Archived from groups: rec.photo.digital.slr-systems,rec.photo.digital (
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Mark Lauter wrote:
> "Alan Meyer" <ameyer2@yahoo.com> wrote
>
> > More importantly, with a larger diameter there is also a
> > smaller depth of field. As the depth of field becomes smaller,
> > minor errors in the shape or surface of the glass become more
> > apparent in the transmitted image.
>
> In which part of the image - the focused or unfocused bits?
On the off chance that this was not a rhetorical question,
and for the benefit of those who didn't follow the original
argument, I'll go ahead and answer it with some elaboration.
[Which is a short way of introducing my long winded claptrap]
All parts of the image are equally affected by imperfections
in the lens, but it's the focused parts that show the problem.
An imperfection in the lens scatters the light or directs it
to some place other than where it belongs relative to the
original scene. If a point in the scene is out of focus on
the camera sensor, any imperfections in the transmission of
that point are also out of focus and the imperfections are
less visible. The imperfections give us a bit more blur
within the blur.
But imagine an imperfection that causes some photons to focus
one millimeter in front of or behind the spot where they
should focus. If the depth of field at the sensor is 3
millimeters, that imperfection is invisible. If the DOF is
1 millimeter it's visible. [The right way to explain this
is with a diagram showing the difference between lines
converging from a narrow lens opening vs. lines converging
from a wide one, but the reader will have to use his
imagination for that.]
This won't be true for every type of imperfection, but it's
obviously true for abberations affecting focus, e.g., places
where the lens does not have a perfect curve of the right
parabolic (or whatever) shape.
Also, getting the lens curves exactly right is more difficult
the further you get from the center of a lens. Cheap tiny
cameras sometimes use sections of spheres for the lens because
they're easy and cheap to make and, for small lenses, they're
a close approximation to the right parabolic shape. But wide
lenses made as sections of spheres would produce horrible
results.
Maybe modern computer controlled glass cutting machinery
has eliminated the extra cost of creating more perfect lens
shapes. I don't know. But I'm guessing that it's still more
expensive to shape the wide lenses than the small ones.
Alan
/www.outbackphoto.com/dp_essentials/dp_essentials_01/field_size_3-2.gif>
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