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Long exposure

JointComms
Posted 07/09/2005 - 12:36 Link
Did a three-minute exposure and it must have taken just as long to record to a 80x card.

Is this normal?
MattMatic
Posted 07/09/2005 - 14:20 Link
Yes, it's normal... normally
After a long exposure, the camera takes another shot of the same duration with the shutter closed and performs dark-frame-subtraction to reduce the noise. (The second shot is a black image that has just the noise and isn't recorded directly.)

In the *ist-D you can disable the noise reduction for long exposures in the custom menu

Matt
JointComms
Posted 08/09/2005 - 13:18 Link
Thank you Matt. I glad I finally asked this question.

I had tried my hand at shooting moon and stars. It was quick and without any photographic wothiness - like I said an experiment. So, my question is what would happen if I shut-off the noise-reduction (NR)? I would really hate to waste so much time on a much longer exposure.

Second question - would I need to remember to turn the NR back on?

Thanks
MattMatic
Posted 08/09/2005 - 13:29 Link
It depends what you use for processing...

If you shoot JPG, then it's probably best left enabled. You can use products like NeatImage to filter the JPG afterward, but I've not tried that.

If you shoot RAW, then using Capture One will give you excellent noise reduction already. Adobe Camera RAW can filter noise, but it is quite a trial-and-error approach. Personally I find that Capture One does a great job. It also removes "hot-pixels" automatically (bright green, blue, or red dots). Saves hours of Photoshop time!

If you are taking a series of shots at the same exposure time, with either JPG or RAW, you can do the dark-frame-subtraction manually - take one shot with the lens cap on, then take the rest. In Photoshop you can subtract the dark frame from the rest.

Just do a Google search for "dark frame subtraction" or "digital astronomy" and I'm sure you'll find the info you need.

All I can say is - try it!
I would suggest you setup one of the custom menus for "night shots", and leave your regular custom option alone. (Assuming you are using the *ist-D) I leave (1) for regular shots, and always have (3) for when I want complete control and best quality (ie MTF mode, no ISO boosting etc).

Hope that helps,
Matt
JointComms
Posted 08/09/2005 - 14:50 Link
Thanks again Matt - your input and information is priceless.

Yes, I am using the istD.

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