Re: Marks on EOS20D photos - tricky diagnosis...

In article <Pine.LNX.4.60.0507091824590.14389@xxxxxxxxxxxxxxxxxxxx>, Stu Carter <stu@xxxxxxxxxxxxxxxxxxx> writes 
Hi,

I've been seeing marks on all photos I've been taking with my EOS20D, but the diagnosis is proving very tricky. They appear to be related to the lens (taking the lens off removes the marks), but not physically attached to the lens (rotating the lens in its bayonet does not move the marks).

It's probably easier if you go and look at the entry I made with sample images here: http://blog.pygmygoat.net/index.php?p=586

Can anyone suggest what might be causing these marks, and how to stop it. Unfortunately, I don't have any other lenses to swap out and test.

Thanks in advance for any help you can offer.

Stu 

Stu,

I'm fairly certain that they are marks on the sensor. Firstly, they look very similar to some I got on mine (10D); secondly, it is almost* impossible for marks on the surface of a lens to show up sharply on a photo; thirdly, this is confirmed by the fact that the spots do not move when the lens is rotated.

Why no spots when you remove the lens? Well - and you will understand this is somewhat speculative on my part - when you take a photo normally (i.e. with a lens in place) the light striking the sensor is quite directional; thus if there is a speck of dirt it will cast a shadow. When you take an exposure with no lens present, the light is a lot less directional, thus the shadow cast by the speck will be weaker or non-existent.

Here's a suggestion - repeat the "no-lens" exposure, but using a very small light source a long way away. You will probably see the marks again.

Or just clean the sensor, of course....

David
--
David Littlewood
.

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