What camera settings accentuate dust on sensor?

[BradS]

Occasionally some dust or other debris on the sensor shows up on photos of the uniform clear sky as slightly dark "smudges". My attempts to clean the sensor and check after cleaning are inconclusive...that is, after cleaning, sometimes the smudge is there and sometimes it is not. So, I'm not sure if I need to clean more or if there is some other issue with lens or sensor or .... ???

So, the question is, what conditions accentuate dust on the sensor?

Seems like, small aperture, focus at infinity (does it matter?) and, possibly, longer focal length ?

 

[koraks]

Seems like, small aperture, focus at infinity( does it matter?) and possibly longer focal length ?

Small aperture, focus @ infinity and shorter focal length.

 

[BradS]

Small aperture, focus @ infinity and shorter focal length.

Thanks.
The strange thing is that the shadowy blob shows up with the 40mm lens but not the 28mm lens (both at f/16 and focused at infinity) so, I'm really stumped. Is it at all possible that some defect in a lens shows up as a shadowy blob in the photo under these test conditions?

camera & lenses: Nikon Zfc with 28mm f/2.8 and 40mm f/2.0 Nikkor Z lenses.

 

[koraks]

Is it at all possible that some defect in a lens shows up as a shadowy blob in the photo under these test conditions?

Yes, but you should be able to see it pretty clearly on (most likely) the rear lens element. If you closely study the lens, while light is shining through it, can you see anything unusual?

It may also help if you post a photo of the blob-like artefact. It would be relevant to be able to judge the size and shape.

 

[BradS]

Looks like ISO may also have an effect.
Unfortunatley not a carefully designed experiment but, I think it shows a difference between the two focal lengths. And now that I look at these on the computer screen, I see a small artifact is still visible in the image made with the 28mm as well.
It may be easier to see if you view these on flickr instead. Click on the 'Brad S' link after any photo will take you there.

before cleaning:

ZFC_1307 by Brad S, on Flickr


after cleaning, 28mm, f/16, ISO 100

ZFC_1323 by Brad S, on Flickr

after cleaning, 40mm, f/16, ISO 500

ZFC_1336 by Brad S, on Flickr


after cleaning, 28mm, f/16, ISO 100
ZFC_1338 by Brad S, on Flickr


after cleaning, 28mm, f/16, ISO 1000

ZFC_1339 by Brad S, on Flickr

 

[koraks]

after cleaning, 28mm, f/16, ISO 100

No defect visible on this one, not even if I dramatically increase contrast.

after cleaning, 40mm, f/16, ISO 500

Several blobs:

after cleaning, 28mm, f/16, ISO 100

Clean; no artefacts.

after cleaning, 28mm, f/16, ISO 1000

Clean; no artefacts.

The blobs on that single 40mm f/16 frame looks like dust. Given the aperture setting, I'd expect the dust to be at some distance from the sensor; not directly on top of it, as it would be more sharply defined in that case. Dust on the rear lens element sounds like a plausible cause.

 

[BradS]

...

The blobs on that single 40mm f/16 frame looks like dust. Given the aperture setting, I'd expect the dust to be at some distance from the sensor; not directly on top of it, as it would be more sharply defined in that case. Dust on the rear lens element sounds like a plausible cause.

Hmmm, interesting.

There is some dust on the rear element of the both the 40mm lens and the 16~50mm kit zoom. I never would have guessed that it would show up in a photo. What an amazing time we live in.

Thank you!

 

[koraks]

Let me know if cleaning the lenses actually gets rid of the problem!

 

[BradS]

Let me know if cleaning the lenses actually gets rid of the problem!

Yes, it did. Thanks! It is still amazing to me that what seemed like an insignificant amount of dust on the rear element resulted in those shadowy blobs. It is relatively dry and very dusty here where I live so, keeping up with the dust is yet another challenge of this new technology. Thanks again for your help.

 

[koraks]

Great, I'm happy to hear that!
Yes, one might have hoped that moving away from film would forever get rid of the dust problem - but it still rears its ugly head from time to time.

 

[Snowfire]

High depth of field definitely exacerbates dust problems. And if you really want to see all the dust on your sensor, shoot with a pinhole (you can improvise one from common foil if you do not wish to purchase a photo-grade pinhole.)

 

[abruzzi]

Great, I'm happy to hear that!
Yes, one might have hoped that moving away from film would forever get rid of the dust problem - but it still rears its ugly head from time to time.

in some ways it makes it worse. I was traveling with a Pentax DSLR. One day I got dust on the sensor, probably when changing lenses. Unfortuantely I didn't notice, partly because being primarily a film shooter I don't look at the photo immediately after taking the shot ("chimping"). So end of the day I moved the files to the laptop I brought and I saw 50+ shots that all had dust on the sensor. If it had been a film camera dust on the film would have affected a single shot then pulled out of the gate and the next shot wouldn't have seen that mote of dust.

 

[BradS]

... one might have hoped that moving away from film would forever get rid of the dust problem - but it still rears its ugly head from time to time.

in some ways it makes it worse. ....

Funny thing is, with film, I never really cared....except when it was something like an eyelash stuck in the film gate that showed up as a dark squiggly line on the edge of ever single exposure.

 

[grain elevator]

Getting rid of the last bits of dust is near impossible. I think a few key ideas here are: not stopping down more than needed, not sharpening more than strictly needed, and: ignoring everything that isn't visible at the intended display size. Also making use of the variety of options in Photoshop and the like. Again, the pragmatic have an easier life, e.g. for small display sizes, automatic removal may be perfectly acceptable, and one may reserve careful spotting for when large prints are needed. For most of us, that isn't very often.