I've done a 2 minute exposure with the lens cap on to check for noise - very little , a few pixels with rgb values in the range of 6, 5, 5.
Just a quick note from my experience testing long exposures...
The lens-cap test is only so useful for a variety of reasons:
- doesn't show the effect of behind-the-scene noise reduction on texture/detail
- doesn't show drifts in color accuracy (especially shadow color) or per-channel
- if you don't cover the view finder light can make it's way in (only important when testing many-minute exposures in a normally lit room) and futz with the outcome
It's much better if you can shoot with and without a strong, high quality, ND filter and compare those two images. That's not a perfect test since you'll be adding optics which can soften detail but it's a better indication of real world performance in my opinion.
I've tested some cameras that do better in the real-world then their lens-cap test would indicate, and I've tested some cameras that do worse. I don't know and am not commenting on which is the case with the Pentax. I just though in a thread about Digital Long Exposures that a note about testing methodology might be relevant.
Doug Peterson
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