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Old Nov 11, 2011, 10:36 PM   #1
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Default Stuck pixels problem on a330

Hi...

I'm new here, also newbie in dSLR photography.
On the photo below, you can see many stuck pixels.
I've tried to fix it as this guy says (change time one month forward, etc.), but nothing's changed. I've also tried with the pixel mapping option.
Is there any solution for fixing those pixels?

This photo is taken with the lens cap on, 30" shutter speed, F3.5, ISO 100:
http://s9.postimage.org/r5vy9c7bh/DSC01902.jpg

This is the picture where I saw those pixels for the first time. Taken in low light conditions, 10" shutter speed, F3.5, ISO 100:
http://s12.postimage.org/3qtggm3ej/DSC01889.jpg
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Old Nov 12, 2011, 5:45 AM   #2
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The pixel remapping functions are only designed to remove pixels that would be visible on normal exposures (shutter speeds faster than 1 second), not long exposures.

Pixels that are always hot (too bright) regardless of camera settings are considered to be "stuck", and remap routines can locate them and update a bad pixel map in EEPROM so that they don't show up in typical shots.

But, if the hot pixels you're seeing with your A330 are only occurring on longer exposures, that's normal (and not what the remap routine is designed to correct by updating a bad pixel table in EEPROM, because you wouldn't want them being mapped out at typical shutter speeds).

Removing hot pixels on longer exposures is what the long exposure noise reduction feature is for (so, I'd make sure it's turned on so it can perform a dark frame subtraction on longer exposures).

With most cameras, it only engages on photos taken longer than 1 second (where hot pixels usually show up). Most cameras are going to have a lot of hot pixels on longer exposures. You just don't see a lot of them because they're being mapped out by dark frame subtraction. With some non-dSLR models, it's not uncommon to see hundreds of them on exposures much over a second if long exposure NR is turned off. Most dSLR models do better.

Virtually all sensors will have them in some conditions. The longer the shutter is open, and the less light a photosite gets, the more likely a photosite won't be linear with it's fill factor, and it will record a higher value faster than it should. You just don't see a lot of them with most newer cameras because of built in dark frame subtraction noise reduction removing them on longer exposures (if you have it turned on), since the ones that occur at typical shutter speeds have already been mapped out by the pixel remap functions (which update a bad pixel map in EEPROM).

The way long exposure NR works is by taking two photos. The first photo is of your subject. Then the camera automatically takes a second image using the same settings with the shutter closed. The camera then notes the positions of any hot (bright) pixels in the "dark frame" (second) exposure, and maps them out of the actual exposure by interpolating values from surrounding pixels to replace the hot ones.

It knows where to find them because the dark frame exposure was taken at the same time, using the same settings, with the camera at the same temperature. In most cases the hot pixel locations will be the same in both images using this technique. But, the longer the exposure, the greater the chance it will miss some of them. This type of noise reduction is known as "Dark Frame Subtraction".

But, the pixels will need to meet a certain threshold of brightness before they're mapped out. Also, taking photos with a lens cap on is not a good way to test for what you'd see in real world conditions (because the sensor gets no light that way, increasing the chance that a photosite will not register the correct value).

You'll find Long Exposure Noise Reduction on the second page of your record menus (right before High ISO Noise Reduction). Make sure it's turned on for Long Exposures.

It's entire purpose is to map out hot pixels on Long Exposures (anything over 1 second) using a Dark Frame Subtraction method.
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Old Nov 12, 2011, 5:57 AM   #3
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BTW, since the A330 has a remap function in it's menus, there is no need to mess around with moving the date back and forth to trigger it. That function is designed to map out stuck pixels (pixels that are always hot, regardless of exposure length).

But, it's not designed to get rid of pixels that are only hot on longer exposures (you wouldn't want to update the bad pixel map in EEPROM with pixels that are only hot when exposures are long, since you'd end up mapping out a lot of pixels that you'd want to be used at faster shutter speeds).

Note that the photos you tried to embed don't allow hot linking. But, we'd prefer that images that large are not embedded in posts here anyway (please keep the longest side to 1024 pixels or less). So, I removed the img tags you placed around them and just left the links instead.

I can see from looking at the EXIF information for those that you had Long Exposure Noise Reduction turned Off. As mentioned my first post, you'll want to Turn Long Exposure NR On so that the camera can use Dark Frame Subtraction to map out hot pixels on longer exposures (that's what that feature is designed to do).
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Old Nov 12, 2011, 1:24 PM   #4
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Hi Jim!

I've just found out that the long exposure noise reduction is automatically turned off while the alpha is set to continuous shooting... probably this was my first time using long exposure and continuous shooting together in low light conditions.

So, thanks very much for the advanced explanation! You taught me few important things about my dSLR.
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Old Nov 12, 2011, 2:26 PM   #5
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Aaaah... I didn't think about that (automatically turns off Long Exposure NR that way).

That makes sense (since Dark Frame Subtraction needs to take two photos and you'd probably be looking for faster frame rates with Continuous Drive Mode enabled).
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Old Nov 12, 2011, 5:28 PM   #6
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Very interesting. I always turned long exp nr off on my sony a550 and never noticed any issues but i didn't look for that. I'll do a test and look carefully at the results. Thanks for explaining this so well.
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