Forgotten in its old age

by Aditya

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Here and there I read of people who set their lens to a small aperture while testing their sensor cleanness, supposedly to get the best image of dust speckles. However, the image of the on-sensor dust particles should not, to my understanding, be affected by the sharpness induced by the lens settings. Same is true for dust particle on the lens elements themselves. It makes me wonder - do these people misunderstand the theory of how the optical system work, or am I missing something?

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Perhaps this question could be extended to cover other aspects: Does it matter if I use a wide-angle lens or a telephoto lens? A zoom or a prime? Defocus to infinity or very close? (At least with some quick experiments that I did, all other things being equal, dust was easier to spot with a shorter focal length – however, the combination of a short focal length and small aperture meant that the background was not defocused enough...) –  Jukka Suomela May 19 '11 at 10:06
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@JukkaSuomela - which is why, I guess, the best method will be to test aiming at clear blue skies. –  ysap May 19 '11 at 13:43

2 Answers 2

up vote 12 down vote accepted

If the dust was really on the sensor proper, you'd be absolutely correct.

At least in the normal case, it's virtually impossible for dust to get on the surface of the sensor itself, because there's a couple millimeters or so of filters directly in front of the sensor. The front-most of these is (at least in the usual case) the AA filer.

The important thing is that all of this transparent glass. Therefore, with a wider aperture, there's more light coming at the sensor from various angles. Since the light can travel at whatever angle through those filters (because they're all at least mostly-transparent glass), the dust spots won't normally block all the light. With a smaller aperture, the light comes nearly straight back from the small aperture, so the edges of any dust spots are clearly defined.

In practice, the difference is pretty obvious. Here's a shot at f/1.7, then shot taken a few moments later at f/22 (same camera, same lens, etc. -- all that's changed is the aperture and shutter speed):

f/1.7: enter image description here

f/22: enter image description here

Like usual for this situation, I've also boosted the contrast to make the dust more visible -- since it's a plain, low-contrast subject, the histogram starts out like this:

enter image description here

To make the dust more apparent, you adjust the levels something like this:

enter image description here

If you apply it to the f/1.7 picture (as I did above, to keep things fair), that will also overemphasize the light falloff at the corners of the picture. Though there is some anyway, it's not normally even close to as bad as it looks in the first shot above.

Note that this is a pretty rigorous test. This sensor is clean enough that on typical shots, there's no sign of dust on the sensor at all. Between the total lack of contrast/detail in the subject, tiny aperture, and extreme boost of contrast, we're seeing quite a bit that we'd never see in any normal picture (not to mention that the two worst spots here are near the bottom of the frame, where there's nearly always at least a little detail to hide these problems anyway). If you test this on your own camera, don't be too surprised if it looks (quite possibly a lot) worse than the ones above. From what I've seen, I'd guess that most dSLRs (and a pretty fair number of P&S cameras too) are at least a little, and often a lot, worse than this.

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Mine is a lot worse than that. :P I've been afraid to do a wet cleaning...but maybe I have to. You have one hell of a CLEAN sensor! –  jrista May 16 '11 at 6:47
    
What did you shoot in these two pics? –  Lazer May 16 '11 at 14:01
    
@Lazer: The wall of my office, mostly out of focus. There's enough DoF at f/22 that you can see a little of the texture of the wall (though it's still out of focus, and something like a 2s exposure, shot handheld, so it's a long ways from sharp). The more common subject is a clear, blue sky. –  Jerry Coffin May 16 '11 at 14:07
    
Thanks for the practical examples! –  ysap May 16 '11 at 15:53

If I understand your question correctly, stopping the aperture down to its narrowest ensures that light is focused as tightly as possible. If you take a photograph at a wider aperture, excess non-incident light will still make it to the sensor, and mitigate the effects of sensor dust.

To put that in more precise technical terms...with a narrow aperture, the light that strikes the sensor is from as narrow a field as possible, and as close to perpendicular as possible (90° to sensor plane). That causes dust particles to create a "sharp shadow" on the sensor. At a wider aperture, the light that strikes the sensor is from a wide field of view, and light may not always be perpendicular (could be anywhere from 90° to say 70°), since the entire lens surface plays a role in focusing light. Those off-axis rays of light cause dust particles to create a "soft shadow" on the sensor.

enter image description here

You could demonstrate the effect at a macro scale, if you need a visual exemplar. Hold your hand up a foot or two from your wall, and point a bright but narrow beam of light at it...say from a flashlight about 10 feet away. The shadow from your hand should be clear and sharp. Perform the same experiment again, however this time, set up several shaded lamps that emit light in a broad field in a line parallel to the wall about 10 feet away. The shadow from your hand should be soft and dim, if visible at all (except under closer scrutiny.) Narrowing your aperture is akin to using the flashlight, while widening it is akin to setting up a wider row of lights.

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OK, there are a couple of concepts here. First, the angle of incidence is perpendicular only at the center of the sensor. This is not affected by the aperture, but merely the plain geometry of the problem. It is true, though, that with a wide aperture, the rays approach from a wider angle (cone) than with a narrow aperture. Second, by definition, the sensor is at the plane of focus, so all rays should converge on the dust particle. Third, your experiment with the shadows assume the fingers are far from the wall, however, the dust is on the sensor (give or take the IR filter). –  ysap May 16 '11 at 6:02
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@ysap: The dust is on the filters in FRONT of the sensor, NOT on the sensor. Given the scale were talking about with a modern DSLR, my macro example (which is admittedly VERY rough) is attempting to take that into account. It should also be noted that when using this technique, it brings out dust in the center of the sensor best, and dust along the edges of the sensor worst...for the very reason that light is not entirely "perpendicular" in those regions. So, we don't really disagree, however perhaps to test in macro scale, keep your hand only half a foot from the wall. ;) –  jrista May 16 '11 at 6:14
    
Also keep in mind that with modern DSLR's, there is an air gap between the filter component in front of the sensor and the sensor itself to allow for automatic sensor cleaning via vibration (at least, for most...I think those with in-camera vibration reduction may not.) That adds to the distance between dust and sensor. Another note...the "sharpness" that is usually gained by stopping down doesn't play a role here...usually, I defocus the lens to infinity, so that the light coming through contains no relevant "detail"...its just flat, unfocused, and largely "perpendicular" light. –  jrista May 16 '11 at 6:15
    
Helpful word for this: "Collimated". en.wikipedia.org/wiki/Collimated_light –  mattdm May 16 '11 at 13:40
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Because the distance from the subject to the lens is typically much greater than the distance from the lens to the sensor, the small distances in front of the sensor are much more critical. Edge rays that are almost parallel to each other when they strike the objective are highly angled when projected by the lens onto the sensor (especially if the objective is significantly larger in area than the area of the light circle it projects at the sensor's plane). –  Michael Clark Feb 15 '13 at 18:28

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