Update 6:46pm: After reading Tom’s comment about it being sensor dust, I googled around about sensor cleaning. I think he’s right, so I edited this article. The original blamed dust on the lens.
I just spent hours cleaning up some photos I took at the beginning of the month. I was experimenting with some of the “stuff” I had learned in class and was setting my aperture manually to see the difference in the effects.
Before this experiment, I could tell you what was supposed to happen from what I had read: large aperture, shallow depth of field (small part of picture in focus)… small aperture, deep depth of field (large part of picture in focus). I took some shots at Griffis Sculpture Park that show this concept well.
With a wide open aperture (f4.0), I can choose to focus on the foreground:
or the background:
With a smaller aperture (f22), I can get both in focus:
Now, back to my cleanup job. Do you notice in the last photo above there are spots in the sky. Apparently when you shoot with a small aperture, EVERYTHING is in focus: even the dirt on your sensor!
Here was another experiment in which I shot a broad landscape twice varying the aperture.
Here is aperture wide open (f4.0):
Here is with a small aperture (f22):
Once again, even the dirt on my sensor is in focus in the second photo shot with the small aperture. These examples also serve to demonstrate another factor that contributes to depth of field: distance from subject. The DOF is not affected that much when the subject is far from the lens. Both scenes seem to have an equivalent DOF.
Interestingly, I took my camera to have the sensor cleaned the day after I went to Griffis… Now to perform the test suggested at the website below:
Here’s an article about sensor cleaning: