Boyd Hobbs , Jun 28, 2009; 11:40 p.m.
G Dan Mitchell , Jun 28, 2009; 11:49 p.m.
Actually the results are absolutely normal. You need to turn on the long exposure noise reduction feature. Without it you will get exactly the result you describe - with it enabled the 5DII can do very wonderful night photography.
Boyd Hobbs , Jun 29, 2009; 01:17 a.m.
A while ago, I shot night photography with my 20D. The noise reduction feature was on and caused the camera to process a 30 minute exposure for another 30 minutes before it would even preview. I turned it off just to save time, but even then I never saw one hot pixel.
G Dan Mitchell , Jun 29, 2009; 01:23 a.m.
Boyd, that is just plain astonishing. I've shot a lot of night photography - and once or twice when I neglected to enable the long exposure noise reduction feature the results were (from my perspective) unusable.
You are correct that this feature creates a second "blank" exposure after the actual exposure. This is so that it can capture an image in the blank frame that contains only the hot pixel data, which it then subtracts from the first exposure containing your image.
Do try this. I am confident that the noise/hot pixel problem will disappear.
Dan
Alan Bryant , Jun 29, 2009; 01:45 a.m.
I have to say Boyd's results look excessive to me. I just shot a 120 second exposure on my 5D, at ISO 100, and I got this -
That's without noise reduction. Did I just get lucky and get a particularly clean sensor? I've only occasionally noticed bad pixels, even at high ISO.
G Dan Mitchell , Jun 29, 2009; 01:54 a.m.
I should have mentioned another common problem with long exposures. Many people who haven't done some night photography try for an exposure that looks as dark as the actual subject. This is almost always a big mistake - and one symptom will be excessive noise. Instead, whatever the image looks like in the LCD, shoot for a well-balanced histogram and then make any needed corrections in post.
I have a quick guide to night photography stuff posted here: http://www.gdanmitchell.com/2009/02/11/hints-for-night-photography
Dan
Peter Wang 
, Jun 29, 2009; 01:59 a.m.
I suggest that we all start posting pictures of a 2 min. exposure with and without dark frame subtraction enabled, on cameras that support it. Post 600x600 100% center crops--or better yet, 200% crops because then it'll be easier to see the hot pixels. This way, we can avoid debates over pixel density and sensor area. A hot pixel is a hot pixel. Posting the same image size and magnification will allow us to better estimate hot pixel frequency.
Include the following information: (1) Camera model (2) Dark frame subtraction enabled/disabled (3) exposure time, if not 2 min., and (3) whether you posted 100% or 200% crop.
I'll post mine when I get the chance, which should be sometime Monday evening.
Kasper Hettinga , Jun 29, 2009; 04:16 a.m.
Don't know if it matters here...but I noticed lightroom removing hot pixels from nights shots I took with my 20D without long-exposure noise reduction. The picture in Lightroom (and the jpeg-export) looked quite clean. But when I opened the original JPG recorded in-camera (recording RAW+Jpeg using RAW in lightroom) I saw many more hot pixels...
Alec Myers , Jun 29, 2009; 05:36 a.m.
It's not difficult. Shoot 10 images. Including some with the body cap on, at different shutter speeds. If they same pixels are red/blue/not-black in each image then they might be hot. If they move from frame to frame, they're noise.
Alan Bryant , Jun 29, 2009; 10:05 a.m.
