How to identify dust location: on lens or sensor?
SLR camera and other camera with interchangeable lens are prone to dust. Some of the dust might get stuck on the DSLR sensor and some dust other might stuck on the lens. On this article I will show you on how to identify dust location, whether the dust is in the lens or is it in the DSLR's sensor.
Dust on sensor
Dust on sensor will only appear in higher f stops (e.g. the aperture opening is small at f/22 and smaller). Sensor dust will appear on same location in the image anytime you took picture. Dust at f/30 is not a problem, however some larger dust will appear even when shooting at f/5.6, and this is a real distraction! I'm sorry that I only have 1 dusty sensor picture, so I can't show you that the dust stays in the same in my picture.
To see the dust on the sensor, you can put the camera in manual or aperture priority (Av) mode. You will need to use F/22 or smaller aperture. F/36 will show some more smaller dusts. Then you need to shot to a white wall with adequate illumination from bright white light source from above. But stay clear from your shadows. Use low ISO speed to make sure that the noise is low so you won't made mistake between noise and dust. Then make the correct exposure by setting EV to 0 so the image will be well illuminated.
This usually result in 1 seconds or more exposure time. Don't worry that you'll shake the camera, since the dust will stay on the same location. You should even vibrate it a little bit while taking picture to make sure the dust on the sensor is the one that is recorded, not the texture or dust on the wall. The image below shows my sensor with dust in it. Not many of dust are visible, but it is pretty annoying since the dust can be seen from f/8.
So the conclusion is some dust on lens is OK as long as it is not visible on real world shooting, which means at f stop smaller than f/11. If you do need to shot at f/22 or smaller aperture - which is unlikely due to diffraction issues - then you'll start to worry about dust issue.
Dust on lens
While the dust on the lens can be seen by physically staring the lens from its mount. Dust on lens usually can't be seen on 99% of the picture. You won't notice that there is dirt in the lens. One of the way to show it on picture is by deliberately taking out of focus picture of several light source with black background in your image. f/5.6 should be enough to see the dust. The dust will then appear in the bokeh. If you have several light sources, the dirt will appear on some of the bokehs at the same location. One thing you should remember is not to overexpose the image, since overexposing them will render the dust invisible.
Please see the image below (cropped) for the reference of what a dusty lens deliver:
On my case, the dust appears to be located inside my lens - may be from production phase - so I can't clean it myself. However since my lens is still under warranty, Canon fixed this for free.
If you looks the images on my gallery, you can see some of my images made using EF-S 18-55 is made by lens with dust inside. Some others images is made with dusty sensor, and some others with both dusty lens and sensor. You will never now this fact until I told you. So I believe dust are not an all important factor in digital photography.