DSLR optical path: where is that spec of dust?

After a recent trip to Hurst Castle we noticed dirt spots on the photos from 2 cameras.

While giving them a clean, it occurred to me that there are quite a few places where dust and dirt can be a problem.

Both of the cameras in question are Pentax DSLR (a K3 and a K30) so the optical paths are different when viewing a subject, as compared to taking a photo.

When looking through the viewfinder, the light enters the lens and is diverted via a mirror, through a pentaprism to the eye piece.

When the shutter button is pressed, for the duration of the exposure the mirror swings out of the way, and the light passes from the lens direct to the image sensor.

I reckon there are 8 surfaces that are cleanable and may be liable to dust deposits:-

1. the viewing lens in the eye-cup
2. the focusing screen
3. the mirror
4. the image sensor
5. the rear glass of the lens
6. the front glass of the lens
7. the rear glass of the lens filter
8. the front glass of the lens filter

With this in mind, here are a couple of simple deductions:-

• if dust is visible in the view finder, but not on a test photo; check viewfinder, focusing screen and mirror
• if dust is visible on the test photo; check (or swap out) lens & filter before checking the image sensor

I would suggest you only clean the image sensor if you have eliminated problems with the lens and filter.

Is it a bird? ...is it a plane? ...is it fluff?

Check my earlier post on how to take test shots and clean the image sensor.