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Old Feb 28, 2006, 10:06 AM   #1
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I recently got myself a 20D and for most part it has been an awesome performer except for one thing. Hot Pixels! I tried a long exposure shot at 5 secs iso 100 I saw several red dots ( one seemed larger than the other) and several white spots. I then went on to test it for each iso at 5, 10 20 and 30 secs. I stopped at iso 1600, At tiso 1600 there were around 15- 16 of them. I know it doesn't show up at faster speed but knowing that its there is driving me crazy. Is this common? Can Canon totally remap it out?

I tried several brandnew ones in the store and most of them performed even worse than mine. Is this a bad batch or what? I tried a D70s just for the heck of it and was amazed at even at iso 1600, 30 secs, there were no hot pixels!!! Grandted I am not about to trade my 20D for a D70s but it just blows my mind that the Nikon's Sony made ccd can outperform the Canon'c CMOS when it comes to hot pixel. Any advice will be most gratefully welcomed. Thanks in advance.
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Old Feb 28, 2006, 10:38 AM   #2
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Actually, the EOS-20D has a bit of an advantage on longer exposures compared to the models using Sony CCD sensors like the Nikon D70s.

Most models use a "Dark Frame Subtraction" noise reduction system on exposures over about 1 second.

What happens is that when you take a photos, another image is taken at the same settings immediatley thereafter with the shutter closed. That's why it takes twice as long with many models (two photos are taken).

Then, the camera's image processing locates the hot pixels in the dark frame exposure and removes them from the actual image you took (usually by replacing the hot pixels with values taken from adjacent pixels).

It knows where to find them in the actual exposure, since the dark frame exposure was taken at the same time, using the same camera settings, with the camera at the same temperature. So, any pixels that register as bright with the shutter closed, shouldn't be there, and are likely to be in the same place on your actual image.

Most models already have dark frame subtraction noise reduction enabled by default with longer exposures.

It's my understanding that you need to turn it on via Custom Function 2 with the EOS-20D.

Hot pixels are perfectly normal with longer exposures. The longer the exposure, the higher the ISO speed, and the warmer the camera temperature, the more of them you'll have.

That's why digital camera systems used for astronomy have cooling systems for their sensors (to help reduce their occurance).

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Old Feb 28, 2006, 12:09 PM   #3
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JimC,

Thanks very much for the fast response. I am just curious as to why the D70s doesn;t seem to be suffering from the same problem. I have 2 friends who are nightscape/long exposure freaks and both of them use D70s and have no such hot pixel problems. And as I've said I even tried one in the store and found their claims to be true as I tried a brand new D70s with noise reduction off and no hot pixels
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Old Feb 28, 2006, 12:22 PM   #4
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danielg wrote:
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I am just curious as to why the D70s doesn;t seem to be suffering from the same problem. I have 2 friends who are nightscape/long exposure freaks and both of them use D70s and have no such hot pixel problems.
JimC wrote:
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Most models already have dark frame subtraction noise reduction enabled by default with longer exposures.

It's my understanding that you need to turn it on via Custom Function 2 with the EOS-20D.
Added:


Check custom function 2 and see if it's enabled or not by default (or if it's been changed). If it's on, it should already be using dark frame subtraction.

If it's off, it won't be doing it.

But, dark frame subtraction can't always be perfect, and the longer the exposure and higher the iso speed, the less likely it will find all of the same hot pixel locations in two consective exposures based on the locations of the hot pixels in the dark frame exposure taken to find them.

The more you use it testing, the warrmer the sensor will get too, which can increase your number of hot pixels on longer exposures, too (as it can increase nose levels aside from pixels bright enough to be considered "hot enough" to be out of tolerance).

All cameras have them on longer exposures. Users just don't see them because of dark frame noise reduction processing subtracting them out.

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Old Feb 28, 2006, 10:52 PM   #5
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But how come the D70s fared alot better even with NR turned off? When Canon says they'll remap it what exactly does it mean? will a remaped sensor affect regular shots (non long exposure)?
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Old Mar 1, 2006, 7:24 AM   #6
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danielg wrote:
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But how come the D70s fared alot better even with NR turned off? When Canon says they'll remap it what exactly does it mean? will a remaped sensor affect regular shots (non long exposure)?
Most tests I've seen show the EOS-20D to be a very good performer on long exposures.

It's not just about hot pixels either, as random noise impacts image quality, even if the pixels are not bright enough to be considered hot.

As far as remapping pixels, most digital cameras contain a list of bad pixel locations in EEPROM. The cameras replace these pixel locations with values from adjacent pixels (so that you never know they're bad).

You can even find some software around to do this yourself for some camera models (remap bad pixels). I know of a package that works on some of the older Nikon and Olympus Digital cameras that shared some of the same supporting chipset and firmware. I also know of one that works with the Nikon D100.

Some Olympus models have a menu choice to remap them.

Some Konica Minolta models automatically remap them at the first of every month, in case any more bad pixels have shown up.

IOW, sensors are not perfect, and the manufacturers typically find and remap any pixels that are going to be hot enough to show up in typical exposures (not long expsoures), before the cameras leave the factory.

Most manufacturers would not remap pixels that are only hot at longer exposures. That's why they have dark frame noise reduction built in for that purpose. You don't want to map out all of the pixels that show up hot on longer exposures, expecially at higher ISO speeds (and it's not advisable to use higher ISO speeds for long exposures).. You use the built in noise reduction to find them and map them out at the time you take the photo if you need longer exposures.


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Old Mar 1, 2006, 9:46 AM   #7
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Guys

Doesn't this have anything to do with CMOS vs CCD?

CCD pixels are read out as absolute before the A/D conversion and it's the same conversion for all the elements. CMOS on the contrary process the electrons locally at the pixel level, and ouput digitized values so when some elements saturated on long exposure that's what you get...
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Old Mar 1, 2006, 10:05 AM   #8
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Why would it matter?

I suspect that the bad pixel mapping is probably happening after the A/D conversion anyway (look up bad phototsite locations in a table, and replace their values with values based on adjacent photosites).

With tests on KM models I've seen, it's even been done in raw files (so it's not even part of the demosaic algorithms). IOW, raw is not necesarily raw. But, then again, their dark frame subraction has been known to leave "black pits", so it doesn't appear to be as sophisticated as their bad pixel remapping (for pixels that have been mapped out because they're also hot at typical shutter speeds).

I've seen the same thing said about Canon's dark frame substraction, too (black pit when replacing a bright pixel, instead of a more sophisticated process to interpolate values from adjacent pixels).

Phil Askey mentioned it in his long exposure tests with and without noise reduction enabled (which is a dark frame subtraction type system, comparing the actual exposure with the dark frame exposure).

With some models, bad pixel mapping does appear to be part of the demosaic algorithms (stuck pixels may be in raw, but not jpeg images).

Canon's got a custom function that controls dark frame subtraction noise reduction. So, if he's getting stuck pixels on longer exposures, I'd just turn it on (unless he wants to use a more sophisticated method during PP for replacing them).


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Old Mar 1, 2006, 11:35 AM   #9
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JimC

I tried turning on the LENR it did help some but therre were still some left. At iso1600 my supposedly black shot looked like the milky way. Canon service treied to remap it but to no avail, the tech guy said I had 3 dead pixels to boot. I just returned the camera for a credit memo and will see what come out of that- I might even swing Nikon's way after being stunned by the hot pixel free image I got from a D70s iso 1600 @ 2 MINUTES!!! Thanks for your help. Appreciate it.
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Old Mar 1, 2006, 11:43 AM   #10
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danielg wrote:
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I might even swing Nikon's way after being stunned by the hot pixel free image I got from a D70s iso 1600 @ 2 MINUTES!!! Thanks for your help. Appreciate it.
But, how soft was the detail? There are always tradeoffs when manufacturers make design decisions. ;-)


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