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Equipment is Canon EOS 5D3 (5760x3840) with a 24-105 f/4 L lens at 105mm, ISO 200.

The following are crops from the same small (676x449) section of a couple of blue sky shots I did to test for sensor dirt. The first image is unmodified other than Raw->JPEG conversion in Lightroom with no setting changes, and shows some faint dark circles.

f/22 Unedited

f/22 unedited

To enhance the circles I cranked up contrast and clarity to 100%, giving this:

f/22 Contrast-Enhanced

f/22 contrast-enhanced

However, the same area of an image taken 1 second earlier at f/7.1 shows no evidence of these spots, even after cranking up the contrast/clarity to 100%.

f/7.1 Contrast-Enhanced

f/7.1 contrast-enhanced

Other Information

  1. The spots seem to be more prevalent towards the edges of the original (full-size) image, although there is one spot near the center. None of the spots are visible at f/7.1.
  2. All the spots appear to be the same size, but have different densities.

Given all this evidence can I deduce that these are lens defects (or dirt on a lens element)?

Since they're completely undetectable at "normal" apertures I don't consider this a problem. I just would like to know what I'm seeing.

BTW, the obvious test is to shoot the same image with a different lens and compare, but I won't be able to do that for several days and thought this might be an interesting question for photo.se.

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  • \$\begingroup\$ Try on multiple lenses at max aperture, if the spots are still there, its on the sensor, if not, its the lens. \$\endgroup\$
    – thebtm
    Commented Oct 28, 2015 at 17:52
  • \$\begingroup\$ @thebtm I think you mean minimum aperture (highest f-number). Maximum aperture is the widest setting with the lowest f-number. \$\endgroup\$
    – Michael C
    Commented Oct 28, 2015 at 22:20
  • \$\begingroup\$ See also photo.stackexchange.com/questions/12087/… \$\endgroup\$
    – Michael C
    Commented Oct 28, 2015 at 22:20
  • \$\begingroup\$ @thebtm It's rarely, if ever, dust on the lens. lensrentals.com/blog/2011/08/the-apocalypse-of-lens-dust \$\endgroup\$
    – Michael C
    Commented Oct 28, 2015 at 22:30
  • \$\begingroup\$ almost certainly sensor dust. just clean the sensor and you're good. \$\endgroup\$
    – Jakub Sisak GeoGraphics
    Commented Oct 29, 2015 at 19:21

2 Answers 2

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That is dust on the sensor (well the IR filter over your sensor), dust specks in the lens are not capable of causing spots on the image like this.

The reason you can see them at a small aperture and not at a large one is that the light rays are far more linearly aligned at f22, hence they cast a shadow - remember the surface of the filter where they sit is not ON the sensor, but close to it.

At a wider aperture the incident light basically goes AROUND the dust particles, and therefore cannot cause such a defined shadow.

Edited to add: Dust on the surface of FILM will always cause spots on an image, at any aperture.

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  • \$\begingroup\$ Beautiful answer! \$\endgroup\$
    – icor103
    Commented Oct 28, 2015 at 19:27
  • \$\begingroup\$ I can't edit your post because it doesn't have enough errors (lol), but it should be speck and shadow. \$\endgroup\$
    – icor103
    Commented Oct 28, 2015 at 19:31
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This is the typical appearance of sensor dust.

Dust on the lens rear element will never be so well defined, won't be aperture dependent, and will be visible throuh the viewfinder, while sensor dust, of course, is only in the picture.

Dust on the front element won't be visible as specks at all, it will decrease contrast in a more vaguely defined area.

You can see some examples of all of these eg. here: https://photographylife.com/how-to-spot-dust-on-your-dslr-sensor

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  • \$\begingroup\$ Why is sensor dust aperture-dependent? \$\endgroup\$
    – icor103
    Commented Oct 28, 2015 at 19:00
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    \$\begingroup\$ @icor103 because the dust isn't really on the sensor itself, but a bit in front of it on the AA filter, see photo.stackexchange.com/questions/12087/… \$\endgroup\$
    – ths
    Commented Oct 28, 2015 at 19:09
  • \$\begingroup\$ Why are they all the same size? \$\endgroup\$
    – Jim Garrison
    Commented Oct 28, 2015 at 19:15
  • \$\begingroup\$ Thats a bit more complicated to explain, its a "circle of confusion" thing, note that they arent all the same darkness... \$\endgroup\$
    – Digital Lightcraft
    Commented Oct 28, 2015 at 19:28
  • \$\begingroup\$ According to this blog entry from Roger Cicala, owner, CEO, and chief lens guru at lensrentals.com, how much dust on the rear lens will show up in a photo is aperture dependent. lensrentals.com/blog/2011/08/the-apocalypse-of-lens-dust \$\endgroup\$
    – Michael C
    Commented Oct 30, 2015 at 6:00

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