focal length changed for a prime lens ?

today i looked though my prime lens and was astonished. while i
changing the focus, the subject changed its size a alittle in the
viewfind! actually, i found that when focus changes nearest distance
to infinite, the subject size reduces a very small scale. i am not
sure if it was that before. is this a problem?

my lens is a Ziess 50mm prime. thanks.

-
woody

Steven Woody wrote:
today i looked though my prime lens and was astonished. while i
changing the focus, the subject changed its size a alittle in the
viewfind! actually, i found that when focus changes nearest distance
to infinite, the subject size reduces a very small scale. i am not
sure if it was that before. is this a problem?

No. All lenses exhibit this "effect" as focus is achieved by moving
lens elements fore and aft causing a very minor change in magnification
between the subject plane and the focus plane. (I might not be
explaining it in the best technical manner...).

Cheers,
Alan

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Steven Woody wrote:
today i looked though my prime lens and was astonished. while i
changing the focus, the subject changed its size a alittle in the
viewfind! actually, i found that when focus changes nearest distance
to infinite, the subject size reduces a very small scale. i am not
sure if it was that before. is this a problem?

my lens is a Ziess 50mm prime. thanks.

-
woody

This happens in lenses that focus by extending the front element. This
means the extension is altered, which literally changes the focal length.
This is especially noticeable in true macro lenses that extend a great
length during close focus. For this reason, focus rails are used to achieve
focus (a focus rail is designed to move the entire camera/lens set-up
forward or back) without changing the focal length...and the
enlargement/framing.

Internal focusing lenses help this problem because there is no over-all
extension change.

If you compare the older Canon 100mm 2.8 macro lens with the newer USM
version of the same lens, you'll find that the older one extends in this
way, while the newer version has internal focusing. I've considered selling
my older macro and buying the new...just to avoid this very issue.

-Mark²

--
Images (Plus Snaps & Grabs) by Mark² at:
www.pbase.com/markuson

Mark² wrote:

Steven Woody wrote:

today i looked though my prime lens and was astonished. while i
changing the focus, the subject changed its size a alittle in the
viewfind! actually, i found that when focus changes nearest distance
to infinite, the subject size reduces a very small scale. i am not
sure if it was that before. is this a problem?

my lens is a Ziess 50mm prime. thanks.

This happens in lenses that focus by extending the front element. This
means the extension is altered, which literally changes the focal length.
This is especially noticeable in true macro lenses that extend a great
length during close focus. For this reason, focus rails are used to achieve
focus (a focus rail is designed to move the entire camera/lens set-up
forward or back) without changing the focal length...and the
enlargement/framing.

Ah, interesting. I have tried merging bracketed focus for macros and
it's very tough with this effect. Not only the enlargement changes but
the overall perspective and proportions of things.

Also, I was doing some closeups with an (internal focus) 70-200 zoom
with closeup lens attachment and I noticed there are a variety of ways
of achieving the same enlargement using the zoom and focus. I want to
study this more.

Internal focusing lenses help this problem because there is no over-all
extension change.

If you compare the older Canon 100mm 2.8 macro lens with the newer USM
version of the same lens, you'll find that the older one extends in this
way, while the newer version has internal focusing. I've considered selling
my older macro and buying the new...just to avoid this very issue.

Mark² (lowest even number here) wrote:
Steven Woody wrote:
today i looked though my prime lens and was astonished. while i
changing the focus, the subject changed its size a alittle in the
viewfind! actually, i found that when focus changes nearest distance
to infinite, the subject size reduces a very small scale. i am not
sure if it was that before. is this a problem?

my lens is a Ziess 50mm prime. thanks.

-
woody

This happens in lenses that focus by extending the front element. This
means the extension is altered, which literally changes the focal length.
This is especially noticeable in true macro lenses that extend a great
length during close focus. For this reason, focus rails are used to achieve
focus (a focus rail is designed to move the entire camera/lens set-up
forward or back) without changing the focal length...and the
enlargement/framing.

Internal focusing lenses help this problem because there is no over-all
extension change.

If you compare the older Canon 100mm 2.8 macro lens with the newer USM
version of the same lens, you'll find that the older one extends in this
way, while the newer version has internal focusing. I've considered selling
my older macro and buying the new...just to avoid this very issue.

-Mark²

--
Images (Plus Snaps & Grabs) by Mark² at:
www.pbase.com/markuson

thank you!

On Sat, 3 Jun 2006 11:53:25 -0700, "Mark²" mjmorgan(lowest even number
wrote:

Internal focusing lenses help this problem because there is no over-all
extension change.

Except all internally focusing lenses _do_ change focal length as
they focus. The effect is most notable at macro and near-macro
ranges.

A lens system which focuses by extension alone keeps the same
focal length, but you can still see some change in subject
size.

--
Michael Benveniste --
Spam and UCE professionally evaluated for $419. Use this email
address only to submit mail for evaluation.

Michael Benveniste wrote:
On Sat, 3 Jun 2006 11:53:25 -0700, "Mark²" mjmorgan(lowest even
number wrote:

Internal focusing lenses help this problem because there is no
over-all extension change.

Except all internally focusing lenses _do_ change focal length as
they focus. The effect is most notable at macro and near-macro
ranges.

A lens system which focuses by extension alone keeps the same
focal length, but you can still see some change in subject
size.

Whatever you claim...the fact is that the internal focussing 100mm 2.8 USM
macro doesn't behave the way the older extending 100mm macro does in this
regard.

--
Images (Plus Snaps & Grabs) by Mark² at:
www.pbase.com/markuson

"Mark²" mjmorgan(lowest even number wrote:

Whatever you claim...the fact is that the internal focussing
100mm 2.8 USM macro doesn't behave the way the older extending
100mm macro does in this regard.

See: http://www.photozone.de/2Equipment/r...anon100usm.htm,
which states, "The effective focal length decreases a bit
toward closer focus..." By my calculations, at 1:1 it's about
72mm.

--
Michael Benveniste --
Spam and UCE professionally evaluated for $250. Use this email
address only to submit mail for evaluation.

Steven Woody wrote:
today i looked though my prime lens and was astonished. while i
changing the focus, the subject changed its size a alittle in the
viewfind! actually, i found that when focus changes nearest distance
to infinite, the subject size reduces a very small scale. i am not
sure if it was that before. is this a problem?

my lens is a Ziess 50mm prime. thanks.

-
woody

The effect is called breathing, and it afflicts essentially all still
camera lenses. It afflicts lenses whether or not their focal length
changes slightly during focusing. In your case, the focal length is
clearly constant for all focus distances.

Its fairly easy to understand what happens with a lens having simple
focusing, as in your 50mm Zeiss. As you focus to a close object you
move the lens further from the image plane. Therefore, the field of
view must become smaller since the film/sensor subtends a smaller
angle. Conversely, as you go from close focus to infinity the field
angle gets bigger, and therefore the subject size would get smaller,
which is what you noticed.

The effect is more pronounced in wide angle lenses.

Breathing can be corrected in the optical design, but typically this is
only done for cine lenses, where dynamic focusing effects can be very
critical. Its one reason why cine lenses are so expensive.

Brian

"Mark²" mjmorgan(lowest even number wrote in message
news:YpKgg.6059$rS6.3814@fed1read11...
Michael Benveniste wrote:
On Sat, 3 Jun 2006 11:53:25 -0700, "Mark²" mjmorgan(lowest even
number wrote:

Internal focusing lenses help this problem because there is no
over-all extension change.

Except all internally focusing lenses _do_ change focal length as
they focus. The effect is most notable at macro and near-macro
ranges.

A lens system which focuses by extension alone keeps the same
focal length, but you can still see some change in subject
size.

Whatever you claim...the fact is that the internal focussing 100mm 2.8 USM
macro doesn't behave the way the older extending 100mm macro does in this
regard.


He's not 'claiming' anything - simply stating a fact. Actually, lenses that
focus solely by moving the whole lens, en bloc, relative to the film plane
don't change their focal length at all, though the image size does change.
Lenses that use internal focus pretty much always alter focal length, though
how much varies - FL seems to _decrease_ as they are focused closer, at
least in every case I've encountered.


Peter