Dust and Aperture

[mcg]

<p>I have some dust on my D300's sensor. But what mystifies me is that it only seems to be visible when I shoot at a small aperture (usually f/11 or smaller). At first I thought it was dirt on a (rear) lens element, but is present after changing lenses, and I did tests at various apertures (long exposures, defocussed, moved back and forth over a white wall, so the only thing that would be visible on the image would be dust).<br>

As I understand it, if the dust resides on the filter in front of the sensor, then lens aperture should make no difference at all to dust being recorded on the sensor. Why does it though? All I can think of is that the light rays are so "parallel" with a small aperture that the don't have a chance to "go round" the specks.<br>

(By the way, the D300's ensor cleaning has failed to shift it, so I'll have to elve inside I guess.)<br>

Here's a comparison of the dust taken at different apertures so you can see.<br>

<img src="http://img.photobucket.com/albums/v285/bollyscot/0106-11416web.jpg" alt="" /></p>

[godfrey]

<p>The dust settles not on the sensor but on a cover glass or antialiasing filter in front of the sensor. So there is indeed space between the plane where the sensor is and where the dust is. When the lens is not closed down, the effective light source is broad: light diffuses around the edges of the dust and makes it soft, difficult to see. When the lens is closed down, the light is much more akin to coming from a point source so the edges of the dust specks become distinct. </p>

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