Sensor

A sensor is an array of pixels to capture the light in digital cameras, rather than film. Pixels are very, very small. Sensors are made of millions of pixels. A 3888 x 2592 pixel sensor (Canon 40D) has 10,081,584(=3888x2592) pixels, or 10 megapixels.

Size matters

A larger sensor allows more pixels for higher resolution, but also allows for a greater range for the depth of field, especially on the shallow end. Point and shoot digital cameras have a small sensors. Even small sensors can be packed with a huge number of pixels (currently over 12megapixels). Those same 12 megapixels spread out over the area of a DSLR sensor will produce a different quality to the image, but also the larger area allows for larger pixels - more light gathering capability. That can mean less noise at the same ISO setting.

Dust

A speck of dust on the sensor can block many pixels from receiving light. This will cause a blur and dark spot on the image and every image until the sensor is cleaned. This is most noticed at small apertures (large f-stop numbers). In the samples below, the circled dust specks can be easily cloned with photo editing. When the specks appear on the subject, less easy to correct, but also less likely to be noticed. Many photographers just ignore these until the pain of editing is too time consuming, then they clean the sensor very carefully as to not trade a few dust spots for many spots.

• Example of Dust on the Sensor
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  Circles indicate dust on sensor

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  Shows up in the same place on every image