Alternate to double exposure for long exposures

I wondered why, when taking long exposures, the camera would take 2x as long to load the image onto the SD card when someone explained that the camera (Olympus OMD EM-5, original version) takes a second exposure with the shutter closed (a “black” image) to reduce the noise created by long exposures.

There also was a suggestion that I could disable that option (save battery life) and do something similar to the “black” image in post.

Using either (or both) darktable and gimp, how would I accomplish this?

Apparently UFraw has a portion of the UI where you can select your darkframe. RawTherapee also has this. I don’t believe darktable does.

The technique you’re after is called “dark frame subtraction”.

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You’d still need to shoot a black frame at the time you did the image exposure, but you could then use it in post for all the images from that scene.

This is bread-and-butter for astro photographers; they’ll shoot multiple dark frames, as the statistical median of all those makes for a better noise subtractor than a single dark frame.

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Oooo, you opened the door!! LoL

So, how do I shoot dark frames? Lens cap on?

Do I shoot these the same length of the long exposure as the “real” image or can I just use the same “10sec” dark exposure for all?

Can I keep a “collection” of dark frames for use at any time or should they be shot during the same evening as the “real” images I take?

How do I average and then, how do I subtract? I suspect that the answer to the second is about one of the blending modes in gimp.

Yes, lens cap on and (if you use a DSLR) also cover the viewfinder.

In RawTherapee: http://rawpedia.rawtherapee.com/Dark-Frame

Same exposure and ISO as the images. If you’re exposures are really long, same ambient air temp, to which the camera should be acclimated.

Astrophotographers tend to take a number of dark frames and use them to create a “master dark frame” using median blending in general. Since it takes a lot of time to collect all those individual dark frames (they often use 20-50 individual frames), if their shooting conditions don’t change significantly (exposure parameters + ambient air temperature) they keep using the same set of dark frames or master dark frame for something like 1 year.