Dust bunny physics.

[Vernon t]

Physics is phun...remember.

Ok...question.

Shoot the sky with your lens open wide and you'll get the blissful impression that your sensor is clean.
Stop down to f16 or 32 or whatever and sit on your digital toches and view the file. For the real masochistic rush auto-level it in photoshop.

Why the sharp visibility of the insidious little devils stopped down?
Is it something analogous to depth of field in reverse, that is, are the offensive contaminants blurred out with a lens opened up because, in reality, they are out in front of the plane of focus in front of the sensor held out there by the protective glass and AA filter and who knows what other layers?

fysics anybody???

 

[Chris101]

Hee hee hee

With a small aperture, it is like the light inside the camera comes from a point source. With a large aperture, a wide source.

Look at the shadow cast by a single light bulb hitting your hand a foot or so above a piece of paper. Now hold your hand the same distance above a paper under a large fluorescent tube bank. Notice the bulb will cast a distinct dark shadow, while the light bank will make a diffuse shadow with no real edges.

 

[heiko]

Thanks Vernon and Chris for posting / answering. I keep forgetting such things and and Chris' explanation is just to the point.

 

[Vernon t]

Yes, thanks Chris.

This principle frustrates me on the occasions I want to shoot wide and back-lit. A way-stopped down lens can get a neat star-like diffraction pattern of the sun, but the bunnies come out to play. Most recently I shot back-lit skiers trying this after a week of frequent lens changes. Ouch.

My Eclipse and Pec-pads came yesterday. Wish me luck...I'm off to taunt my sensor and flirt with Murphy's law!