Once upon a time, when photographers still loaded and unloaded strange little metal cans into their cameras, nobody paid any real attention to changing lenses in the field. After all, you’d get a new sensor with each shot you take. Not true with DSLRs. Even worse, most imaging sensors carries some degree of static charge, making them a dust magnet when the mirror box is exposed during a lens change.

So, how do you check for sensor dust? This is my method:

* Mount a lens that supports a small aperture – say, f/22 or even better, f/32
* Pop the flash.
* Set your camera to Manual, sync speed, f/32 and use the ISO to adjust your exposure.
* Aim camera at white wall, then defocus the lens.
* Shoot a frame so that the exposure is just shy of clipping in the histogram.

Because the lens is defocused, the only thing that will be in focus in the image would be debris sitting on top of your sensor casting shadows onto the shot – the sensor dust that you’ll need to clean.