Infinity... isn't

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

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.

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

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.

--
m-m

This is normal for long focal length lenses. Some atmospheric
conditions can require focusing past infinity. At least this is what I
read somewhere. Not sure if I ever focused passed infinity since
there's no way to know.

John

"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...

That last little bit takes you into ... The Twilight Zone. This setting is
used by people taking blurred photos of UFOs and of Elvis through the window
of McDonalds.

Seriously though, didn't lenses in the good old film days have an 'IR'
setting because infra red light focuses differently? (Maybe wrong on that!)

Paul

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.

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.

But we hates it. It means that I cannot even manually focus
for infinity.

--
--- Paul J. Gans

"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

Hi.

The best explanation I have heard of for the extra travel built into lenses
is to allow for extremes of temperature, causing the barrel to alter its
actual length.

Without that bit of spare, it might then not be possible to get infinity
focus.

Roy G

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.

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

On 22 Jan 2007 16:32:48 -0800, Annika1980 wrote:

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.

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

You've *got* to start getting your own stereo equipment. Buying
Nigel Tufnel's used gear just doesn't add up, in more ways than one.
[mpa!]

"The last release from the FreeBSD 4.x stable branch, FreeBSD 4.11
(which followed 4.8, 4.9, and 4.10), was known as the "Spinal Tap"
release" - - http://en.wikipedia.org/wiki/Up_to_eleven

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