Dust on CCD

[billsol]

Why does the dust on my CCD only appear when I use a small apeture (f22)? When I shoot wide open (f1.8) you can not see the dust. Dust on my CCD should have nothing to do with my lens right? In theory I should be able to see when taking a picture with no lens (assuming the exposure values are the same in all three cases). Somebody please help me figure this out.

Thanks,

Bill
--
http://www.wrsphotography.com

 

[psychmike]

Hmmm, good question. Let me take a stab at it, but I'd like to hear what others think too.

The dust diesn't actually sit on the sensor, but on the AA filter above the sensor, so there is SOME distance between the dust and the sensor. With small apertures, the light is coming through a smaller hole and is travelling in a narrower path that converges on the sensor with a less acute angle. Consequently, anything in its path will leave a more distinct shadow. With larger apertures, light is coming through a larger hole. As it passes through the lenses, it converges at a greater angle and can 'get around' the dust a bit, making the shadow less distinct.

I'm just speculating though! Anyone else?

M.

 

[Thomas Comerford]

Your lens affects how light reaches the sensor, and that will determine how obvious the dust is. Try this:

Set your camera to aperture priority, f/22.
Hold a pen up against the front of the lens.
Look through the viewfinder.
Press the DOF-preview button.
Spot the difference!

Why does the dust on my CCD only appear when I use a small apeture
(f22)? When I shoot wide open (f1.8) you can not see the dust.
Dust on my CCD should have nothing to do with my lens right? In
theory I should be able to see when taking a picture with no lens
(assuming the exposure values are the same in all three cases).
Somebody please help me figure this out.

Thanks,

Bill
--
http://www.wrsphotography.com

 

[timeFreezer]

Use the on board flash on the camera and take someones picture making him/her stand in front of a white wall. You will notice a shadow on the wall. This is analogous to a small aperture. i.e. we consider the person in the picture as dust and the wall as the sensor.

Now if you were to take the same picture with 2 or more lights, you will hardly notice a shadow on the wall. As shadow cast by one light source is cancelled by another light source. This is what it is like when the aperture is set to wide open. For the dust wide aperture is like a big light source that cancels shadows cast by itself from another point.

Hope you got it.

TF

--
Photography: Mankind's most successful effort in freezing time!

 

[rschoemaker]

http://forums.dpreview.com/forums/read.asp?forum=1034&message=12257625