Sources of noise:

● External: “shot” noise.● Random fluctuations in photons striking the

sensor● Sometimes looks OK.

● Internal: sensor noise● Increases linearly with exposure time● Increases exponentially with temperature ● Usually looks terrible (e.g. banding)● Every sensor also has “hot” and/or “dead” pixels

due to manufacturing defects and aging.

Noise:

Noise is always present, but at high ISO signal:noise worsens, making noise more noticeable.

Foveon sensor, ISO 3200

Luminance + chroma

Luminance only

Chroma only

Noise removed

Noise characteristics:

● Luminance (brightness) noise: Sometimes looks OK or “film-like”● Chroma (color) noise: generally looks bad

Noise removed

General guidelines for noise reduction:

● Apply noise reduction before sharpening● Remove as much chroma noise as possible● Reduce luminance noise according to taste, while minimizing loss of detail

No noise reduction Just enough Too much :(

Sources of image blur:

● Camera/subject movement● Lens blur (even in “perfectly” focused images, because no lens is perfect)● Post-processing (raw conversion, resampling, noise reduction)

Sharpening:

Every photo needs at least a little sharpening before printing

● Special case: deconvolution.● Recovers sharpness information buried within the blurred image.● Requires knowledge of the process that caused the blur.● Case study: the Hubble space telescope.

● The usual case: unsharp masking.● Enhances contrast at light/dark transitions.● Creates the optical impression of sharpness (acutance), without restoring

information.● A pre-digital technique.

Types of sharpening:

● Amount: The magnitude of the contrast increase at a light/dark transition. 50-100 percent is typical.

● Radius: The width (in pixels) of the area affected by the sharpening. Should be about equal to the final resolution divided by 250.

● Example: your image is 5000x4000 pixels. It will be printed at 10x8 inches. Final resolution = 500 px/inch. Sharpening radius = 500/250 = 2 px.

● Threshold: Limits sharpening to areas with strong light-dark transitions. Can minimize enhancement of skin blemishes, for example. Keep it pretty low to avoid unnatural effects.

Parameters for unsharp masking:

No sharpening Just enough Too much(image is “crunchy”)