Cheapskate's Technical Photos Page

Technical Photos

Digital Noise

Digital cameras produce noise in every photo they take. Amp noise, bias noise and hot pixels are always present and increase dramatically with time exposures and high ISO speeds. Two of these, amp noise and hot pixels, have the same pattern in each exposure. Bias noise is random and changes with each exposure. All of these can be reduced or removed from your images using Photoshop through the use of dark frames and bias frames. The principle is very simple: Take a picture of the noise itself and subtract it from the image. This process is called dark and bias frame subtraction.

For a given ISO and exposure time, amp glow and hot pixels will be virtually identical in successive frames. Reduce them by using a lower ISO and/or shorter exposure.

Temperature Affects Digital Noise

The temperature of the camera's imaging chip has a dramatic effect on amp glow and hot pixels. A cooler chip results in less noise. There are various ways to reduce heat in the camera. Allow a cool-down time between exposures or turn the camera off for a few minutes between shots. Use an ac-adapter instead of the camera's internal battery (which heats up during the exposure). Shooting in cold weather keeps the chip cooler. I've seen photos of cold packs strapped to cameras, but I can't recommend this. It could cause condensation inside the camera body. CCD cameras designed for astro work often have cooling units built into them. I haven't verified this, but I've heard digital noise is cut in half for every 6 degrees you can cool your camera.

Amp Noise and Hot Pixels in Time Exposures

The following exposures using a Canon 10D are 10 minutes in length with a 10 minute cool-down time between them. Different camera models will have varying amounts of in-camera noise. The enlarged area (left) shows a sampling of hot pixels from the center of the frame. Full frames (right) show amp glow along the right edge and an overall lightening of the background. For the same exposure time, amp glow increases with higher ISO. The 10D is known for producing lots of noise. Newer models, such as the XTi, produce far less.

Note: The Canon 10D does not have an actual ISO of 3200. Expanded ISO is not a doubling of sensitivity, but a mathematical function performed by the camera after the exposure, which is actually made at ISO 1600.

Bias Noise
If you take a photo with the lens cap on, using the fastest shutter speed and lowest ISO, the resulting "black" image will not be entirely black. Some of the pixels will be non-zero (not black) due to bias noise. Note the example below: 1/4000th second at ISO 100. The unaltered frame on the left appears completely black. Using Photoshop to enhance this image shows the bias noise (at right).

Bias noise is subtle but it's presence becomes pronounced in astrophotos when subjected to the processing extremes they require. Bias noise is removed by using subtraction, the same technique for removing amp noise and hot pixels.

Broadband Light Pollution Filter Comparison
Unaltered images using no filter (left) and a broadband light pollution filter (right). Despite the shorter exposure times there is more detail. This is due to the higher ISO used with the filter, which keeps the background light pollution from washing out the nebula. Broadband filters are very useful for photographing emission nebulae. These emit relatively few frequencies (colors) of light which the filters are designed to transmit while blocking other frequencies.

The California Nebula without a filter is not even visible because of light pollution caused by streetlights and ionized oxygen in the atmosphere. These two photos are unmodified.

The California Nebula with a filter is visible in center. The reddish tint on the right edge is amp glow. Processing this image will darken the background, lighten the nebula and reduce noise.

The Orion Nebula, no filter. The low ISO produces a smooth background, but lessens the visibility of the nebula. Imperfect lens coatings cause the blue halos around stars.

The Orion Nebula with filter. Note extra detail and lessening of halos around brighter stars. The blotchy red background is caused by high ISO and is removed by processing. Amp glow is visible at bottom.

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Broadband Light Pollution Filter Comparison Unaltered images using no filter (left) and a broadband light pollution filter (right). Despite the shorter exposure times there is more detail. This is due to the higher ISO used with the filter, which keeps the background light pollution from washing out the nebula. Broadband filters are very useful for photographing emission nebulae. These emit relatively few frequencies (colors) of light which the filters are designed to transmit while blocking other frequencies.
The California Nebula without a filter is not even visible because of light pollution caused by streetlights and ionized oxygen in the atmosphere. These two photos are unmodified.	The California Nebula with a filter is visible in center. The reddish tint on the right edge is amp glow. Processing this image will darken the background, lighten the nebula and reduce noise.

The Orion Nebula, no filter. The low ISO produces a smooth background, but lessens the visibility of the nebula. Imperfect lens coatings cause the blue halos around stars.	The Orion Nebula with filter. Note extra detail and lessening of halos around brighter stars. The blotchy red background is caused by high ISO and is removed by processing. Amp glow is visible at bottom.