Detecting dust on a DSLR sensor

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
 >> Stay informed about: Detecting dust on a DSLR sensor 

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 >> Stay informed about: Detecting dust on a DSLR sensor 

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
 >> Stay informed about: Detecting dust on a DSLR sensor 

On 2008-06-27, Alfred Molon <alfred_molon.RemoveThis@yahoo.com> 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
 >> Stay informed about: Detecting dust on a DSLR sensor 

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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-- e-meil: Remove FreeLunch.
 >> Stay informed about: Detecting dust on a DSLR sensor 

In article <g45i2h$8on$1@news.acm.uiuc.edu>, 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
 >> Stay informed about: Detecting dust on a DSLR sensor 

Alfred Molon wrote:
> In article <g45i2h$8on$1@news.acm.uiuc.edu>, 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
 >> Stay informed about: Detecting dust on a DSLR sensor 

Alfred Molon wrote:
> In article <g45i2h$8on$1@news.acm.uiuc.edu>, 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.
 >> Stay informed about: Detecting dust on a DSLR sensor 

Alfred Molon wrote:
> In article <g45i2h$8on$1@news.acm.uiuc.edu>, 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
 >> Stay informed about: Detecting dust on a DSLR sensor 

user.RemoveThis@domain.invalid wrote:
> Alfred Molon wrote:
>> In article <g45i2h$8on$1@news.acm.uiuc.edu>, 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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 >> Stay informed about: Detecting dust on a DSLR sensor 

In article <g4627q$dbb$1@news.acm.uiuc.edu>, says...

> The dust is not on the sensor. Its on the glass in front of
> the sensor, well in front.

Why do then manufacturers recommend not to use a brush to remove the
dust giving as a reason that the sensor might be scratched?
--

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
 >> Stay informed about: Detecting dust on a DSLR sensor 

In article <MPG.22d0dc46faed64fb98bd12.DeleteThis@news.supernews.com>, Alfred
Molon <alfred_molon.DeleteThis@yahoo.com> wrote:

> In article <g4627q$dbb$1@news.acm.uiuc.edu>, says...
>
> > The dust is not on the sensor. Its on the glass in front of
> > the sensor, well in front.
>
> Why do then manufacturers recommend not to use a brush to remove the
> dust giving as a reason that the sensor might be scratched?

they mean the glass can be scratched.
 >> Stay informed about: Detecting dust on a DSLR sensor 

nospam wrote:
> In article <MPG.22d0dc46faed64fb98bd12.DeleteThis@news.supernews.com>, Alfred
> Molon <alfred_molon.DeleteThis@yahoo.com> wrote:
>
>> In article <g4627q$dbb$1@news.acm.uiuc.edu>, says...
>>
>>> The dust is not on the sensor. Its on the glass in front of
>>> the sensor, well in front.
>> Why do then manufacturers recommend not to use a brush to remove the
>> dust giving as a reason that the sensor might be scratched?
>
> they mean the glass can be scratched.

And I think they exaggerate the danger. A wet cleaning seems just as
likely to drag a spec of diamond dust across the glass as a clean dry
nylon brush.

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

all google groups messages filtered due to spam
 >> Stay informed about: Detecting dust on a DSLR sensor