alanS
Dr Dust
Reged: 30/09/2005
Posts: 3653
Loc: Up North, England.
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Well, as far as I can gather if you have a large aperture there's plenty of room for the light to get around any obstructions so if there are any they either don't appear or appear to be fuzzy.
If you have a small aperture the light is less able to avoid the obstruction and therefore it is defined more sharply.
So, contamination may not show up at all at f1.4, may appear and be fuzzy at f11 and may be sharply defined at f18.
I often do a test at the smallest possible aperture before going out for the day and if something is sharply defined I clean it away. I find this preferable to cloning out later and risking a shot I may want to keep being ruined, although to be honest I've only ever had to delete maybe half a dozen shots because I wasn't happy with them after cloning out contamination.
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Alan's defence lawyer claimed that "Booze played no part in his typo's."
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El Sid
Going potty
Reged: 14/04/2003
Posts: 9470
Loc: Sussex-by-the-Sea
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Quote:
Why does stopping dows show up dust? I can see why it would if it was on the lens, but I've never understood the phsyscs/science behind it to highlight dust on the sensor?
A sort of explanation...
The sharper the light source, the narrower the beam of light becomes and the less it spreads. At small apertures the light source is a very small point, the light rays are then very close to parallel and as a result any speck on the sensor either blocks them more or less totally with respect to the photo-sites underneath or doesn't affect them at all. As a result the speck appears very clean edged.
With a large aperture the light is far less parallel as the source is less of a point. As a result the dust interacts with the light much less effectively. Because the incident angle of the light is much more varied and shallow the speck casts a shadow over more photo-sites, making it appear larger than it is, while at the same time other light rays pass behind the speck reducing the intensity of the shadows. This reduces the contrast of the shadow effectively rendering it semi-transparent.
The net result of all this is that at wide apertures dust appears as a large fuzzy blob becoming a small sharp speck as aperture reduces. The greater the edge contrast the sharper it appears which is also why it's far more visible in large plain toned light areas such as sky and virtually invisible in dark and or heavily patterned areas...
I hope this explanation is slightly clearer than mud.....
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Nigel
Completely BSRIPN 
ElSid Gallery
A camera in the hand is more fun than one in the cupboard........
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beejaybee
Marvin
Reged: 18/07/2007
Posts: 4980
Loc: Really Here In Name Only
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Quote:
Why does stopping dows show up dust?
Because the front surface of the anti-alias filter is some distance ahead of the focal plane ... there's the microprism array and the Bayer filter matrix to squeeze in as well before the light reaches the actual light sensor array.
When well stopped down, dust casts a small sharp shadow because the rays of light which just miss it are almost parallel, whichever part of the lens aperture they came through. When the lens is opened up, the shadow cone becomes wider and shorter, so that the dust shadow is large and diffuse - much harder to spot by casual inspection.
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Terrywoodenpic
A whiff of silicon...
Reged: 21/01/2006
Posts: 370
Loc: Saddleworth UK
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I suspect one day every lens will come with its own sealed and attached sensor that is also matched for CA, distortion and vignetting. Of course It will also need an inbuilt semi silvered mirror pellicle. for directing the image to the viewfinder. The shutter would also need to be purely solid state electronic.
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63 happy photo years from amateur to professional and back to amateur
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Bone_Idle
Mr Maybe
Reged: 28/07/2006
Posts: 1392
Loc: Bradford
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Thanks for the explanations, it makes sense now!
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Thanks
Nick
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