[cdw2000]

This morning I was testing a lens I just received. I was taking multiple shots of my living room, testing all the aperture settings. As I transitioned to smaller apertures, I began noticing a small fiber particle showing up against the white wall of my living room. I switched lenses and the dust fiber was still showing up in the resulting shot. I then cleaned the sensor and tried again. The fiber was gone.

So here's the question: Why does the aperture size of the lens make a difference as to when dust is visible on the sensor? It seems to me that once the light hits the sensor it shouldn't matter what lens aperture was used to get it there.

The only explanation I can think of is that both lenses were wide angle (16mm and 18mm, respectively) and at the smallest aperture, there is a DOF effect at the sensor surface that causes particles on the sensor surface to come into focus. I'm an electrical engineer, not an optics expert, though, so don't know if that is correct, but it seem plausible.

[Mr. B]

there is a DOF effect at the sensor surface that causes particles on the sensor surface to come into focus

That's it

[carabez]

Why Sensor Dust is More Visible at Small Apertures
Do you wonder why sensor dust is not visible at very large apertures, but starts to show up at small apertures like f/8? Below you will find an excellent illustration that demonstrates how aperture size affects the shape and the size of dust particles.



https://photographylife.com/why-sensor-dust-is-more-visible-at-small-apertures

[cdw2000]

Thank you!

[swip]

Now I know. Thanks for asking.

[AstroDave]

carabez's illustrations are good, but a tad incomplete - not only is there a "sensor filter" above the sensor (usually to limit IR light (wavelengths longer than about 700 nm) from hitting the sensor) but there is yet another layer of glass over top of the sensor pixels themselves, as part of the sensor assembly (i.e. when you (Pentax!) buy a CCD from, say, Sony it comes with this glass glued at the top of the chip assembly. So, the IR filter has to go above this cover glass).

Thus "sensor dust" is several mm above the actual sensor pixels, and the shadow angle effect in carabez's pictures can be quite substantial.

[pschlute]

The established method for checking for dust on the sensor for me has always been uniform blue sky and f22

[DWS1]

The established method for checking for dust on the sensor for me has always been uniform blue sky and f22

That will for sure find the dust spots. I might add to have it slightly out of focus.

[clickclick]

I've also found stopping down and taking pictures of light bulbs - do multiple shots to put the bulb in different areas for full coverage, will reveal the dust bunnies too

[Philoslothical]

I generally just stop down and shoot a blank wall with a flash, while the lens is kept out of focus. A sheet of white paper works well, too, and so does a computer monitor displaying pure white. You can get as close as you want to the white surface used as long as there's room for the flash to work if you're using one. The surface should be kept out of focus, but it doesn't matter whether this is accomplished by focusing the lens out of range, or just moving the lens too close to the screen or paper. I think it's generally best to try to expose the surface optimally, so a spike in the center of the histogram since it's a pure white surface. Then it's easy to bring out the dust in Lightroom or Photoshop by playing with contrast and such, or with the "visualize spots" setting in the Dust Removal feature of Lightroom. This is useful if you're making a dust mask to apply to later photos, though I've noticed the dust removal startup action of the camera tends to move dust around more than it removes it, and this can undermine the accuracy of your dust mask in a hurry if you're using the startup action frequently.

For removing dust I've been doing about an annual wet cleaning with generic swabs and Eclipse fluid. No problems yet, though sometimes it takes a few swabs to get it right. I think that's more on me. It's simpler than it sounds, but it does provoke some anxiety. There are lots of good Youtube videos demonstrating how to safely wet clean a sensor.

[pentaxus]

A wide lens shows it easier. Even stuff on the rear element at small apertures.

[ehrwien]

To find dust I just overexpose a mostly uniform bright surface at narrow apertures. No need for fast shutter or tripod, just move around the camera for an exposure time of a few seconds if necessary. The background will disappear in a more or less uniform blur while the dust on the sensor will not change place for the whole exposure, so you can make out each particle in the resulting picture.

I always had the "sensor shake cleaning" set for engaging when I power off my K-S2, but I feel it did not help much with dust and cost a lot of battery power with me powering the camera on and off quite often. Second level of cleaning is in cleaning mode with a rocket blower, but that does not help with tougher particles. Next I'm going to try the O-ICK1 stick. Last resort would be wet cleaning, but I've had my K-S2 for many years now and never done it or had it done. I always make sure to minize the time there is no lens on the camera when changing lenses and keeping the opening pointing down when changing lenses so nothing can fall into the mirror box in that time.

For cleaning up images where dust particles are visible I like the repair tool in FastStone ImageViewer.