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

Re: focal length changed for a prime lens ?

very clear explaination! i fully understand. thank you BC!


Steven Woody

Re: focal length changed for a prime lens ?

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


BC

Re: focal length changed for a prime lens ?

See: http://www.photozone.de/2Equipment/reviews/canon100usm.htm,
which states, "The effective focal length decreases a bit
toward closer focus..." By my calculations, at 1:1 it's about
72mm.
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Michael Benveniste

Re: focal length changed for a prime lens ?

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.
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MarkČ

Re: focal length changed for a prime lens ?

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

Re: focal length changed for a prime lens ?

eve
ing
thank you!


Steven Woody

Re: focal length changed for a prime lens ?

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.


Paul Furman

Re: focal length changed for a prime lens ?

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.
-MarkČ
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MarkČ

Re: focal length changed for a prime lens ?

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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Alan Browne

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