Dear Bart,
You are correct, my first name is Rohit.
I am attaching a shot for your reference. It is the 77th shot from the camera at ISO100 and 2.5 sec shutter. (I recently went to a night safari and a 2.5 -3 sec shutter was common in pictures.)
The image is a test picture so that while testing the hotpixels come out separately. You will see bight green and purple pixels. Further the camera has gone to the serice centre 3-4 times for software upgrades and pixel mapping but it has not helped at all.
Okay,
First of all, as exposure times get longer there will be a gradual increase of noise caused by thermal effects. This noise should be minimal with exposures shorter than appox. 1 seconds. For longer exposures the noise will increase, that's why some camera's allow to subtract a dark frame which will eliminate most of the pattern noise, but not the random noise (the procedure will probably increase the random noise).
Hot pixels are pixels that are systematically always brighter than they should be, and there will be more of them and with higher intensity with longer exposure times. There is 1 characteristic that stays the same though and that sets them apart from random noise, they have the exact same pixel coordinates with each (long) exposure. So that would take at least 2 exposures to prove.
There exist also "stuck" pixels which always have more or less the same value, or "dead" pixels which are alway more or less zero.
In your example there are a couple of pixels that are relatively hot, there are 16 with an amplitude of more than 100, and a total 63 are hotter than 50. Whether that's a high number or not, depends on how other cameras with the same sensor do. Also important is how your DRO settings, contrast, and sharpening levels were, as that may influence the behavior of hot pixels.
When you feel that the amount of hot pixels is excessive, you should build a good case for Sony so they know what to look for. I suggest to make several exposure pairs with increasing exposure time (e.g. 1/15s, 1/8s, 1/4s, 1/2s, 1s, 2s 4s). Cover the viewfinder, and the lens, or better yet put the body cap on and shoot without lens. That will exclude light from outside the camera. For each exposure pair, the hot pixel positions should be the same, and approx. at the same level, thus excluding random noise. Thoughout the sequence the intensity will increase and the positions will be the same.
When you see the intensity increasing, and the hot pixels do not show at the shortest exposures, then mapping them out will probably be problematic. Stuck, and very hot pixels, may be candidates for mapping out, and you will know the pixel coordinates from the test sequence.
For images where the hot pixels are still objectionable, you can resort to postprocessing.
There are 2 free utilities you could try:
http://www.mediachance.com/digicam/hotpixels.htm and
http://www.mediachance.com/digicam/blackframe.htm
Again, whether the amount of hot pixels is substandard for the sensor in your camera model is hard to say without comparison to other units. However, with a bit of methodical testing you can at least pinpoint the worst offenders, and to some extent repair them in postprocessing.
Apparently (according to DPReview) your camera offers a noise reduction option for exposures longer than 1 second, it would probably help to reduce some of the systematic hot pixels at longer exposure times.
Hope that helps a bit,
Bart
