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Is your zoom varifocal?

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Anonymous
February 2, 2005 4:31:36 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

To paraphrase an estimable poster who undoubtedly knows, "Your 'zoom'
lens is in fact 'varifocal', meaning that the point of focus changes as
you zoom. ... A true zoom lens does not do this."

Alternatively, how many "True zoom" lenses are in use by contributors to
this group?

I posted a quotation from Mr Canon's polyglot leaflet included with the
EF24-70mm f/2.8L USM lens (not cheap):
"(!) · Be sure to finish zooming before focusing. Changing the zoom ring
after focusing can affect the focus."

There's one instance of non-true zoom confessions. Any others?

Is the effect of any practical importance in the performance of
photographic tasks by an intelligent biped?



--
Frank ess

More about : zoom varifocal

Anonymous
February 2, 2005 7:11:58 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

"Frank ess" <frank@fshe2fs.com> writes:

> "(!) · Be sure to finish zooming before focusing. Changing the zoom ring
> after focusing can affect the focus."

I don't read that as saying changing zoom changes the focus. I read it as
saying that less than careful users may find that they changed the focus
when they moved the zoom ring.

I'm not commenting on whether the lens involved is a one-true zoom or a
varifocal, as I don't care. I think it's warning users that they can screw
up the focus if they mess with the zoom.

My practice with my manual camera is to focus fully zoomed, then back off
the frame the shot as I wish.
--
Phil Stripling | email to the replyto address is presumed
The Civilized Explorer | spam and read later. email from this URL
http://www.cieux.com/ | http://www.civex.com/ is read daily.
February 2, 2005 7:27:45 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

Frank ess wrote:

> To paraphrase an estimable poster who undoubtedly knows, "Your 'zoom'
> lens is in fact 'varifocal', meaning that the point of focus changes as
> you zoom. ... A true zoom lens does not do this."
>
> Alternatively, how many "True zoom" lenses are in use by contributors to
> this group?
>
> I posted a quotation from Mr Canon's polyglot leaflet included with the
> EF24-70mm f/2.8L USM lens (not cheap):
> "(!) · Be sure to finish zooming before focusing. Changing the zoom ring
> after focusing can affect the focus."
>
> There's one instance of non-true zoom confessions. Any others?
>
> Is the effect of any practical importance in the performance of
> photographic tasks by an intelligent biped?


Is that the same thing as Internal Focus (IF) type? My IF doesn't spin
the zoom but it seems not possible to zoom in for close focusing then
zoom out. That would be handy.
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Anonymous
February 2, 2005 9:55:39 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

On Wed, 2 Feb 2005 13:31:36 -0800, "Frank ess" <frank@fshe2fs.com>
wrote:

>There's one instance of non-true zoom confessions. Any others?

Both the Nikon 24-120 models are varifocal and noted as such
in the small print. Kiron made a number of openly varifocal 35mm
lenses, which were also marketed under the Vivitar brand.

--
Michael Benveniste -- mhb-offer@clearether.com
Spam and UCE professionally evaluated for $419. Use this email
address only to submit mail for evaluation.
Anonymous
February 3, 2005 12:39:05 AM

Archived from groups: rec.photo.digital.slr-systems (More info?)

On Wed, 2 Feb 2005 13:31:36 -0800, Frank ess <frank@fshe2fs.com> wrote:
>
> Is the effect of any practical importance in the performance of
> photographic tasks by an intelligent biped?

We've already seen that one poster to this newsgroup was taught to
focus at full zoom and then pull back, for better focus accuracy.
For what it's worth.

I didn't know until today that there was such thing as a zoom that
kept focus as the focal length changed, though I had occasionally
wondered.

--
Ben Rosengart (212) 741-4400 x215
Sometimes it only makes sense to focus our attention on those
questions that are equal parts trivial and intriguing.
--Josh Micah Marshall
Anonymous
February 3, 2005 4:30:57 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

On Wed, 2 Feb 2005 21:39:05 +0000 (UTC), Ben Rosengart <br+rpdss@panix.com>
wrote:

>On Wed, 2 Feb 2005 13:31:36 -0800, Frank ess <frank@fshe2fs.com> wrote:
>>
>> Is the effect of any practical importance in the performance of
>> photographic tasks by an intelligent biped?
>
>We've already seen that one poster to this newsgroup was taught to
>focus at full zoom and then pull back, for better focus accuracy.
>For what it's worth.
>
>I didn't know until today that there was such thing as a zoom that
>kept focus as the focal length changed, though I had occasionally
>wondered.

Presumably the "focus tight then pull back" technique would only work if
the lens _did_ keep its focus unchanged as it zooms/widens (i.e. if it were
a "true zoom" as someone else called it). Conversely, focusing when at the
wide end [of a "real zoom"] and then zooming in (without rechecking focus)
would be unwise, not because the focus might change in doing so, but
because the original focus may not have been precise, and this only shows
up at tight zoom.

My guess is that in the days when manual focus was the only option (or AF,
if available wasn't used much), keeping constant focus so the above trick
could be used would have had a sufficiently large "perceived benefit" to
make it worth designing in.

Now that the use of auto-focus is much more common [a possibly unfounded
assumption], it matters less if the focus changes as the lens is zoomed.
If it's easier to build a lens without worrying about keeping constant
focus, then lenses can be made cheaper.


Regards,
Graham Holden (g-holden AT dircon DOT co DOT uk)
--
There are 10 types of people in the world;
those that understand binary and those that don't.
Anonymous
February 3, 2005 7:36:25 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

Phil Stripling wrote:
>
> "Frank ess" <frank@fshe2fs.com> writes:
>
> > "(!) · Be sure to finish zooming before focusing. Changing the zoom ring
> > after focusing can affect the focus."
>
> I don't read that as saying changing zoom changes the focus. I read it as
> saying that less than careful users may find that they changed the focus
> when they moved the zoom ring.

That's an odd assumption, and an unlikely one. It means that *zooming*
can change the focus, specially from one end to the other. Minor zoom
changes may not matter so much.
>
> I'm not commenting on whether the lens involved is a one-true zoom or a
> varifocal, as I don't care. I think it's warning users that they can screw
> up the focus if they mess with the zoom.
>
> My practice with my manual camera is to focus fully zoomed, then back off
> the frame the shot as I wish.

I'd check that you don't lose focus when doing that, Phil. From a lens
design point of view, allowing the focus to change somewhat with zooming
is common, as maintaining accurate focus while zooming puts considerable
constraints on lens performance. Particularly with AF lenses, it
doesn't really matter if the focus changes, and if it provides a better
lens performance, why not let it change?

Colin
Anonymous
February 4, 2005 4:13:46 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

"Ben Rosengart" <br+rpdss@panix.com> wrote in message
news:slrnd02i3p.hrp.br@panix5.panix.com...
> On Wed, 2 Feb 2005 13:31:36 -0800, Frank ess <frank@fshe2fs.com> wrote:
>>
>> Is the effect of any practical importance in the performance of
>> photographic tasks by an intelligent biped?
>
> We've already seen that one poster to this newsgroup was taught to
> focus at full zoom and then pull back, for better focus accuracy.
> For what it's worth.
>
> I didn't know until today that there was such thing as a zoom that
> kept focus as the focal length changed, though I had occasionally
> wondered.
>
> --
> Ben Rosengart (212) 741-4400
> x215
> Sometimes it only makes sense to focus our attention on those
> questions that are equal parts trivial and intriguing.
> --Josh Micah Marshall

On the Olympus E20, i always zoomed right in, then zoomed out to achieve
good composure & clicked the shutter...all without loosing focus! Eyes
crystal sharp, yet the 28-105 on my canon 20D seems soft no matter what I do
:( 
!