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20D Canon focus advice



 
 
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  #21  
Old August 22nd 05, 08:31 AM
Brian Baird
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Or you can just turn up the in-camera sharpening...

....Did you ever bother to look at the custom functions, Doug?
--
http://www.pbase.com/bcbaird
  #23  
Old August 22nd 05, 10:48 AM
Tony Polson
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Randall Ainsworth wrote:

In article , pixby
wrote:

Since buying Canon gear I've seen many photographers with 20D problems
and experienced some severe problems with my own gear which should not
have happened. People seem to think quality is related to price. Canon
have disproved that with their 20D. Those who don't get the faults,
don't push their cameras.


There are too many people with money who buy cameras like the 20D and
have no knowledge of photography.



'Twas ever thus.

A great many people bought high end 35mm SLRs like the Canon EOS 1N/V
and the Nikon F5, only to use them with lousy 28-200mm lenses for
taking family snapshots.

It's like people who buy a Porsche and use it only for shopping trips.

It's called "freedom".

;-)

  #24  
Old August 22nd 05, 11:36 AM
Colin D
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David Littlewood wrote:

In article , Colin D
writes

The lens stops down to f/22 at 18mm, and f/38 at the 55mm end - where
you had the lens set - and at that aperture the image will be badly
affected by diffraction, so it will not be anywhere near as sharp as at
f/11 or f/8. Basically, that sort of aperture is unusable for normal
photography.

True-ish; f/38 will be very much below optimum for anything other than a
pinhole or bottle end. However, it may well be obligatory to get the
depth of field required in high ratio macro work (don't forget that that
diffraction is determined by effective aperture (which is measured
aperture x (1+m)).

Also, in large format work, f/45 is quite routine. Yes, it is
sub-optimum, but at least you have a lot of resolution to spare.

snip

True also, but the lens the OP is talking about is a zoom 'macro', not
capable of high-ratio work, 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; and two, there is less magnification of the image required from
the larger negative.

regards,

Colin D.
  #25  
Old August 22nd 05, 12:43 PM
David Littlewood
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In article , Slack
writes
David Littlewood wrote:

Unfortunately, current AF bodies (except top-of-range models like
Canon's 1-series) are stuck with screen almost totally unhelpful for
manual focussing. This is, BTW, a major problem with using DSLRs for
photomicrography, one of my main interests.
David



That should've read, come stock with an AF screen
http://www.keoptics.com/Canon20D.htm

Although, it will add some $$ to the orig purchase price, at least
there is an option available.


Thanks for this. Unfortunately, split prisms and micro-fresnel screens
are not much improvement for what I want to do. The only satisfactory
screen is the clear spot/cross hair type, which gives precise focussing
in extreme macro and photomicrography. The "I" screen in the 1-series
film cameras is the one I use in those.

If it doesn't have a changeable screen, bugger.

David
--
David Littlewood
  #26  
Old August 22nd 05, 12:50 PM
David Littlewood
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In article , Colin D
writes


David Littlewood wrote:

In article , Colin D
writes

The lens stops down to f/22 at 18mm, and f/38 at the 55mm end - where
you had the lens set - and at that aperture the image will be badly
affected by diffraction, so it will not be anywhere near as sharp as at
f/11 or f/8. Basically, that sort of aperture is unusable for normal
photography.

True-ish; f/38 will be very much below optimum for anything other than a
pinhole or bottle end. However, it may well be obligatory to get the
depth of field required in high ratio macro work (don't forget that that
diffraction is determined by effective aperture (which is measured
aperture x (1+m)).

Also, in large format work, f/45 is quite routine. Yes, it is
sub-optimum, but at least you have a lot of resolution to spare.

snip

True also, but the lens the OP is talking about is a zoom 'macro', not
capable of high-ratio work,


True

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.

and two, there is less magnification of the image required from
the larger negative.

True, which is why I said "but at least [with LF] you have a lot of
resolution to spare". This is probably why you think (and I used to
think) resolution depends on diameter: the lower magnification factor
for a large film means that a higher f-number will be satisfactory.
Rules of thumb exist, along the lines of "anything above 4mm (i.e. f/38
on a 150mm lens) is fine". They don't entirely cover the position
though.
--
David Littlewood
  #28  
Old August 22nd 05, 08:50 PM
John Passaneau
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"David Littlewood" wrote in message
...
In article , Brian Baird
writes

snip

Unfortunately, current AF bodies (except top-of-range models like Canon's
1-series) are stuck with screen almost totally unhelpful for manual
focussing. This is, BTW, a major problem with using DSLRs for
photomicrography, one of my main interests.

It (interchangeable screens) is also one of the more exciting (to me)
features of the EOS 5D, if the leaks prove accurate (and if one of the
screens is a useful one with clear spot and cross-hair).

If you still have problems, send the camera with your lenses to a Canon
repair facility. People have reported success after the techs re-
calibrated their body and lenses.


David
--
David Littlewood


Boy do you have that right. The screen in my Digital Rebel sucks for
photomicrography. I can still use it but its not a pleasant experience. I
miss the screens I used in my F1's. But on the other hand the sensor crop
means my bellows mount macro lens give me a higher magnification.



--
John Passaneau
State College Pa.



  #29  
Old August 22nd 05, 09:01 PM
John Passaneau
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"Cockpit Colin" wrote in message
...
Thanks all,

My suspicions were aroused when I took a photo of a stream and surrounding
vegetation whilst on a walking track (hand held) - it just looked plain
blury. I went back later on that day and set the camera up on a tripod. On
my 20D with kit lens (zoomed all the way out) At 100 ISO and F29? (or
there
abouts - stopped down as far as it would go) I ended up with a exposure of
2.5 sec (not a breath of wind, so not worried about anything moving in the
breeze).

Back on the PC EVERYTHING just didn't appear to be as sharp as I would
have
expected (shot in RAW too). The whole thing just appeared hard on the
eyes -
it wasn't until I ran an unsharp mask at 200% over it that it started to
look "normal". Not sure if this is the camera, or if I'm expecting too
much.
I did take some photos of some graph paper later on that night, and it
"sort
of" appeared OK (difficult to tell).

I could send it in for checking, but it probably means doing without it
for
a couple of weeks

Cheers,

CC



"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




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.


--
John Passaneau
State College Pa.



  #30  
Old August 22nd 05, 11:10 PM
Cockpit Colin
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"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,



 




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