Long exposure = long delay

I was playing with long exposures on the K10D last night, and took some shots of a dark scene where the shutter was open for about 30 seconds.

I noticed that after the shutter closed, it then took about as many seconds to display the shot as the shutter had been open. For instance, after a 30 second exposure, it was 28 seconds before the shot came up and it was ready for another one.

Presumably it's processing the data it's recorded, and the longer the exposure, the longer it takes. Is it aggregating a number of 'time slices'-worth of information?

The write-to-card doesn't seem any longer, and it doesn't seem to make any difference whether it's creating a JPEG or a RAW file.

Can anyone shed any light on this (pun intended )?

The 'delay' is due to the noise reduction system. Turn it off in the menu and you get a much noisier image displayed faster. Suggestion: leave it on.

The camera takes a second exposure, pure black except for sensor noise. It then does some clever electronic wizardry to subtract the noise in the second image from that in the first. The result is as noise free an image as you'll get.

Long exposure = long sensor run time = heat = noise.

Spot on. Many thanks.