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Old Jan 6, 2005, 10:46 PM   #1
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I just won a Canon Powershot S30 on eBay, and the seller provides a 30 day no-questions-asked money back guarantee. This is not a refurbished camera. The seller has two S30's, and is willing to send me whichever camera is better of the two. What tests do you recommend to determine which of the two cameras is the better one?

Thoughts that occur to me are as follows: Check the photo counter, see how many shots the camera has taken. Check for stuck/dead pixels on the CCD by taking a shutter priority shot with lens covered and shutter set to the max time of 15 seconds, and also set to 1 second. Check for dead pixels on the LCD by taking a photo of a white piece of paper, and looking at it on the LCD to check for black specks. Look at the lens and see which is in better shape, checking for scuffs or scratches (if any) and checking the reflective color of the lens to see if the coating has been damaged. Maybe take a photo of the exact same item with both cameras and see which one has better focus?

Any other checks you suggest that I have overlooked? Any suggestions on a better or different way to check for the possible problems listed above?

Thanks for your response!!

PS. Greetings everyone, this is my first time on this forum!
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Old Jan 9, 2005, 11:30 AM   #2
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Hot pixels are normal for a camera if exposure times are longer than about 1 second (i.e., your 15 second test scenario).

Many Digital Cameras will have hundreds of hot pixels for 15 second exposures. You just don't see them in most cases, because modern Digital Cameras have a "dark frame noise reduction" system built in. This type of system takes two photos (one of your subject, and another with the shutter closed).

Then, it looks for hot pixels in the dark frame exposure, and maps them out in the actual exposure (by interpolating values from adjacent pixels to replace the pixels thatare hot). It knows where to find them, since two exposures of the same length, taken about about the same time, usually have hot pixels in the same locations.

But, the longer the exposure, the less accurate the results. Although hot pixels usually show up at the same place in both exposures (since the temperature of the CCD is likely to be close, and the exposure times the cameras uses for both frames is the same), on longer exposures, sometimes you have pixels that are hot in the dark frame, but not in the actual frame, and vice-versa. CCD Temperature also plays a big role on how many hot pixels show up -- with warmer temperatures resulting in more hot pixels.

The Canon S30 automatically invokes a Dark Frame Subtraction Noise Reduction system on exposures longer than 1/3 second (taking two photos, then mapping out bad pixels in the final exposure, by using the coordinates of hot pixels found in the dark frame exposure).

Up until the last few years, most digital cameras did not have a built in dark frame subtraction system. So, you had to do it yourself (take a photo of the subject, then takeanother exposure with the lens cap on using the same settings). Then, you used software to map out the hot pixels by comparing the two frames. Some utilities have the ability to compare an actual frame with a dark frame and perform the subtraction process for you. Here is a utility that can detect and map out hot pixels, with or without a dark frame:


Because of the large number of hot pixels a camera will have at longer exposures, combined with the way dark frame subtraction works,manufacturers don'tusually consider a CCD to be defective, unless you have hot pixels on typical exposures (1/30 second or faster). Then, these pixels are known as "stuck" (always hot, regardless of exposure times).

In any event, you can download a free utility to check for both hot and dead pixels here:


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Old Jan 9, 2005, 11:41 AM   #3
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Check that buttons and dials work properly. Some cameras develop contact problems after some time...and faulty buttons/switches/dials.
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