Infinity... isn't

Suddenly, without warning, Paul Rubin exclaimed (22-Jan-07 10:49 PM):
jmc writes:
I've been noticing that on my Tamron 17-50mm lens, in manual focus
mode if I run the ring out to the infinity end, it goes out of
focus. Infinity focus is just in from the end of the ring's turn. If
that makes sense... Is this normal?

It's not that uncommon, and it's normal in certain types of fancy
lenses and telescopes, but for a random consumer lens like that 17-50
it means the infinity stop slightly out of adjustment.

Anyway, it shouldn't be a real problem with normal MF technique.
Basically focus the lens all the way to the infinity stop (which can
mean slightly past actual infinity). You can develop a reflex for
doing that simultaneously with lifting the camera to your eye. Camera
makers like Nikon and Pentax make their lenses all consistent in the
direction of rotation towards infinity so that you can do that by
reflex. However they didn't all make the same choice, so clockwise
and counterclockwise are sometimes called "the Nikon direction" and
"the Pentax direction" (I'm not sure which one Canon uses).

Once the camera is at eye level, focus away from infinity while
watching the finder screen. You can do this pretty quickly (go for
speed and smoothness rather than precision) and you will see the image
come into focus. Stop turning when that happens, which means you will
overshoot a bit. Then turn the lens in the other direction more
carefully, stopping just at the point of correct focus using the split
image prism (oh yes, you need a focusing screen with split image
prism, normal on MF SLR's but a special upgrade for most DSLR's, sigh).

You should not overshoot in this second operation. Focusing is three
steps: 1) set lens to infinity by feel, without having to look at it;
2) coarse focus, overshooting focus point; 3) precise focus, just a
slight adjustment to the coarse focus. You should not need to hunt
back and forth or otherwise mess around like beginners often do. This
technique takes a little bit of practice but once you get the hang of
it, at least on non-moving subjects you can focus almost as fast as an
AF system can.

Interestingly, I do consider myself a DSLR beginner (haven't had a real
lens since 1998 and that camera was 20 years old and full manual).

Anyway, interestingly, this is just about the exact method I was using
last night, 'cept by the end I could get to what looked like the correct
focus (no split image, and very very dark) without overshooting when
starting from "beyond infinity".

jmc

Suddenly, without warning, Dave Cohen exclaimed (23-Jan-07 2:36 AM):
M-M wrote:
In article ,
jmc wrote:

Infinity focus is just in from the end of the ring's turn. If that
makes sense...


Same with my Nikon lenses. I was wondering about that also.

I used to see that with a third party zoom on my canon slr. Surprised it
would happen with original lenses though.
Dave Cohen

It's not an original lens, it's a Tamron 17-50mm.

jmc

Suddenly, without warning, M-M exclaimed (23-Jan-07 10:14 AM):
In article ,
Paul J Gans wrote:

It is done on purpose. I believe that the autofocus mechanism
has to be able to go past the best focus in order to know
that that's the best focus.


This appears to be the most sensible explanation so far.


Agreed. So it's not broke after all Man, getting a lot of user
errors with this new camera!

Maybe I should put it away and just use my backup cam (coolpix 5600)

....

Nah! I'm having too much fun with the Rebel! Helps that I love to
learn new things, and this camera is that!

jmc

jmc wrote:
I've been noticing that on my Tamron 17-50mm lens, in manual focus mode
if I run the ring out to the infinity end, it goes out of focus.
Infinity focus is just in from the end of the ring's turn. If that
makes sense...

Is this normal? I'm new to DSLRs and their lenses. It's really
annoying when trying to do sky photography, makes it nearly impossible
(and somewhat a matter of luck) to get sharp night images.

Thanks for any help.

jmc
A zoom lens zooms. If you focus on a particular object and then alter
the zoom, you will lose focus. Zoom lenses for still cameras aren't
designed to maintain focus with zooming, unlike video or movie cameras,
where the lens is required to maintain focus while zooming.

A side effect of this is that the infinity setting will shift slightly
depending on the zoom position. It's not a fault, it's just
characteristic of still camera zooms. My 17-85 mm USM IS lens does it
too - there's about a 3mm difference in the infinity position between
the 17 and the 85 mm ends. 85 mm agrees with the mark, the 17 mm is
about 3mm short of the mark.

Focus carefully on a distant object in daylight and note or mark the
position of the focus scale at the zoom setting you want to use at
night, and use that.

Colin D.

--
Posted via a free Usenet account from http://www.teranews.com

"Annika1980" wrote in message
oups.com...

jmc wrote:
images.

Tell me about it! My stereo goes all the way to 11.


Oh ! No !

There must be something wrong with mine, it only goes to 5.

I really will have to take it back, and get it exchanged for one that goes
to at least 10.

Roy G

Toby wrote:
This should not happen on a short lens like the 17-50

Toby

"jmc" wrote in message
...
I've been noticing that on my Tamron 17-50mm lens, in manual focus mode if
I run the ring out to the infinity end, it goes out of focus. Infinity
focus is just in from the end of the ring's turn. If that makes sense...

Is this normal? I'm new to DSLRs and their lenses. It's really annoying
when trying to do sky photography, makes it nearly impossible (and
somewhat a matter of luck) to get sharp night images.

Thanks for any help.

jmc



I found that the only cure (on a Canon 30D) was test shots,
checked at extreme blowup on the back viewing screen. This works,
so long as the lens (manual setting of course) does not slip.

Doug McDonald

"jmc" wrote in message
...
I've been noticing that on my Tamron 17-50mm lens, in manual focus mode if
I run the ring out to the infinity end, it goes out of focus. Infinity
focus is just in from the end of the ring's turn. If that makes sense...

Is this normal? I'm new to DSLRs and their lenses. It's really annoying
when trying to do sky photography, makes it nearly impossible (and
somewhat a matter of luck) to get sharp night images.

Not unusual for a telephoto, because thermal expansion is a factor in
focusing lenses with a long focal length. For a short lens though it's
either a problem with the lens itself, as others have mentioned, or less
likely a problem with the camera's flange focal distance. Swap lenses with
another camera and see if the problem follows the lens or not.
--
Mike Russell
www.curvemeister.com/forum/