why storing a long exposure photo takes almost as long as the exposure time itself?

Question

last night I took some night sky shots, and I've noticed that with long exposure times [15s;30s] the time it took to store the images in the camera memory card (jpeg) took roughly as long?

Whilst the file size is fairly small at 1 to 3MB / file.

Any idea why this is?

iso were between 200 and 800.

EDIT: thank you for pointing out the Long Exposure Noise Reduction functionality, this indeed explains the extra time taken after the true exposure.

from Canon:

Noise reduction will be automatically activated when noise is detected in any exposure over one second. Noise reduction is subtractive, so after the initial exposure, a second exposure of identical length is made with the shutter closed. Noise from the second exposure is subtracted from the first image leaving a pristine, noise-free image. The only downside is the exposure takes twice as long.

Answer 1

I believe this has to do with Long Exposure Noise Reduction. To cancel out noise the camera will close the shutter and take an equally long exposure again, this time capturing a black image with only the electrical noise on it. This information is then used to reduce the noise on the original exposure.

In the camera settings you can disable the Noise Reduction for long exposures. Check the manual or google to find out how to enable and disable it.

Answer 2

Look up Long Exposure Noise Reduction in your camera's manual. Depending on the model, you may or may not be able to continue taking photos. Some models do the processing immediately after the exposure. Some allow you to continue to take exposures, and then do the processing.

This feature uses dark frame subtraction. The idea being that if you expose a dark frame for the same length of time, then you can subtract away all the static noise.

It also makes sense as to why the camera spends as long processing after the shot as the initial exposure. If I made a 30 second exposure, it spends another 30 seconds so it can find the noise in a 30 second exposure.

I have a Canon 1D IV, and I ran into this last fall when I started to take photos of star trails.

I tried playing around with this. The buffer count goes down with each image I take before allowing the processing part to complete.

If I take a sequence of several shots with the same exposure time, at the end, the processing time is equal to that exposure time. So if I'm taking two 30 second shots, I have about 30 seconds processing at the end.

If I vary the exposure time, the processing time is equal to the sum of the times. So if I take a 15 second shot followed by a 30 second shot, I have about 45 seconds processing at the end.

My conjecture to this is that if I take a series with the same exposure time, the camera is really only doing one dark frame, and then re-using that information. If I take a series with different exposure times, the camera does indeed take multiple dark frames. This makes a lot more sense to me.

Answer 3

The extra time that the camera takes to store long exposure shots is caused by the extra processing that the camera has to do to reduce noise. It may help to turn off long exposure noise reduction if you are using that.