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Old Dec 2, 2006, 7:48 AM   #1
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I have a Nikon Coolpix 8700... it's been a good camera, but recently I've been noticing an increasing number of dead pixels, or possibly dust.

When I take a pic with the lens cap on, at screen resolution I see a black image with a scattering of white dots (they're strong enough that they show up in any darkish areas of my photos). When I up the image to the actual resolution, it's not true black (as I'm fairly certain it used to be), but composed of a mix of true black and slightly not-black pixels. The "dead pixels" are sometimes, and sometimes not, surrounded by a bit of color as well I'm wondering - is this dust? The front of the lens is clean...

So, dead or dust is the first question. Fixable? is the second, and Worth it? is the third.

I'm using these pixels as an excuse to look at new cameras, but if it's possible to take the thing apart and clean a mirror or CCD or something, I can deal with that (yea, it's way out of warranty).

Any advice is appreciated. I've included a crop of one area of the image, complete with what I think is a dead pixel.
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Old Dec 2, 2006, 8:37 AM   #2
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If the "specs" or "dots" are showing up with the lens cap on as you say, it sure sounds like dead pixels. I'm not sure how the imager would display dust if there was no light coming in. The other test, in my mind,is if the "specs" are showing up consistently in the same location over several days or even a week or so. I don't think dust would sit still that long for a series of pictures.

Couple of options come to mind. Does your Nikon have "pixel mapping" available? My Oly C-730 has this as a built in tool and it works well. Secondly, you might try to see if you can squirt in some compressed air to remove any dust that's there. Otherwise, it's check with a dealer or repair center or as you say, it might be time to look for a new camera. Oh, the decisions one has to make...

Hope this has helped a little.

- Paul
Olympus E- 500 C- 730
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Old Dec 2, 2006, 9:05 AM   #3
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That was a 2 second exposure with the lens cap on. If that's the only time you're seeing it, consider yourself to be very lucky.

Heck, that pixel was so dim that it probably wouldn't even register as bright enough to be considered hot by most software designed to detect hot pixels.

It's not unusual for some cameras have hundreds of hot pixels on a 2 second or longer exposure if noise reduction is turned off.

Speaking of Noise Reduction.. Is it turned on? It's entire purpose is to remove hot pixels on long exposures. It's on the third page of your shooting menu.

BTW, that's a hot pixel, not a dead pixel. Big difference. A dead pixel will always be black (does not respond to light). A hot pixel is one that is brighter than it should be. If it's bright at typical shutter speeds (i.e., 1/30 second or faster), it's considered to be "stuck" (always bright).

Remove your lens cap. You'll get better photos that way. ;-)

Leaving it on will cause slower shutter speeds, and since you don't have any light hitting the sensor, you can get some hot pixels testing one that way.

To be frank, I find it amusing when someone notices them with lens cap on type tests. At one time, these types of tests were helpful, since cameras didn't have any type of system to get rid of them. So, you wanted to try and find a camera that didn't have any at exposures of around one or two seconds. Even then, you were "rolling the dice" on whether you'd get one that good or not.

With some newer non-DSLR cameras with smaller photosites, you can have hundreds of them with exposures of only a few seconds if noise reduction is turned off.

Hot pixels are when photosites record a higher value (brighter) than they should, usually with longer shutter speeds. When the problem occurs at faster shutter speeds, they're considered "stuck" (always bright).

Virtually all CCDs will have hot pixels in some conditions. The longer the shutter is open (and the darker the conditions since not enough photons are hitting the sensor) the more likely a photosite won't be linear in it's output when responding to light, and it will record a higher value than it should.

Temperature also impacts it (which is why you see cooling systems for sensors in digital cameras used in astronomy). So, the longer you test for hot pixels, the more you'll likely have, especially without any light hitting the sensor, since the camera will warm up while you're testing for them.

You just don't see hot pixels with most newer cameras because of built in noise reduction to subtract them out on longer exposures, since the ones that occur at typical shutter speeds have already been mapped out at the factory.

Make sure your noise reduction is turned on. That's what it's there for (to remove hot pixels on longer exposures). It's not designed to remove the typical noise you see. It's entire purpose is to remove hot pixels on longer exposures.

Hot Pixels are normal on longer exposures. Your camera has a built in dark frame noise reduction system to try and locate them and map them out on longer exposures.

It works on photos taken longer than 1/30 second with your camera. The way it 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 pixels in the "dark frame" (second) exposure, and maps them out of the actual exposure.

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. If it's not very hot, it may not be bright enough to meet the threshold built in either (and that example was very dim).

If you have a pixel that is always hot (bright), even on shorter exposures in good light, then it's referred to as a "Stuck" pixel. The Long Exposure NR feature won't help for that.

In most cameras, when a camera develops a stuck pixel, a trip back to the manufacturer is needed. The manufacturer then updates a bad pixel table in EEPROM. When a photo is taken the image processing automatically checks the camera's bad pixel table and maps the pixel out (replaces it with values determined from adjacent pixels using sophisticated interpolation algorithms).

Some cameras allow the user to perform a bad pixel map themselves (a number of Olympus models have a menu choice that calls a procedure to locate and map out bad pixels).

KM DSLR models have an undocumented feature that checks and maps out bad pixels on a monthly basis that you can force by setting the date up one month if you don't want to wait after you see a bad one.

I've got software that can remap hot pixels for a number of Nikon and Olympus non-DSLR models. But, from what I've been able to determine, it won't work with the 8700 (although it will work with the 5700). I've also got software that can update the bad pixel table in the D100. But, I'm not aware of anyone (except for Nikon) that has software to work with the 8700.

But, note that these remap utilities are *not* designed to remove hot pixels that occur on longer exposures (they're normal, and the noise reduction should get most of them if turned on, as it should kick in on exposures of around 1/30 second or longer with your CP 8700).

The remap is only designed to map out pixels that are going to be hot on typical exposures (faster shutter speeds) You don't want it remapping pixels that only show up on long expoures since the noise reduction system's entire purpose is to remove those, and you can end up with a lot of them on a long exposure without noise reduction turned on with many cameras.

Do any show up at faster shutter speeds? If so, there are some utilities designed to find them during post processing. But, as dim as that one was, I'm not sure they'd find it if that's typical of what you're talking about.

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Old Dec 2, 2006, 9:09 AM   #4
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Are there issue with your images from this camera?? The "lens cap" test you describe is fairly controversial, and from what I understand is not a truly reliable way to find dead or hot pixels. If dead or hot pixels aren't showing up in your images, then I wouldn't worry about it. All camera's have a few dead or hot pixels but they are routinely mapped out by the cameras internal software. These pixels also change and shift over time. That being said, a new camera is always a nice thing.:G
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