Re: Canon EOS350D/Rebel XT - Depth of field problems

Today commented courteously on the subject at hand


All Things Mopar wrote:
JPS wrote:
What you are saying is true at wide apertures, but when
you stop down enough that diffraction is very strong,
then quite a bit of depth is at the maximum sharpness
possible, even though it is less than the sharpness of
less depth at a larger aperture.

What I say is true at /any/ aperture from a /mathematical/
standpoint, as I said in the snip above. Focus is focus is
focus, easily "proven" with a simple magnifying glass
setting a blade of grass on fire by concentrating the rays
of the sun on it. There is really only one point of true
focus. The concept of "more" or "less" goes to the
definition of "depth of field" and varies not only by
aperture but by the lens focal length as has been
discussed elsewhere.

No. What you are saying would be true if geometrical optics
was correct, ie if the wavelength of light was zero.
However, it's not, and this results in diffraction effects.
A point source is imaged by a circular aperture as an Airy
function. see, for example, a nice and simple explanation
here:
http://hyperphysics.phy-astr.gsu.edu/Hbase/phyopt/cirapp2.ht
ml Things are not as clear-cut as you say.

another elitist response, i see. if you stick with the real
world, dof is quite easy to understand: it gets larger with
the bigger cof and smaller with smaller cof, it gets larger as
the distance focused increases and vice versa, and it gets
larger as the lens (any lens) focal length gets shorter and
vice versa. all the rest is theoretical bull*** that normal
people cannot measure outside a lab. non-elitists want to take
pictures and want to know how to get the best
composure/exposure, want to know the most reliable way(s) to
get good af lock, want the most of the main subject to look
in-focus, want to know how to improve dof for situations where
no obvious solutions occurs to them, and they want to know
what can be done to improve slightly blurry pictures. only
elitists want to grovel down in the dirt about diffraction
effects.

lest you think i don't understand this, you're wrong. i do.
but, i neither follow nor care about diffraction or max lines
a lens can resolve or the best/worst sensor size/type as i
cannot measure it when i am in the field taking pictures and i
cannot do anything about it when buying a camera or lens other
than to not buy one with bad qualities. so, i /still/ stand by
my earlier pronouncement: buy good glass, learn to use your
camera well, use aperture and shutter consistent with light
available and the photographic effect you want to achieve and
decide if there is or is not enough dof.

if you want to play theorticican, you could load your pda with
millions of lines of specs from all the lenses in the world,
including their diffraction and resolutions characteristics,
their barrel/pincushion distortion specs, chromatic aberration
as focal lenght and aperture change, as well as dof tables
compiled over the ages using standard and altered sizes for
cof, then - in the heat of battle - consult your pda to find
out how best to determine point-of-focus and best estimate of
dof.

or, you could just push the little button on your camera or
lens that shows an approximation of dof, decide if that works,
and move on with your life.

when people like you take me on, you really never know exactly
what my knowledge and experience is across a very broad
spectrum of subjects, even a broad spectrum of subjects under
the general heading of "photography" or the narrower one of
"digital photography" when you decide i'm full of ***. as
such, you "expert" legends-in-your-own-mind elitists are never
quite sure if i am what i say i am - a pragmatist - or a
bull*** artist or stupid or ignorant or just an ordinary
***.

but, i do know what you are - fodder for the dust bin. enjoy
talking to your buds in the exclusive elitist club.

--
ATM, aka Jerry

"Whether You Think You CAN Or CAN'T, You're Right." ? Henry
Ford
.