Pixel problems with Canon 40D.

[chrisb]

Hi, anybody encountered problems with 40D taking night shots?  I've had this camera for less than a year now and last weekend I shot night shots and as I was looking at the results I noticed some had red and blue pixels against the dark sky.  Not all shots got the problem but some had more than 3 or four of those pixels totally irrelevant of the scene.  Now I don't suspect it could be caused by a defective CF card, I use a Transcend 8 Gb card, I don't understand why it only happened on some shots and not all.  I've looked on the net and different forums and nobody seemed to have reported a similar problem.  Could it be that my sensor is showing signs of weaknesses or eventual problems?  I hope not, because I'm going in Europe for 3 weeks in mid September and it's now too late to have it checked by Canon.  Do you have an idea Bob?
PS: seems like they appear on long exposure setting and high ISO.

[Bob Atkins]

What you are seeing is perfectly normal. These are called "hot" pixels and all sensors have them. They are caused by sensor defects, but if you have only one or two out of 10 million, that's not a bad number. In fact the sensor may have more than that, but they may be "mapped out" of the sensor readout.

They are pixels which develop charge at an accelerated rate and show up as bright in long time exposures even if no light hits the sensor. My 40D has them and my 5D has them. They are not a sign of the sensor "failing" or aging. My giess is that they were there on the day you bought the camera but you've only just noticed them because you were shooting under unusual conditions (long exposures in no light).

If you were seeing dozens of such hot pixels, it could be worth contacting Canon and seeing if they can "map them out", which is basically a firmware process which fills them in with information from the surrounding "normal" pixels. One or two (or even three or four) aren't really enough to be considered a "defect", though if you ask nicely perhaps Canon will map them out. It's worth asking if they bother you

If you want to try some "witchcraft" I have seen reports that the followng procedure will remove hot pixels. I've also seen reports that it does nothing at all. At the very least it can do no harm. Here it is....

Wiith the lens on the camera and the lens cap on the lens and the room dark take a shot with lens cap on at something like 1/5s @ 1600 ISO (Autofocus OFF). Then  activatethe 'Sensor Clean' function for about 30 seconds.  Then switch the camera off to close the shutter and return the mirror.

It's reported to work on the 5D and there have been some reports it works on other EOS models. You can try it and see if it does anything for you.

[chrisb]

Thanks Bob it did work at 1/15 th sec...  don't see the pixels at that exposure and ISO 1600, but if I do a test at 1 sec and ISO 1600, they reappear at the exact same spot.  So they seem to remap for that speed 1/15 sec. somehow I can't explain, but if I take a shot slower that 1/15 sec, they reappear and I've tested different speeds.  I've go 5 hot pixels.  I'll give it a try for a while, I'm going on vacation and is now too late to take for repair, hopefully it won't get worst, but like you say, they may have been there from day one.

[Bob Atkins]

The longer the exposure the more likely it is that they will show up. They just build up charge faster than a "normal" pixel. If they don't map out, you can ask Canon to do it for you. Whether they will and how much they will charge I don't know, but it's worth contacting them if the hot pixels are distracting you.