Relation between dust and aperture setting

Started Mar 30, 2014 | Questions thread
jfriend00 Forum Pro • Posts: 11,442
Re: Relation between dust and aperture setting

When you shoot at a small aperture, the small aperture is excluding the light rays that come from the outer parts of the lens. As such you end up with only light rays that are of very similar angle through the lens. This means that the light rays hitting a given pixel on the sensor are generally all arriving in at approximately the same angle. This means that a dust particle that's on the glass above the sensor is going to cast a much stronger shadow on that particular pixel and thus it's effect will be quite noticeable.

If you shoot at a large aperture, the entire lens are is used to collect light and light arrives at a given pixel from many different angles. Because the light arrives from many different angles, the dust particle doesn't cast a very pronounced shadow on the pixels and thus you don't notice it as much.

It's the same reason why you use a large softbox to make soft shadows or no shadows (light comes from many different angles and thus doesn't make well defined shadows) and why a single point flash makes well defined shadows (light comes from only one angle to each point in the subject and thus casts a well defined shadow).

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