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Registered Member #16
Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554
I'm getting to know our new Olympus E-510 a bit better, and I was out tonight figuring out settings for satellite/meteor/iridium photos. I finally got a match of ISO, F stop and shutter time that seemed to have yielded a good image without too much noise, but when I opened it in photoshop I noticed something strange.
There are a number of small, vivid red and blue dots, about the same size as stars throughout the image.
Below you can see a small section of one image, and you can see the Full Size Image (Warning, 1MB file size).
I was shooting with a 30 second shutter speed, Aperture was a 5.0 or a 5.6, and 400ISO with noise filter and cancellation on. Is this just CCD noise?
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
Yes, those are hot pixels that have higher than normal leakage currents that become significant at long exposures. it is common on all cameras but the different brands have different ways of coping with it.
One common method is called long exposure noise reduction that genertally makes the camera take two pictures, the second one with the shutter closed. Then it subtracts the dark picture and removes all fixed pattern noise in the picture.
All Olympus cameras tend to have a function to map out hot pixels. Look for "Pixel Mapping" in setup menu 2. That will make an exposure with the shutter closed and update the table of bad pixels. The camera will replace the values of the bad pixels with the average of the pixels around them after each exposure. That usually fixes most hot pixels, if not the pixel mapping table might be full and the camera should probably be replaced under warranty.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
wrote ...
One common method is called long exposure noise reduction that genertally makes the camera take two pictures, the second one with the shutter closed. Then it subtracts the dark picture and removes all fixed pattern noise in the picture.
Actually, the noise Dave is seeing in his camera is random thermal noise which is called "dark current" typically. This causes the random red, blue, green pixels speckled over the image. The method you describe, Bjorn, is for removing hot-pixels only which are fixed.
The only way to remove the dark current (random) noise in the image is by integrating an image many times, which when taking single event shots like an Iridium flare is not possible. But for fixed astronomical objects, you would typically take many shots, same exposure, etc.., and then average all frames together which increases signal/noise ratio.
Looking at your full size image, the red,green,blue speckles you see are dark current noise (random), while the bright white pixels you see here and there are most likely "hot pixels" or fixed pattern noise which can be subtracted out with a dark frame.
BTW, what was the ambient temperature you took those photos at?
Registered Member #16
Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554
I'd say outside temperature was somewhere around 72-75 degrees while I was taking the photos last night.
I tried a few more exposures after posting this thread. Dropping down to ISO 200 and opening the aperture to maximum (3.2 with this lense) reduced the overall noise by probably 75%, and cut the 'dark current' by closer to 90%. There were still one or two bright blue pixels which were consistent over a couple exposures. This I assume would be a 'hot pixel'.
Thanks for the hint about pixel mapping Bjørn. I had read through the manual and had seen something about it, but its exact purpose didn't really register.
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Strange camera noise
