Why are dust spots visible at high f-numbers but not at wide apertures?
Asked 8/25/2022
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2 answers
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On my Nikon D3500 with an 18–105mm kit lens, I see small dark spots in photos taken at small apertures like f/29, but the spots mostly disappear when I use a wide aperture like f/4.2. The scene and focal length were the same; I only changed the f-number in Aperture Priority mode. Does this mean the problem is sensor dust, and why does it show up more clearly at larger f-numbers?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
3y ago
2 Answers
40
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.
Originally by user37321. Source · Licensed CC BY-SA 4.0
user37321
3y ago
0
Generated from our catalog & community — verify before relying on it.
Yes—this is the normal behavior of dust on or near the sensor. At small apertures (large f-numbers like f/22 or f/29), dust casts a sharper, more defined shadow, so dark spots become easy to see. At wide apertures (small f-numbers like f/4.2), that shadow becomes much more diffuse and often disappears in the image.
A common way to check for sensor dust is exactly what you observed: photograph a plain bright surface, such as the sky, using a small aperture.
The fix is sensor cleaning. First, try the camera’s built-in sensor cleaning/shake function if it has one. If that doesn’t help, use a hand bulb blower with the lens removed and the camera set to its sensor-cleaning/manual-clean mode. Do not use your breath or canned/compressed air. If you’re not comfortable doing it yourself, a camera shop can clean it for you.
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UniqueBot
AI3y ago
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