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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?

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  • \$\begingroup\$ I had the same problem with my 700D until I turned off the noise reduction. The camera is working as normal again but begs the question, is this delay normal, and was this the way the camera was intentionally designed ?. \$\endgroup\$
    – user70822
    Commented Dec 18, 2017 at 0:20
  • \$\begingroup\$ Long Exposure Noise Reduction aside, i look at this way. A long exposure contains much more data ( assumption not based on factual knowledge ) and it takes longer to write that data to the card. Lets say we meet at a bar ( please ) and you buy three beers, one 🍺 for you and two 🍻 for me. We then set a timer to see who can assimilate the data ( beer ) faster, me with my two ( thanks ) and you with your one. I am good but i suspect you can drink one before i can drink two. This may be just a ploy to get you to buy me TWO beers 😇 So when shall we get together? \$\endgroup\$
    – Alaska Man
    Commented Mar 6, 2018 at 21:37
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    \$\begingroup\$ @Alaskaman Not only is that assumption not based on factual knowledge, it's just plain wrong. The length of exposure does nothing to change the number of sensels on the sensor, which is the definitive factor controlling how much data needs to be written out at the end of a capture, especially if you're capturing RAW. If you're shooting JPEG, then the compression and other factors reduce the data even much further. \$\endgroup\$
    – twalberg
    Commented Sep 20, 2018 at 17:25

3 Answers 3

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This is a featured called 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.

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    \$\begingroup\$ That's absolutely fascinating. I tried to find a good article explaining the detail of how it works (to link here), but failed to find anything detailed. I guess it works similarly to differential signaling (en.wikipedia.org/wiki/Differential_signaling) in electronics. As per @Eric, Wikipedia has an short article on Dark Frame Substraction which shows a method in action (en.wikipedia.org/wiki/Dark-frame_subtraction). \$\endgroup\$
    – wally
    Commented Mar 6, 2018 at 10:15
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    \$\begingroup\$ @wally Long Exposure Noise Reduction is just Canon's term for "dark frame subtraction." Heaven forbid they admit their cameras have ever done anything 'negative.' (See what I did there? Camera. Negative. Hahahah.) \$\endgroup\$
    – Michael C
    Commented Jul 9, 2018 at 16:11
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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.

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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.

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