Hey, Woodstock was 40 years ago this weekend - hence the play on words in the title of this blog post... which is actually about long exposures.
Uh, if you don't get the play on words, do a search on CSN. If you don't know who CNS is, do a search on Crosy, Stills and Nash. :-)
This info comes from my buddy at Canon USA, Rudy Winston. Take it away Rudy!
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How long it takes to write an LONG EXPOSURE image to the memory card in any EOS camera, regardless of specific model, depends upon whether the user has activated the Long Exposure Noise Reduction. There are three possible settings on most models: Off, Auto, and On.
If the user turns Long Exposure NR Off, he or she should find writing to the memory card pretty quick -- little effective difference vs. taking a conventional shot at normal shutter speeds in daylight. In other words, the red Card Busy light on the back of the camera will appear lit for only a second or two, if that.
However, if Long Exposure Noise Reduction is active, things change. Long Exposure NR is achieved via the "dark frame subtraction method". This in essence means that the camera shoots the actual long exposure picture, closes the shutter, AND THEN ACTIVATES THE SENSOR FOR THE SAME LENGTH OF TIME AS THE ACTUAL EXPOSURE. It then compares the level of sensor noise generated by this second, "blank" exposure to what's in the actual image file. Armed with this info, it then processes the actual image and removes the fixed-pattern noise where it's seen in both files. The blank file, of course, is not saved to the memory card.
What this means, though, is that the camera is literally tied-up for double the length of time of the actual exposure. A Bulb exposure of, say, 60 seconds means that the camera will then blink the red Card Busy light for an additional 60 seconds, as it reactivates the sensor to generate this blank image. You cannot shoot a picture during this period -- so for someone shooting, for example, fireworks, where they might want to take a series of Bulb exposures in quick succession, it might be preferable to put up with a little noise and turn the NR feature off. This is something the user who posted this inquiry will have to decide for him- or herself, usually on a case-by-case basis.
Long Exposure Noise Reduction ONLY impacts images taken at shutter speeds of one full second and longer (2 seconds, 4 seconds, etc), regardless of whether they're set by the user, automatically set by the camera, or the BULB setting is used. You can certainly leave the feature active at all times; it's simply ignored at shorter shutter speeds (like 1/125th second, etc).