Bob Meyer wrote:
>
> --- "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
> >
Agreed.
> > 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.
This isn't entirely true. DOF depends on focal length whenever
you are not in macro range, IOW, if you start to approach
hyperfocal distance. The DOF of a 20mm and a 200mm, both at f/8
and at a magnification of 1:100 is extremely different. Looking
through the viewfinder or your average DOF calculator will
show this without a shadow of doubt.
> > 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.
If this is meant to be done with "equivalent" focal lengths, ok.
> > Now most people think that if you switch to a wide-angle lens you
> > will get more depth of field, that's wrong.
Hmmm. See above.
> > 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.
Yes.
> > However, if you move closer to have the same subject fill the frame,
> > you depth of field will be the same.
True only for close ups. (= big magnifications)
>
> 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).
>
This is true, kind of anyway. But if you change your viewing
distance according to the size of the print, it won't matter.
Obviously this will not work for very small prints, as your
eyes can't focus close enough.
> 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?
It should, but it isn't. Actually, even at same print size, the
apparent DOF of the print made from the CCD image is bigger,
although it needs more enlargement in the printing step.
Thats also why large format photographers don't get more DOF
on a print, even though their lenses can be stopped down much
more than 35mm lenses. Ok, not really, they can and will use
the movements of their camera to their advantage.
Thomas Bantel
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