I know that aperture affects the appearance of dust on a camera's sensor (or, technically, the filter in front of the sensor), but does focus matter at all?

Since the dust is behind the lens, I wouldn't think it matters at all whether the light rays hitting the dust happen to be coming from an object that's sharply rendered.

My camera has an automatic "Dust Alert" feature. I noticed that this feature is unavailable when the focus mode is switched to manual mode, and that when activated, it immediately racks the lens out to the farthest slightly-past-infinity focus (regardless of what I'm pointing it at). Is there something about this infinity-and-beyond focus that makes dust stand out more clearly? (And does that mean that sensor dust would be less apparent in macro photographs?)


1 Answer 1


With the focus at infinity, rays hitting the sensor are more parallel which makes the edges of dust appear crisper. This probably helps with the filter Pentax uses to produce the dust alert image shown on your camera.

This is the same reason why depth-of-field is larger when focusing further, so what the camera is doing reaching for a huge depth-of-field. It also sets the aperture to a small value.

Whether sensor-dust appears more or less on a macro photo is not clear. Certainly, there is less depth-of-field at macro distances but by focusing closer it may extend closer to the sensor even if there is less of it.

  • \$\begingroup\$ So, light from even close sources is more parallel when the focus is at infinity? \$\endgroup\$
    – mattdm
    Feb 16, 2013 at 18:08
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    \$\begingroup\$ The lens does not know how far light comes from. When focused at infinity, the light reaching the sensor is as parallel as can be for the given lens. \$\endgroup\$
    – Itai
    Feb 16, 2013 at 19:38
  • \$\begingroup\$ I think that's not true. Firstly because focusing at infinity doesn't mean you are aiming at an object at infinity, where rays reaching the sensor would be more parallel when reaching the lens, not when reaching the sensor. And second because the closer to the lens the object aimed and focused, the wider the light cone angle. physicsclassroom.com/Class/refrn/u14l5c1.gif \$\endgroup\$
    – Andrestand
    Feb 16, 2015 at 12:26
  • \$\begingroup\$ Interesting. This reminds me that the sensor is sensitive to light angle and the most significant change in designing lenses for digital is to make most rays hit the lens perpendicular to it. \$\endgroup\$
    – Itai
    Feb 16, 2015 at 17:35

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