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Old Aug 30, 2004, 11:45 AM   #1
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Recently i bought W1 and i think i have expected too much from it. After some tests, i really wonder : Quantity ofbright grain noise will be different in each camera ? ThenIs it just like wedraw a lucky lot when picking up a camera at the shop ?

I make a test for this camera for checkingnoise level by take 2 shots in a very dark condition.

1 shot :shutter speed8 sec , F 10

anothershot: shutter speed 30 sec, F 2.8

The first shot is fine, there is just4 or 5smallbrightgrains (which are often called dead pixels) that appear on thedark scene. Thesecond shot (30 sec) is quite noisy with many bright dots or grains. I attached the second image here.

the two imagesare uploaded here:



I really need your experienceon this.
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Old Aug 30, 2004, 6:04 PM   #2
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You're not seeing dead pixels, you're seeing hot pixels (which is perfectly normal on longer exposures).

Your photos have the following (using a luminous threshold of 60 to only pickup hot pixels versus noise):

8 second exposure:

2 bad pixels (both hot),0 dead pixels

x positon, y position, luminous value


30 second exposure:

10 bad pixels (all hot), 0 dead pixels

x positon, y position, luminous value


ADead Pixel is a pixel that reads zero or is always off on all exposures. This state produces a black pixel in the final image.A dead pixel will occurs regardless of shutter speed, aperture size or any other user settings. It will occur on normal exposures and tends to be more obvious under bright condition as it tend to stand out.

A Stuck Pixel is a pixel that always reads high or is always on to maximum power on all exposures. This produces a bright pixel usually of red, blue or green in color in the final image. A Stuck pixel will occur regardless of shutter speed, aperture size or any other user settings. It will occur on normal exposures and tends to be more obvious under dim condition as it tend to stand out.

A Hot Pixel is a pixel that reads high (bright) on longer exposures, and can produce white, red (orange) or green (yellow green) pixels in longer exposures. The longer the exposure the more visible the hot pixels. Hot Pixels may also occur when the camera is hot after prolonged usage.

Here's the deal:

Your camera has a noise reduction system that is designed to subtract hot pixels from the final image on longer exposures. It actually takes 2 photos, using the same exposure settings (that's why it takes twice as long to process the images after a certain shutter speed or slower). One image simulates having the lens cap on (shutter remains closed). On longer exposures (anything much over a second), this image will contain lots of hot pixels. The second image is of your actual subject. Then, it compares the images, and uses interpolation techniques to replace the hot pixels in the final image from values found in adjacent pixels. This has the effect of masking out the hot pixels. This is known as "dark frame subtraction".

It knows where to find them in the actual image of your subject, because most of the time, hot pixels will show up in the same locations if the camera settings are the same. But, it can't always be right. That's why it can miss some (some pixels may show up as hot in your subject photo that don't in the black frame photo, and vice-versa).

On a typical 5 Megapixel Sensor, with noise reduction disabled (as you can in some models), it's not uncommon tohave adozen or more hot pixels on a 1 second exposure. With a 10 second exposure, it's not uncommon to have hundreds of hundreds of hot pixels.

That's why the CCD's have cooling systems when used for astronomy (a cooler CCD will have less hot pixels on longer exposures).

Basically, if you do not have any pixels that are hot at typical shutter speeds (about 1/30 second or faster), then you have nothing to worry about. These are known as "stuck pixels" when they remain hot at all exposure settings.

Up until recently, cameras did not even have noise reduction systems to help find and remove hot pixels on longer exposures. So, you had to do thedark frame subtraction using software. You took a photo with the lens cap on. Then, you immediately took the photo of your subject using the exact same settings. You can't wait too long between them, because the pixels that are hot can change rapidly with temperature and other factors).

Then, you used software to compare the images to remove the ones that showed up in the "dark frame" photo from the photo of your subject.It can't always work perfectly (but it's usually close enough to remove most of them on exposures that are not too long).
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Old Aug 31, 2004, 1:00 AM   #3
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Thank alotfor your help. I canfeelsatisfied with my camera now as it is not uncommon to see hot pixels on a newly bought Sony camera.

My camera yields hot pixels only when used at exposure longer than4 sec.

From 4 sec or shorter, it's good without hot pixels. Sony is good anyhow.

Thanks again.
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