Sat 23-Jul-11 01:24 PM | edited Sat 23-Jul-11 01:46 PM by jj99999
I am a new D7000 owner and was playing with some low light photo shooting, and I noticed in one of my RAW photos shot on manual where i severely underexposed it, that when I lightened the exposure some pixels (maybe eight or nine or more, some much brighter than others) jumped out in the shot. Is this a defect or is this normal? I am not sure how to post the RAW photo so someone could take a look at it. Thank you! I added an example to my identical post http://forums.dpreview.com/forums/read.asp?forum=1034&message=38962353 since I cant figure out how to post here yet.
I would say this is normal. In a severely underexposed image, the signal (the desirable image) is as strong as the noise (the undesirable). If you brighten the whole image in post processing, the ugliness of the noise (some prominent pixels included) will become highly visible. Peter
With long exposures like the 30 second exposure in your sample, you need to use Long Exposure Noise Reduction. This feature essentially takes a second image and compares the two to eliminate the noise. With earlier cameras you need Long Exp NR for anything 8 seconds or longer. With the D7000 I believe you need it at 2 seconds and longer.
Note that the "second image" means your actual time to complete a single exposure will be much longer than just the 30 seconds of your timed exposure. I would expect to have the elapsed time something over a minute per exposure. I find that with the long time period per image you spend a lot more time on composition and making sure each exposure is correct.
The other thing about your post is that you have done some post processing. In camera sharpening or sharpening in your post processing can highlight noise. With low light and long exposures, you want to get the exposure right and not rely on post processing to recover an overly dark image. While 30 second exposures are within the realm of "normal" photography, severely underexposed images are an effect that is expected to have some noise. If you like the effect, just be prepared to clone out the bright pixels rather than expecting the camera to perform outside differently.
Also be sure your firmware is up to date. There were some early reports of hot spots in low light video that were corrected with a firmware release. Those issues are not related to your post.
Sun 24-Jul-11 11:31 AM | edited Sun 24-Jul-11 01:23 PM by ChrisPlatt
It is normal. Those bright pixels are called "hot pixels" and as metioned above, they are one type of noise (thermal noise) that can be introduced into the image during capture.
Each pixel site or well on a sensor is a sort of tiny capacitor that collects electrons that have been excited from a lower energy state by photons hitting the sensor. Unfortunately, heat can also excite electrons and cause them to move and be captured in those wells. That type of electron movement is called dark current. The amount of dark current generated is proportional to both exposure time and temperature. Due to minute variations in physical structure and impurity levels, some pixel sites are prone to much higher levels of dark current than others and collect more thermally generated electrons than their neighbors. Their "well" fills up faster and you get a bright pixel.
As exposures get longer and the sensor gets hotter, those variations in pixel sensitivity to dark current will be more obvious.
Long exposure noise reduction is the cure. Or you can also do dark frame subtraction in Photoshop during post processing. That may be faster than using long exposure noise reduction if you are taking multiple images.