Relation between dust and aperture setting

Started Mar 30, 2014 | Questions thread
Leonard Shepherd
Leonard Shepherd Forum Pro • Posts: 14,288
Re: The first correct answer?

dwight3 wrote:

The dust does not sit on the sensor. There is a Bayer filter and an AA filter sitting on the sensor with a coating above that. The dust is therefore separated from the sensor by a little distance.

As we sometimes say in the UK - this hits the nail on the head.

At the sensor there is depth of focus - not exactly the same as depth of field but similar in principle.

Any dust in front of the sensor is outside the narrow depth of focus at f3.5 and is not rendered sharp What is not rendered sharp cannot be seen.

By f22 there is a lot more depth of focus and often enough to render dust sharp. At f22 if you try hard enough you can often see it.

Other factors such as direction of light and subject lighting range, particularly dark dusts shadows against a sunlit white wall (maybe 20 stops dynamic range) emphasise dust when there is enough depth of focus to show it. Soft light or dark backgrounds disguise it.

Most older lens have some dust inside which is rendered outside the zone of depth of focus at the sensor and does not affect image quality. Excessive dust in a lens and on a sensor can affect image quality.

Keeping lenses, lens caps, body caps and camera bags clean helps keep lens surfaces, lens internals and sensors free of dust.

The sensor clean facility in recent Nikon cameras deals with most dust issues.

"Pushing photographic boundaries" can introduce issues, such as getting lens flare when including the sun in the picture area.

It is up to each photographer to decide if some dust occasionally showing at an aperture rarely (as distinct from never) used at a contrast range beyond the sensor dynamic range (perhaps as high as 20 stops) at an image magnification around 50 inch wide print size (D600 viewed at 100% on an average laptop) is a major issue, or a way to check if it is time for a rocket blower or a wet clean.

If you think viewing at 50 inch print size from 24 MP is relevant to normal photographic quality I suggest using a macro shot of your facial skin texture is not a wise comparison.

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Leonard Shepherd
Producing good quality photographs, or being good at sport or art, involves a little more than buying appropriate equipment. Practice, some learning and perhaps natural talent often play a bigger role than the equipment in your hands.

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