40D slow processing of timed exposure?

[Jabba]

Okay- I'm in need of some guidance here, folks. I've noticed that my 40D takes a LONG time to process a lengthy timed exposure, show a preview on the LCD, and then allow me to take the next shot. It's pretty darned fast when there's a fast shutter speed, but when I use the bulb setting (like my efforts to capture some lightning last night) it takes nearly as long as the exposure time to show the image and return the shutter to a ready state. 15 seconds or so to process the shot? What gives? Why would it take longer to process than a shorter exposure shot using the same card and the same file size? I'm using a fast 4 GB card, shooting in RAW or RAW+S Thanks for your help-

[DsrtVW]

If it is anything like my D300 you probably have on long exposure noise reduction selected in your menu. I sometimes turn mine off when getting setup and then back on when I feel the adjustments are right or just clean it up later in Post processing. It is aqnnoying waiting to see the shot, the longer the shutter is open the longer the wait

[ziggy53]

Okay- I'm in need of some guidance here, folks. I've noticed that my 40D takes a LONG time to process a lengthy timed exposure, show a preview on the LCD, and then allow me to take the next shot. It's pretty darned fast when there's a fast shutter speed, but when I use the bulb setting (like my efforts to capture some lightning last night) it takes nearly as long as the exposure time to show the image and return the shutter to a ready state. 15 seconds or so to process the shot? What gives?...

This is normal behavior if you have the "Long Exposure Noise Reduction" turned on. The camera builds a "dark frame" of just the imager noise and the exposure for the dark frame is the same as the timed image. The dark frame is then inverted to subtract the noise from the image.

If you have a lot of long exposure images to do and you want the advantage of a dark frame you can make your own by doing the same thing the camera is doing automatically, but do it manually on a single frame, and then invert the dark frame and use it as a mask to remove the noise. (Turn off the "Long Exposure Noise Reduction" first, of course.)

As long as the time exposures are all about the same length, they should have similar noise patterns, so the single dark frame will work fairly well against all the time exposure images.

[Robinivich]

ziggy beat me to it! (just!)

I find stuck pixels are such a PITA in post processing that I leave it on, but if you're running tons of exposures during a thunderstorm you'll quickly get to a point where it takes less time to DIY like ziggy suggested. Just use the same settings and take a frame with the lens cap on. There are also programs out there that will do the subtraction for you, but ziggy's idea to use a mask is a good one.

[Jabba]

Thanks guys!~ (now I've gotta bird dog that setting out of the menu...)

[mmmatt]

This is normal behavior if you have the "Long Exposure Noise Reduction" turned on. The camera builds a "dark frame" of just the imager noise and the exposure for the dark frame is the same as the timed image. The dark frame is then inverted to subtract the noise from the image.

If you have a lot of long exposure images to do and you want the advantage of a dark frame you can make your own by doing the same thing the camera is doing automatically, but do it manually on a single frame, and then invert the dark frame and use it as a mask to remove the noise. (Turn off the "Long Exposure Noise Reduction" first, of course.)

As long as the time exposures are all about the same length, they should have similar noise patterns, so the single dark frame will work fairly well against all the time exposure images.

that is interesting! I assumed it was a process similar to what PS does. :thumb

Matt

[Jabba]

Made the change- worked like a charm. Thanks again, folks. (now if only we can get another nice night of lightning!...)