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First off, I understand that there is another thread with a similar question but mine is slightly different. I have a Nikon D80 and all of a sudden these black dots appeared on my photos. Quick google search led me to believe that there was dust on the image sensor, so I purchased a blower and tried my luck. However, while cleaning and testing I found that the specs disappeared when I removed the lens. For example,

This is the image without the lens This is the image without the lens

This is the image with the lens and with the black dots (taken at f/22)

This is the image with the lens and with the black dots (taken at f/22)

Does this mean the problem is with the lens or still with the sensor? The black dots are most prominent at narrow apertures like f/22 so I suppose that maybe they are still there when the lens is removed but they're just not visible. I just want to confirm so I know exactly what the problem is and accordingly try to fix it.

Sorry for the long post, and thanks in advance.

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Dust in a DSLR generally ends up on top of a thin layer of glass(like) above the sensor, usually an anti-alias filter. Even on cameras without AA filters there is still usually a layer of glass (or similar) there.

This means the dust is slightly above the sensor.

When you expose with a lens set to a high (small) aperture, this near-point-source casts a very sharp shadow from the dust on the sensor and you see it.

WIth a wide aperture (or no lens at all) the light is coming from a wide source, and so casts a diffuse shadow which might not be visible at all.

It is like your hand at waist height casting a shadow in the sun, but with diffuse light from a cloud, it casts no shadow -- doesn't mean it isn't there, just how the shadow falls.

If there were no glass above the sensor, the dust would almost always show up, just as your hand would cast a shadow even in clouds if held an inch above the ground.

It's dust. Clean it, have it cleaned, or if you mostly show with wider apertures it will not be noticeable. All DSLR's get it. You can reduce how quickly by taking care when changing lenses, but they will will get dusty eventually.

Dust on the lens front or back reduces contrast but generally will never appear as a clear spec on the image (it is so completely out of focus, much as you cannot see a fence with the lens pushed up against it, but the fence color bleeds through). Dots on the image are almost always on the sensor not the lens in any normal situation.

  • I had a hair stuck on the back of a lens once, and that presented itself as a fairly distinct, roughly-hair-shaped shadow. So I think it could easily be a larger blob of something stuck to the back of the lens that is casting a partial shadow on the sensor. – dgatwood Sep 21 '17 at 22:11
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    These are pretty tiny, fairly distinct dots. Something near the lens is pretty diffuse. Try it. – Linwood Sep 21 '17 at 22:23
  • What I'm seeing in that photo doesn't look particularly sharp. Each dot has a dark center and a large bleed area where the darkness tapers off, and doesn't fully block the light even in the center. It looks a lot like water spots or something. – dgatwood Sep 21 '17 at 22:38
  • I completely agree with you… but I notice that there are many more dots than user402292 circled. Although their distribution is irregular, their size, shape, and density are remarkably similar, nearly identical. Dust particles tend to be irregular depending on their source—animal, vegetable, mineral. Wouldn't you agree? – Stan Sep 21 '17 at 22:40
  • @dgatwood Your observation fits the argument made by Linwood. The "penumbra" would be the "fall-off" with the darker centre as the shadow itself. – Stan Sep 21 '17 at 22:44
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The reason you don't see any spots on the bright white image is two-fold:

  • Light is entering your camera at an aperture size equal to the throat diameter of the Nikon F-mount. That is larger than your camera's sensor. The dust spots on your camera's sensor stack are not visible because unfocused light is coming from around the edges at all angles and there are no shadows being projected onto the sensor below the glass cover.
  • The exposure is so bright that the entire image is at maximum saturation/pure white. Any details that might have been visible at a lower exposure level are completely blown out by the overexposure.

Your example is exactly what one would expect with something on your sensor (dust or spots of dried liquid, etc.) when you take a photo with no lens on and the exposure completely blown out.

On the other hand, it takes a fairly large chunk of something on a lens surface to show up in a photo, and they are much more diffuse and larger than spots caused by material very close to the sensor.

The second image looks very much like there are drops of some type of oil that has gotten on your sensor and then small pieces of dust might have gotten stuck to the oil. Nikons have a reputation of coming from the factory with a little too much oil on the shutter mechanisms. Some of the excess oil eventually finds its way onto the sensor. The fact that the spots in your image are very uniform in size, shape, and opacity strongly suggest drops of some kind of liquid, or bits of dust stuck to such drops, are the source of the spots.

In the case of your D80, it's probably safe to say it is not a factory fresh camera. However, if you have recently had some service work done that included oiling the mirror or shutter mechanisms it might explain the recent appearance of oil on your sensor. Another possibility is that the spots have been there a good while and you just recently shot under the conditions that cause them to be most noticeable (narrow aperture, relatively dim exposure). Or it may just be another type of wet substance that has gotten stuck on the front of your sensor stack.

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You have dust on the sensor. The image without a lens is so overexposed that the little shadow the dust makes is overwhelmed. The shadow is also very diffuse due to the light coming from a wide angle.

To see that this is dust on the sensor, expose again without the lens, but this time:

  1. Adjust the ISO and shutter speed so that most of the image is a light gray.

  2. Point the camera at a distant point light source in a otherwise dark area. A single light bulb at the other side of a otherwise dark room would do it.

Remember that when you view a photograph normally oriented, it actually appeared on the sensor flipped about its midpoint. If you seen dust in the picture in the upper left corner, for example, then the dust is in the lower right corner of the sensor when viewed from the back of the camera. Of course you can't see the sensor from the back of the camera, though. When looking into the camera from the front, the dust will be in the lower left corner.

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