On Wed, 19 Mar 2003 23:16:30 -0500, "Sanman" <me@you.com> wrote:

>"David McCall" <davidmccall@attbi.com> wrote in message

>news:t9%da.169840$S_4.92637@rwcrnsc53...

>> "Sanman" <me@you.com> wrote in message

>news:v7g9pfk069a461@corp.supernews.com...

>> > But what I don't understand is how 2 lenses can output the same amount

>of

>> > light, even though one of them has to spread its light over a larger CCD

>> > area. On an old film projector, when you turn the front lens to make

>the

>> > picture larger, it gets dimmer. If there were a CCD instead of a

>screen,

>> > that CCD would be getting less light per pixel. OK, the CCD and its

>pixels

>> > are large, so it is more sensitive, but doesn't that just compensate for

>the

>> > dimmer projected image? The light sensitivity of this system would not

>> > change with a larger chip. It would stay the same.

>> At the same f-stop, the light level at the chip should be the same. Of

>course,

>> a lens for a larger CCD would have to be larger than a lens for a smaller

>CCD.

>> David

>That's how I understood it too.

>Sanman

The lens would not necessarily need to be larger to

cover the larger chip (though it would likely be...),

and even if larger, it could be set to the same

relative aperture, and so pass the same brightness

of light (assuming equal subject brightness...).

A larger lens does not necessarily pass more light

(and, in fact, it is unlikely to...), but it

could possibly cover a larger sensor (though there

are so many variables that you cannot say that even

this is likely...). Best is understanding the basics

of optics, and then you will realize that the lens

is irrelevant for a simple comparison of CCD

sensitivities... (and likely irrelevant even if you

are comparing systems including the lens, since

most have similar light-passing ability, regardless

of size...).