Before the rush

Before the rush
by evan-pak

Submit your Photo
Hall of Fame

Please participate in Meta
and help us grow.

Photography Stack Exchange is a question and answer site for professional, enthusiast and amateur photographers. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

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?

The actual file size is fairly small at 1 to 3MB per file.

Any idea why this is?

share|improve this question
up vote 26 down vote accepted

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.

share|improve this answer

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.

share|improve this answer

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.

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.