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I have an issue with my Nikon D3500 camera which I do not understand. I can show it best using two photos I took with different settings. The lens I use is a kit lens which ranges from 18 to 105 mm.

This is photo #1: it has an F-number f/29, an exposure time of 1/160, and a focal length of 34 mm.

Image resized. Click the image to view in full size (6000x4000)

From photo #1 you can clearly see there are small black spots on the picture that should not be there. So I thought that it must be an issue with the sensor of the camera and I need to go get it cleaned.

However, here is photo #2 (the same scene with different settings): it has an F-number f/4.2, an exposure time of 1/4000, and a focal length of 34 mm.

Image resized. Click the image to view in full size (6000x4000)

In photo #2 you can see that the spots seen in photo #1 are no longer there. For context, I took these two photos in Aperture priority mode, so the only setting I changed was the F-number.

This pattern, where the greater the F-number, the clearer the spots on the photo is something which I have consistently noticed in all my photos. So my question is, what could be the issue with my camera where these spots appear at only large F-numbers?

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2 Answers 2

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Let me just complement Xenoid's answer.

In this diagram, you can see how a bigger aperture produces a diffused shadow, that sometimes is hard to see.

And when you use a smaller aperture, the shadow of the dust is more visible.

enter image description here

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This is typical of sensor dust, the spots are more visible at small apertures. In fact taking a picture of the sky with a small aperture (large f-number) is the usual way to check for dust spots...

So you just need to clean your sensor.

You camera likely has a way to shake the dust off the sensor on command, or automatically when starting up or shutting down.

If these are not enough, blowing some air (with a bulb blower, not breath or compressed air) on the sensor usually works (remove the lens, and lock the mirror up to see the sensor, your camera must have an option for this).

enter image description here

You'll find more information on cleaning you sensor here.

You can also ask a professional to do this for approximately 25 dollar/pound/euros.

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  • Sensor cleaning 25 $/£/€ seems a bit optimistic. I paid £30 in 2018 and €40 in 2022.
    – gerrit
    Aug 26, 2022 at 7:38
  • And if blowing fails, you can buy cleaning kits - I have previously used something that looks like a tiny squeegee to sweep over the sensor
    – Stefan
    Aug 27, 2022 at 10:36
  • Is there a clue that tells you these are from dust on the sensor as opposed to dust on the lens or the mirror?
    – Adrian McCarthy
    Aug 27, 2022 at 15:10
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    @AdrianMcCarthy There is no mirror in the optical path when the picture is taken, so dust on mirror only shows in the viewfinder. Dust in lens is hardly visible (unless it's the front lens and you are doing extreme macro), usually it just makes the pictures lose a bit of contrast.
    – xenoid
    Aug 27, 2022 at 15:18

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