Following a discussion at work today with a professional with a 5D and some current dust issues, we sat scratching our heads as to the technical reason why dust specs appear sharper when the aperture's stopped down then when wide open?
I got to thinking about circles of confusion etc and started banging my head against a brick wall when I realised that regardless of aperture the point of light will only ever be a point when the lens is focusing the image on the film plane, and that those points of light, or if out of focus, circles of confusion, will be the same size on the sensor. Surely dust should be rendered the same whether the light hitting the sensor is coming from something sharp or from something soft/bokeh-tastic as these points of light/circles of confusion will be the same size across the sensor regardless of their origin?
At which point I had to get back to work, stop thinking about camera physics and we agreed to ask the fellow nerds of the interweb if they knew then science behind it? I'm sure it's painfully obvious when it's explained but amongst the million other things I've done today I'm going right in circles.
Over to you dweebs.