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Noise reduction/dark frame subtraction

Charlie Howard (Steve's Digicam, Panasonic forum, 20 January 2004)...

First, to see what I'm (trying to) explain, put your camera in Manual mode and set aperture to f/2.8 and shutter speed to at least 1 second. Then, aim the camera at yourself so that you can peer into the lens and see the small open iris way inside. Zooming back & forth a bit may help you spot the iris.

Next, while watching the open iris, press the shutter button. The iris will remain open for the shutter time you've selected, then close for the same length of time, and then open again. The time during which it was closed is what I'm assuming is the ``dark-frame'' time.

During the dark-frame time, the CCD accumulates a charge in each of its sensors, just as it does while the shutter is open. If the CCD was entirely noise-free, nothing would accumulate and a perfectly black image would be captured. This is the same as what would happen if you left the lens cap on while taking a picture in a darkened closet.

Since no CCD is perfect, some charges accumulate even when there is no light. That's noise.

Since the conditions for taking the dark-frame image were identical in all respects but one to those for taking the actual image (the one difference being the availability of light), the assumption is that both images accumulated the same noise level in each pixel. Both images were taken at about the same time, same temperature, same duration; and the resulting signals were boosted identically to satisfy the ISO selection.

So, by subtracting the dark-frame readings from the exposed frame, we should wind up with an image that has much less noise. Obviously, this method does not work perfectly, because if it did, an ISO 400 picture would be completely noise-free, even better than what NeatImage or NoiseNinja can do.

Each pixel on the FZ10's sensor is about 2.5 micrometers on a side, (1/10,000 inch). It doesn't take much to give them some charge.

If you want to see an example of pure noise on ANY digital camera, take a long-exposure picture with the lens cap on, in a dark room (the lens cap is not perfectly light-tight). Bring it into your computer, then into a photo editor, and slide the bright end of the LEVELS control towards the left. Eventually, you'll begin to see some colored pixels. They tend to cluster. The closer you can get the LEVELS to 0 before you see anything, the less noise there is. If you take two of these black pictures, one at the camera's minimum ISO and the other at its maximum ISO, you will see a huge difference in the images as you slide their LEVELS from 255 towards 0.

{Picture below `Coast Guard helicopter flying past the Golden Gate Bridge'. Taken by Charlie Howard with FZ10}

Coast Guard helicopter flying past the Golden Gate Bridge. Taken by Charlie Howard with FZ10

Adrian Ashfield...

This only comes into operation with exposures longer than 1/2 second. In order to see what the sensor noise is like, without the noise correction, you can set the exposure to 8 seconds in manual mode, and then select the rapid fire `burst' mode. This defeats the dark frame subtraction. Then look at the resulting image in your editing program paying with the histogram & brightness controls to make it more obvious.

{Picture below `Snow' by Adrian Ashfield taken with FZ10}

Snow by Adrian Ashfield. Taken with FZ10

next up previous index Link to 'photography' page
Next: Minimum focus distance Up: Specifications Previous: Shutter lag times   Index
David Fong 2009-09-04