Why sensor dust is more visible at small apertures

Another reader of ours, Frank Di Luzio, sent the below image that explains exactly why sensor dust is more visible at small apertures. While I have explained this phenomenon to some of our readers before (see the comment section), I have not had a chance to write a separate article with a proper illustration, demonstrating how aperture size affects the shape and size of dust particles. Thanks to our generous readers like Frank, I now do not have to do it, because the below illustration is perfect.

Dust on Sensor

In summary, when the size of aperture is large (a small F-number like f/2.8), light rays reach dust particles that are sitting on the sensor filter from different angles. Remember, although I refer to this as “sensor dust”, dust actually never touches the sensor, because there is a thick filter (actually, more like a number of filters packed together to form a single filter) that sits in front of the camera sensor. Therefore, by the time light reaches the physical sensor, it is spread out on a very large area, making dust appear as a large blob with a soft ring. When using very large apertures like f/1.4 on fast prime lenses, these blobs might be so washed out that they might be practically invisible to your eye. That’s why portrait photographers notice dust less often than landscape photographers!

Now when the lens is stopped down and aperture is significantly smaller, say at f/16, light rays coming from the lens diaphragm are perpendicular to the sensor filter. Because the angle is more or less straight, dust specks also cast direct and defined shadows on the sensor. That’s why dust shows up in images much smaller, darker and with more defined edges at small apertures.

Big thanks to Frank for sending the illustration!

Comments

  1. January 22, 2012 at 10:46 pm

    Very clever, thanks, Peter

  2. 2
    ) RMT
    January 23, 2012 at 5:20 am

    Nasim,
    Something does not add up; the drawings would indicate that a wider field of view is “seen” by the lens at larger apertures. Obviously aperture does not affect the area of light exitting the rear element or else you would be able to zoom with the aperture. Althoug the explanation seems correct, there is something missing. Please explain?

    • January 23, 2012 at 2:38 pm

      RMT, I think you are misunderstanding the illustration – lens opening or its focal length are not illustrated above – only the physical diaphragm is. The lines shown can be thought of a single light ray that could potentially travel like that, there would be millions of light particles reaching the sensor after going through the diaphragm. It is all about how the light is bent – when the aperture is stopped down, light beams travel perpendicularly, why is why they are illustrated as a straight line.

      • 8
        ) Frank
        January 24, 2012 at 1:36 am

        Maybe if you think of it as Depth of Field, it might be easier to imagine. DOF is the circle of confusion. At smaller apertures, the light cone reaching the sensor is narrow. Because of this, the light falling on the dust ist also narrower (sharper harder shadow).

  3. 3
    ) Peter
    January 23, 2012 at 11:35 am

    Close, but I think that “Why Downsizing an Image Reduces Noise” is still far more obtuse.

    • January 23, 2012 at 2:38 pm

      LOL Peter, the other one gave me some headache :)

  4. 7
    ) Suhaimi
    January 23, 2012 at 9:38 pm

    I had the experience with black dots on my photos… at small apertures. I sent my D7000 to Nikon for servicing and it was free since the camera is still under warranty. And in the service report; Nikon stated that they replaced the mirror drive unit. It would’ve have costed me around MYR280.00 (USD90++). Hopefully the would be no recurrence.

  5. 9
    ) David H
    January 24, 2012 at 5:51 am

    I have a very dumb newbie question and if it does not belong here, I will be glad to repost somewhere else. As I look at this diagram above it looks as though at smaller aperture openings, there is less of the total sensor covered. This obviously does not happen but I’m not sure why it does not happen.

    Regardless, great explanation on why I need to get my sensor cleaned once and a while and save myself time in post cleaning up the spots.

    • 10
      ) Peter
      January 24, 2012 at 2:44 pm

      I have been shooting digital for 8 years and cleaned my sensor 3 times. If I change lenses I try to do it in a dust-free environment and hold the camera face down . Also, if I know what lens I need, I put in on in the house before a shoot.

      Lastly, on my D300, I have 18-200 lens and never take it off; hence no dust.
      On my D700, I have a 24-120 which is on my camera 75% of the time. hence no dust.

      Nasim has a good article about cleaning sensors. It’s not that difficult to do.

      Also, cleaning spots on images with Photoshop is a no-brainer.

      Forget about dust and have fun.

  6. 11
    ) Bryan
    January 27, 2012 at 3:43 pm

    makes perfect sense when you see the diagram, this also explains DOF well too

  7. 12
    ) Mark
    April 4, 2013 at 12:52 pm

    Great explanation Nasim.

    I came across this affect last week while on a photo trip in Oregon. On my F16 and F22 images I started getting a long curvy fiber on them. I tried cleaning the front element but they were still there. There weren’t in the wider aperture images so I figured it must be something on the rear lens element cleaning that didn’t get rid of it so I tried a different lens and they were still there so I figured it had to be the sensor. So I flipped if the mirror gave the sensor a few puffs from my little giotto rocket and after that everything was good.

    I was having trouble believing that sensor dust could be affected by aperture. I just didn’t believe because of how close sensor crud is to the sensor that it could be affected by aperture… but I guess it does!

    This site is a great resource… Keep up the great work guys.

  8. 13
    ) James Horan
    May 13, 2014 at 5:05 am

    Interesting post, many thanks. Solves the puzzle for me!

  9. 14
    ) Keith R. Starkey
    May 13, 2014 at 10:04 am

    I have discovered that most of the dust shots I’ve seen in my pictures were my…thumb. Lesson learned: use a tripod!

    Thanks for the article. Informative.

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