--- "Mr. Bill" <[EMAIL PROTECTED]> wrote:
> Ok, let's get everything straight here.
>
> 1) If your image sensor area (film or CCD) is
> bigger, you need a longer
> focal-length lens to cover it, so a 24mm lens is
> normal for 110, a 50mm
> lens is normal for 35mm, an 80mm lens is normal for
> 6x6 a 165mm lens is
> normal for 4x5 (need I go on). If you have a 1/3"
> ccd an 8.4mm lens
> would be considered "normal"
>
> 2) Although these normal lenses will fill the
> different sized frames
> with the same object at the same distance they are
> delivering very
> different subject magnifications
>
> 3) Depth of Field is controlled by two things and
> two things only.
> Subject magnification and aperture. It doesn't
> matter whether you
> increase your subject magnification by moving closer
> or by using a lens
> with a longer focal length your depth of field will
> be reduced. In
> addition, if you use a camera with a larger format
> you are magnifying
> the image more so you will have less depth of field.
> Now most people
> think that if you switch to a wide-angle lens you
> will get more depth of
> field, that's wrong. If you switch to a wide angle
> lens and do not move
> closer to your subject, you are decreasing subject
> magnification and
> your depth of field will increase. However, if you
> move closer to have
> the same subject fill the frame, you depth of field
> will be the same.
OK, all this matches what I learned, but I think it
leaves out a final step. Iwas taught, a long time ago,
that DOF charts, and the DOF scales on lenses, are
scaled assuming a certain level of PRINT size.
Magnification doesn't stop with the image recorded on
film (or CCD) but with the final *viewed* image, be it
a print, a projected transparency, or a slide on a
lightbox.
The same image on film, printed at 5x7 and 16x20, will
appear to have significantly less DOF in the larger
print. (Because of circle-of-confusion
magification--a point on the film that appears as a
point at 5x7 may appear as a circle on the 16x20).
Assuming this is true, and I strongly believe it is,
then your conclusion (below) doesn't hold up:
> This explains why digital cameras have a lot of
> depth of field, there
> CCD's are so small, subject magnification is very
> low and depth of field
> is great.
Assuming I enlarge the image on film and from the CCD
to the same final size, on paper, then total
magnification is equal, it seems to me. So shouldn't
DOF be equal, too?
=====
Bob Meyer
Life is uncertain. Eat dessert first.
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