Why is dark-frame subtraction on my Pentax camera shorter than the exposure time?

Question

When my Pentax K-r applies long exposure noise reduction (dark-frame subtraction), the processing time required for this operation is less than the exposure time. For example, my camera may perform noise reduction for 10 seconds after a 15 second exposure. Why?

Answer

The manual gives you your answer:

Determines conditions such as shutter speed, sensitivity, and internal temperature, and automatically applies Noise Reduction as necessary. (default setting)

This implies that the algorithm for noise reduction isn't a simple 1:1 match with the exposure time and, coupled with the quality of the K-r/K-5 sensor family, the amount it would normally have to apply would have been less than with the prior cameras.

For myself, I have it turned off for my K-5s. I would rather deal with the noise after the fact and not have the camera lose time repeatedly doing this action.

Check more discussion of this question.