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UrbanPhotos Dec 15, 2003 1:32 AM

Dark-frame subtraction
My Sony digicams use noise reduction on any exposures longer than 1/30 seconds. The way I understand it, the camera accomplishes this by taking a dark exposure of the same length after the main picture and using the noise in this otherwise blank picture to remove the noise from the actual picture. It works quite well.

I'm wondering, is there any way I can accomplish this same type of noise reduction on shorter exposures? I could put the lens cap on and take a picture with the same shutter speed, hence creating a dark frame. But what would I do then? Is it even possible to accomplish the dark frame subtraction with a photo editor?

voxmagna Dec 15, 2003 5:57 AM

Try Neatimage first, it might save you a lot of trouble. I'm no expert on how the black frame technique works, but since some noise is uncorrelated in time, position and amplitude, a perfect noise cancelling system would need to capture a copy of the black frame at the same instant as the scene picture - or assume that low frequency noise will keep the same structure if two shots are taken, separated in time.

Neatimage allows you to profile to your camera at different ISO's and shutter times, but you're doing this post JPEG. In-camera noise reduction can do this RAW before JPEG encoding. However, Neatimage does work very well, particularly if forced to use 800 ISO and faster speeds.

If you really want low noise, I wouldn't think you'd get more benefit if you were shooting at 100 or 64 ISO and a tripod is a worthwhile investment as it would make sure all those Mpix make sharp pics!

UrbanPhotos Dec 15, 2003 7:28 AM

I've used NeatImage. It works well, but I have a demo version of it that will only save to jpg, and it compresses too much (the pictures go from over 2 megabytes to less than 1). I suppose I'll have to buy a full version of it.

Also, I do use ISO 100 almost exclusively, only increasing to higher levels when I have no alternative but to go handheld and flash isn't an option.

BillDrew Dec 15, 2003 12:17 PM

Depending on the subject, you might want to look at Max Lyons' Stacker ( Gets rid of noise by taking several pictures and "averaging" them. Meant for cases where the noise is random and your subject doesn't move.

Max's site is well worth the visit in any case.

UrbanPhotos Dec 15, 2003 3:00 PM

Thanks, I'll try that program. That sounds interesting. Of course, it will only work for, as you said, subjects that don't move, and with the camera on a tripod. This would be a good time to use the burst mode that captures three frames in rapid sucession with one push of the shutter. Uses for this will be rare, though, because I don't often use a tripod for static scenery in daylight. I use the tripod either for night shots (where the camera is doing noise reduction anyway) or flash/daylight group pictures I want to include myself in.

I don't have big problems with noise, so all of this is motivated more by curiosity than necessity.

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