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Archive 2013 · What exactly is a dead pixel?
  
 
tidalwdave
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p.1 #1 · What exactly is a dead pixel?


Hello, I just got a 6D and took it out over the weekend to test it out. I noticed that on shots at night of 30 seconds, there is a red dot on the same place in each photo.

That red dot does not appear on photos taken under 30 seconds. The photos were taken in during the day, there were several very low light instances in some photos. On those photos in the spot where the red dot showed up at night during the longer exposure, there was not a dot, even if that area was in a dark/black shadow.

Do I need to return this camera for another copy, or would a dead pixel show up all the time?

I have a 60D and didn't notice this problem with that camera after shooting with it for a year and a half.

Thanks for any insight.



Jan 20, 2013 at 10:36 PM
Mescalamba
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p.1 #2 · What exactly is a dead pixel?


Thats hot pixel, not dead. You dont have long time NR enabled? If you shoot RAW, for example LightRoom takes care of that automatically.

Dead pixel is one that shows all time. This one just fails on long exposure. Every camera has those, but mostly people dont see them cause automatic removal takes care of it.

I guess that if it bothers you it could be remapped by service. And no, you cant RMA it.


http://www.kenrockwell.com/tech/hot-pixels/index.htm




Jan 20, 2013 at 10:49 PM
Monito
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p.1 #3 · What exactly is a dead pixel?


tidalwdave wrote:
That red dot does not appear on photos taken under 30 seconds. The photos were taken in during the day, there were several very low light instances in some photos. On those photos in the spot where the red dot showed up at night during the longer exposure, there was not a dot, even if that area was in a dark/black shadow.


You have long exposure noise reduction turned on. It kicks in on exposures from 1 second to 30 seconds. The camera makes a blank exposure and subtracts that from the actual exposure. Since the hot stuck dead pixel is on in both exposures it is subtracted out for those ones where the long exposure reduction applies.



Edited on Jan 20, 2013 at 11:32 PM · View previous versions



Jan 20, 2013 at 11:29 PM
trenchmonkey
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p.1 #4 · What exactly is a dead pixel?


When she won't respond to mouth ta mouth.


Jan 20, 2013 at 11:31 PM
Monito
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p.1 #5 · What exactly is a dead pixel?


Dead, hot, stuck. All different ways of saying the same thing. We don't notice cold dead pixels (dark ones) the way we notice hot bright stuck pixels, but they are just as stuck dead.

Any way you slice it, it is a non-responding pixel.



Jan 20, 2013 at 11:32 PM
rongoe
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p.1 #6 · What exactly is a dead pixel?


I wish canon would put hot/dead pixel removal into DPP. How hard can it be?


Jan 21, 2013 at 02:52 PM
Aaron D
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p.1 #7 · What exactly is a dead pixel?


Most certainly a "hot" pixel.

I thought a "dead" pixel was one that failed to give any signal? ie - it would be black all the time.



Jan 21, 2013 at 02:57 PM
Mescalamba
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p.1 #8 · What exactly is a dead pixel?


Aaron D wrote:
Most certainly a "hot" pixel.

I thought a "dead" pixel was one that failed to give any signal? ie - it would be black all the time.


That or sometimes they light with color all the time. Black is really dead (no current), color means always wrong "response/answer/read".



Jan 21, 2013 at 05:19 PM
tidalwdave
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p.1 #9 · What exactly is a dead pixel?


Thanks for the replies. My noise reduction was set to off. Is this still normal? I gather I shouldn't worry?


Jan 22, 2013 at 03:43 AM
lowa2
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p.1 #10 · What exactly is a dead pixel?


trenchmonkey wrote:
When she won't respond to mouth ta mouth.




Jan 22, 2013 at 03:54 AM
 

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WilliamG
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p.1 #11 · What exactly is a dead pixel?


trenchmonkey wrote:
When she won't respond to mouth ta mouth.


That would be a dead pixie.



Jan 22, 2013 at 11:21 AM
trenchmonkey
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p.1 #12 · What exactly is a dead pixel?


artistic license


Jan 22, 2013 at 11:35 AM
Monito
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p.1 #13 · What exactly is a dead pixel?


tidalwdave wrote:
Thanks for the replies. My noise reduction was set to off. Is this still normal? I gather I shouldn't worry?


Your response is ambiguous. There are several noise reductions applicable. We were quite specific about which one applies. You'll have to be equally specific if you want us to answer this question of yours well.

You have read your manual, right? I found the specific page number in yours, but I think you would benefit more from finding it yourself rather than me spoon feeding it to you, though I will gladly give it to you if you make the effort and would like further assistance.



Jan 22, 2013 at 12:49 PM
tidalwdave
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p.1 #14 · What exactly is a dead pixel?


High ISO NR was the medium setting, which I think is default. Long Exposure NR was set to off.


Jan 22, 2013 at 05:50 PM
Monito
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p.1 #15 · What exactly is a dead pixel?


Hmm, when you initially wrote about the red pixel appearing in exposures longer than 30 seconds, that indicated the Long Exposure NR was on. Have you noticed the camera red light on the Quick Dial active for a long time (up to 30 seconds) when you make exposures between 1 and 30 seconds?

If that is not happening and LENR is turned off, then it could be heat. Try strapping a frozen freezer gel pack to the camera in an exposure longer than 30 seconds and see if the red dot is there.



Jan 22, 2013 at 06:31 PM
tidalwdave
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p.1 #16 · What exactly is a dead pixel?


No, the red light goes off almost right after the exposure is done. Not sure if it would be heat. The red pixel was on photos taken first thing with the camera was turned on. It was about 33 degrees (F) outside.

Thanks



Jan 22, 2013 at 06:37 PM
Monito
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p.1 #17 · What exactly is a dead pixel?


Ok, LENR is definitely off.

Very odd that it only shows up in exposures longer than 30 seconds. Try some more tests to be certain of that, like the same scene with 40 sec, 30 second, and 20 second exposures in that order.

The reason for trying a frozen gel pack is because the camera generates heat during long exposures.

You may have to send it back, but most cameras sent back are in perfectly good working condition. Yours might have a genuine flaw that you are lucky you noticed soon, since it concerns you.

Some Raw conversion software can map out hot/dead pixels. You are shooting Raw, right?



Jan 22, 2013 at 06:47 PM
tidalwdave
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p.1 #18 · What exactly is a dead pixel?


Yes, I shot with the Raw +JPG on. It show up on the Raw and JPG files. I bought the camera to get more into night time/long exposure shots. Guess if I can return it and get a camera without the red pixel I should give it a try.

I have other shots during the late day that were 20 seconds or so, no red pixel in those, just the night time 30 seconds shots. Very weird.



Jan 22, 2013 at 06:56 PM
Access
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p.1 #19 · What exactly is a dead pixel?


With some of the newer canons, if you take off the lens and put the body cap on, then put the camera into 'cleaning mode' and let it sit around a minute before stopping it, they will clear out or recalibrate and the hot pixels will go away.

I had one once on my 5d2 and after doing this, it cleared or mapped itself out.



Jan 22, 2013 at 08:54 PM
Monito
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p.1 #20 · What exactly is a dead pixel?


I found this in another forum, with no attribution, so I can't vouch for it, but it seems legit:

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There is really no mystery about sensors having bad pixels- they all do but the cameras map the true dead and stuck pixels out. The confusion comes from newbies that don't understand how hot pixels behave. As explained by others above hot pixels are just pixels that have greater than average dark current. The visibility of dark current scales by exposure time and ISO, and dark current itself increases very rapidly with increased sensor temperature. This means that for long exposures especially at higher ISO settings "hot" pixels will appear to be stuck on: even though they may work OK for short duration low ISO exposures.

It is true that current Canon DSLR cameras create a new bad pixel map whenever the firmware is updated or the manual sensor clean function is selected (I have personally verified this for my 5D2 which has two hot pixels). Newbies may report that this doesn't fix their hot pixels because the function only maps out pixels that are bad enough to appear stuck for exposures of about one second at ISO 100. Pixels that work at this setting are not mapped out because they work fine for typical photographs. If you care most about long exposures then you can increase the number of pixels mapped out by first heating up the sensor by using live view. If the manual sensor clean is selected right after using live view for at least two minutes then the increased sensor temperature will result in more of the marginal pixels appearing as stuck to (and therefore mapped out by) the mapping out routine. Of course this will need to be repeated after every time the sensor is cleaned or the firmware is updated or the marginal pixels will reappear.

If the number of hot pixels is still too much for your liking then you can turn on the long exposure noise reduction function or better yet just use Lightroom or Photoshop to perform the raw conversion. These programs use Adobe's ACR that does a good job of removing hot pixels. This is much preferable to the Nikon method which always applies a crude hot pixel removal routine to the raw data of images with exposures of about one second or longer. This forced Nikon hot pixel suppression is probably why there are less complaints about hot pixels in the Nikon forums but it also removes real small image details like faint stars in the night sky. This is one reason why Canon cameras are often preferred for amateur astronomy.

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Jan 23, 2013 at 12:27 AM
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