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Re: Detecting dust on a DSLR sensor

 
 
David J Taylor
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      06-28-2008
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


 
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David J Taylor
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      06-28-2008
Alfred Molon wrote:
> In article <g45i2h$8on$(E-Mail Removed)>, 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


 
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Paul Furman
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      06-28-2008
http://www.velocityreviews.com/forums/(E-Mail Removed)lid wrote:
> Alfred Molon wrote:
>> In article <g45i2h$8on$(E-Mail Removed)>, 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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nospam
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      06-29-2008
In article <(E-Mail Removed)>, Alfred
Molon <(E-Mail Removed)> wrote:

> In article <g4627q$dbb$(E-Mail Removed)>, 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.
 
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Paul Furman
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      06-29-2008
nospam wrote:
> In article <(E-Mail Removed)>, Alfred
> Molon <(E-Mail Removed)> wrote:
>
>> In article <g4627q$dbb$(E-Mail Removed)>, 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

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