This is not a depth of field discussion, but it kind of is. A while back a safari participant noted that the best way to check for dust on your sensor was to take a photo with the lens stopped all the way down (eg f/22 or greater, Eg. a small opening). This seemed a little bit peculiar as we were discussing dust particles on the sensor and not on any of the lens elements. The argument for stopping down would be to increase depth of field and therefore make the dust appear sharper, thus helping to identify whether or not there is actually dust on the sensor and where that dust is! But again this seemed wholly preposterous as depth of field has to do with things in front of the lens not on the sensor! Or does it?
So we ran a test! Cause our motto is “When in doubt, test”. So we set out to photograph a plain blue sky with focus set to the middle of the lens w/ three different f-stops and voila! As you can see the participant was correct (see images below)!
Although the results are clear it didn’t make any sense, so we checked in with Dr. Rebecca Theilmann at the University of California (San Diego). Dr. Theilmann’s Ph.D thesis was in Optical Sciences. Dr. Theilmann explained our test results this way. “When light is transmitted through glass the light is scattered about. And although dust particles are small and sit directly on the surface of the sensor, it is still sitting on top of the sensor. With a wide aperture opening the scattered light reduces the shadow areas of the dust particles therefore it can become nearly invisible on the image. On the other hand when the light is focused with a small f/stop (eg. f/22) shadows appear sharper and darker and thus dust particles are now more visible on your image. So the dust did not disappear, it just became more visible.”
In conclusion, this is physics not a "phenomena”. If you are looking for dust on your image then you need to shoot at the smallest f/stop possible. Here are the directions.
1. Turn lens to manual focus.
2. Point your camera at a plain surface like a white wall or blue sky without clouds.
3. Switch to aperture priority mode and set to f/22
4. Over expose the image by 1 or 2 stops by turning your exposure compensation to plus 1 or plus 2.
5. Take a photo.
6. Magnify the image and check every section of the image to find dust.