Detecting dust on a DSLR sensor

I heard of a technique - take a photo at the smallest aperture (F22 or
higher) - but I don't know if it works. What is the best way to detect
dust, short of opening the camera and inspecting the image sensor?
--

Alfred Molon
------------------------------
Olympus 50X0, 8080, E3X0, E4X0, E5X0 and E3 forum at
http://tech.groups.yahoo.com/group/MyOlympus/
http://myolympus.org/ photo sharing site

Alfred Molon wrote:
I heard of a technique - take a photo at the smallest aperture (F22 or
higher) - but I don't know if it works. What is the best way to detect
dust, short of opening the camera and inspecting the image sensor?

Yes, it works, and it may often be the first way you detect dust! I have
found that a clear blue sky is quite sensitive to dust spots.

David

On 2008-06-27, Alfred Molon wrote:
I heard of a technique - take a photo at the smallest aperture (F22 or
higher) - but I don't know if it works.

So screw up your courage and (heavens!) try it.

What is the best way to detect
dust, short of opening the camera and inspecting the image sensor?

Take a shot at your smallest aperture against a plain, light background.
The sky's quite convenient and, I believe, generally available.

--
savvo orig. invib. man

David J Taylor wrote:
Alfred Molon wrote:
I heard of a technique - take a photo at the smallest aperture (F22 or
higher) - but I don't know if it works. What is the best way to detect
dust, short of opening the camera and inspecting the image sensor?

Yes, it works, and it may often be the first way you detect dust! I have
found that a clear blue sky is quite sensitive to dust spots.

David



Or a white sheet of paper that is very very out of focus. This means,
at f/22 or greater, like the lens (preferably a tele) is focused at
infinity and the paper is two inches from the lens. Be sure it comes
out gray, not white.

This works fine.

Doug McDonald

In article , says...

Or a white sheet of paper that is very very out of focus. This means,
at f/22 or greater, like the lens (preferably a tele) is focused at
infinity and the paper is two inches from the lens. Be sure it comes
out gray, not white.

Just a question or two - why do you need to set the camera to such a
small aperture and why is dust less visible at larger apertures?
--

Alfred Molon
------------------------------
Olympus 50X0, 8080, E3X0, E4X0, E5X0 and E3 forum at
http://tech.groups.yahoo.com/group/MyOlympus/
http://myolympus.org/ photo sharing site

Alfred Molon wrote:
In article , says...

Or a white sheet of paper that is very very out of focus. This means,
at f/22 or greater, like the lens (preferably a tele) is focused at
infinity and the paper is two inches from the lens. Be sure it comes
out gray, not white.

Just a question or two - why do you need to set the camera to such a
small aperture and why is dust less visible at larger apertures?

You need to create, as near as possible, a pinhole lens. This is so that
the divergence of the rays between the dust and the focal plane is
minimised.

David

Alfred Molon wrote:
I heard of a technique - take a photo at the smallest aperture (F22 or
higher) - but I don't know if it works. What is the best way to detect
dust, short of opening the camera and inspecting the image sensor?

As you say f/22, out of focus, shoot any flat, regular, bright surface
(wall, sky, whatever).

You can even make it a real long exposure and wave the camera around...
the spots won't move.

Remember that the spot is on the opposite corner of the sensor from
where it appears in the frame.

--
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-- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm
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-- e-meil: Remove FreeLunch.

Alfred Molon wrote:
In article , says...

Or a white sheet of paper that is very very out of focus. This means,
at f/22 or greater, like the lens (preferably a tele) is focused at
infinity and the paper is two inches from the lens. Be sure it comes
out gray, not white.

Just a question or two - why do you need to set the camera to such a
small aperture and why is dust less visible at larger apertures?

Don't be confused.

--
-- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm
-- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm
-- [SI] gallery & rulz: http://www.pbase.com/shootin
-- e-meil: Remove FreeLunch.

Alfred Molon wrote:
In article , says...

Or a white sheet of paper that is very very out of focus. This means,
at f/22 or greater, like the lens (preferably a tele) is focused at
infinity and the paper is two inches from the lens. Be sure it comes
out gray, not white.

Just a question or two - why do you need to set the camera to such a
small aperture and why is dust less visible at larger apertures?

An actual INTERESTING question, with a real answer available!

The dust is not on the sensor. Its on the glass in front of
the sensor, well in front. If you use a wide-open lens,
a large cone of light comes into each pixel on the sensor.
Most of that light from the big cone will miss the spot of dust.

But if you use f/22 or even better f/32 or f/45, etc. only a
tiny pencil of light will come from the lens to each pixel. A
tiny dust spot will block a large fraction of that light.


This would not apply to a spot of dust sitting right on the
surface of film, for example. I should add that it also does apply
to dust, spots, and even scratches on the front of your lens. A
large numerical f/number makes such spots more obvious.

Doug McDonald

lid wrote:
Alfred Molon wrote:
In article , says...

Or a white sheet of paper that is very very out of focus. This means,
at f/22 or greater, like the lens (preferably a tele) is focused at
infinity and the paper is two inches from the lens. Be sure it comes
out gray, not white.

Just a question or two - why do you need to set the camera to such a
small aperture and why is dust less visible at larger apertures?

An actual INTERESTING question, with a real answer available!

The dust is not on the sensor. Its on the glass in front of
the sensor, well in front. If you use a wide-open lens,
a large cone of light comes into each pixel on the sensor.
Most of that light from the big cone will miss the spot of dust.

But if you use f/22 or even better f/32 or f/45, etc. only a
tiny pencil of light will come from the lens to each pixel. A
tiny dust spot will block a large fraction of that light.


This would not apply to a spot of dust sitting right on the
surface of film, for example. I should add that it also does apply
to dust, spots, and even scratches on the front of your lens. A
large numerical f/number makes such spots more obvious.

And strangely, very small f-number lenses with a point source of light
out of focus will also work like a pinhole focusing dust specs inside
the lens and on the sensor.

--
Paul Furman
www.edgehill.net
www.baynatives.com

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