20D Canon focus advice

"Cockpit Colin" wrote in message
...
Hi all,

I have a nagging suspicion that the focus on my new 20D isn't all that it
should be. Do any of you have any suggestions on the best way to test
this?
I was thinking of photographing a sheet of A4 "Math - graph" type paper.

If the camera's focus is out, will it be out consistently when focusing on
close and distant objects? Should I have the lens wide open or stopped
down
for the test. Is it possible for an image to be in focus through the
viewfinder, and yet be out of focus at the CMOS sensor? Any other thoughts
(I only have the one kit lens at this time, so I can't do any lens
comparisons).

Thanks for your input.

Cheers,

CC



Stop worrying. If you cannot tell from normal shots then it is fine. But
'geek and ye shall find' However if the measurebator urge is too strong,
then go on over to www.dpreview.com and search on bf and ff and you'll find
your comrades. Indeed there is a lot of very useful information on dpreview
also try www.photozone.de . There are plenty of charts on dpreview which you
can get off on - shoot. You should know that it if there is a problem it
could be the lens, the camera or both. Some wide angles lenses do not focus
too well at a distance. Things look sharper in the viewfinder than on a PC
screen on which they tend to look soft and even unsaturated and using a
wide-angle cheapo lens will make them look even softer and duller. Also, you
MUST post process the images to reveal their potential - that is an art in
itself.

You need some original images from the *same* camera and lens to compare
with yours. Try on www.pbase.com use their search function - look for
full-size unprocessed images.

Stop worrying. It is most likely you simply don't know much yet...

"Cockpit Colin" wrote in message
...

"John Passaneau" wrote in message
...
At F29 or so almost any lens will be un-sharp. This is due to diffraction
from the small aperture. It is not a lens defect, it is a law of physics.
At
F29 or so the depth of field really means "every thing is fuzzy". The
best
aperture to check for focus errors would be wide open or if the lens not
of
the highest quality, at 1 or 2 stops down from the maximum aperture. Also
with a zoom lens use the longest focal length available. If your basing
the
possible focus problem on a shot at the smallest possible aperture of
your
lens, I would expect it to be fuzzy.

Thanks for that. Unfortunately the "nature scene" wasn't intended as a
focus
test - it was one nice scene that I wanted as a "keeper". I wanted the
best
possible DOF so I went for smallest aperture - lowest ISO - shot in RAW -
put the camera on a tripod - and set the camera for a delayed shot.

I knew that a large aperture means poor DOF - I didn't know that small
aperture has issues as well (something else to add to my bucket of
wisdom!).

So far, using an unsharp mask @ 300% @ 0.3 as suggested by Colin D seems
to
be producing a good result (I didn't know you could select anything under
1
pixel).

I'll have to do some more tests to see it F8 to F11 are any better for me.

Cheers,




Try www.bobatkins.com Dr Atkins has lots of info on lenses, dslrs, etc....
or try www.photo.net which he now runs.

David Littlewood wrote:

snip
and of course f/45 in a 150mm lens is
entirely different from f/45 in a 50mm lens, for two reasons; one, that
diffraction is a function of the actual diameter of the aperture, and a
longer lens has a larger aperture for the same f-number than a short
lens;

Not true. I used to think this until I actually went and studied the
physics behind it, but diffraction is purely a function of f-number, not
absolute diameter.

Very interesting. I dug out my old Ilford Manual of Photography (that
shows how long I've been involved in this caper!) and looked up Airy
Disc calculations, and you're right. I am guilty of not reading the
formula correctly umpteen years ago. Since d, the aperture diameter, is
a component of the equation, I assumed that diffraction was simply
proportional to d - but I overlooked that v (image distance) is also in
the equation. Since v/d is focal length over diameter, and v/d =
f-number, then diffraction is indeed purely a function of F-number.

My old Dad used to say "the more you know, the more you know you don't
know." After 50 years, I am still learning ... thanks.

Colin D.

David Littlewood wrote:




Thanks for this. Unfortunately, split prisms and micro-fresnel screens
are not much improvement for what I want to do.

If this page explains how to remove the factory screen, send to to Maxwell
precision optics and have him make you a new screen. I'm don't know if
you've ever used one of his screens, they are amazing. Very easy to see the
focus plane yet still is a very bright screen. I doubt you'd need any kind
of focus aide with it. Give him a call and tell him what your needs are,
I'm sure he can come up with a very good solution.

--

Stacey