On Wed, Mar 15, 2000 at 09:54:38PM +0100, Wiliam Wagenaar wrote:
> Hello Zuiks and Zuikettes,
>
> Do Internal Focus lenses indeed change focal length while focusing on close
> objects?
Yes.
> How much is this effect?
It depends :).
On the focal length and the distance to focus on.(see explanation below)
> Is it the same for all IF lenses of the same focal length?
I guess (until combined with other optical construction effects z.B. Zoom, ...)
> Is this common for normal focusing lenses also?
No.
> What is the explanation on this behavior?
Because nobody answered this question yet, Im giving a try, although I`m not to
good in English.
First you have to see how focusing works. If the Distance between the Optical
System (Exact the `Hauptebene' -don`t know the English-term-but doesn`t matter)
matches the focal length, then the Lens is focused to an object at infinity.
if the object is moved, the optical-picture of that object moves in the same
direction (a optical Rule). That means, if the object comes closer to you
(=your Camera), the picture also come closer to you, that means it (the sharp
picture of your object) is behind the film-plane. Your film see`s a unsharp one.
So you have to move the film plane to the position where the sharp picture is.
The point is, that you have to increase the distance from the lens to the film
plane, when focusing to a close object.
In use of normal Photography, it`s no matter whether you keep the position of
the lens and move the film (Contax AX ) or you keep the position of the film
and move the lens towards the object.
(Macro-Enthusiasts using Bellows may see this Point very different :-).
A conventional lens is moved away fromthe film-plane by a helicoid when
focused to close objects.
This can easy be verified while watching a normal lens when focused from
infinity to the near point.
The length of my 85f2 increases from 48mm to about 62mm when focused to it`s
closest distance.
You can study the effect of length growing regarding the distance scaling.
There you can see that the effect is marginal from infinity to about 20 meters
and getting more impact reaching the 1m area (depending on the focal-length of
the lens).
A internal Focus lens always keeps the same length. So it can`t change the
distance from the Lens-group to the film-plane. So what to do? - The trick is
to shorten the focal length. When you shorten the focal length and keep the
lens-film distance, the optical effect is the same like a shorter
focal-length-lens moved away from the film plane. - voila.
You also have these two possibilities to extend the close focus range of normal
lenses:
a) Use an extension tube: increase the distance lens to film.
=> normal focusing
b) Use a additional close-up-lens: It shortens the focus length of the combined
optical system.
=> same effect as internal focusing.
Interesting Aspect:
The angel of view depends from the distance Film to the Center of Lens. At
infinity this equals the focal length. So the nominal angel of view (based upon
the focal length) is correct only for infinity. If this distance is increased
due to normal focusing the angel of view is decreased.
This effect is most viewable with a telephoto and macro-photo lens.
A internal focus lens should always have the same angel of view, because this
distance is fixed.
I don`t have any IF-lens, so I can`t verify this.
Can somebody check whether this assumption is correct?
So you can say, that the shortening of focal lens is exact that amount, that
keep`s the angel of view always the same. While the angel of view of the
same-focal-length conventional lens would get a little narrower.
So don`t worry your tele getting a wide angel, because the statement "IF
shortens focal length" sounds so dramatic.
>
> I have read that at closest focus, the actual focal length of the 350/2.8 is
> only 270mm!
>
> I have not checked this myself but can only refer to heresay. My only
> reference to the mentioned lens is that I have held it once (Thanks to
> Frank).
>
> Well, let's see if you techs can find a challenge in this one!
>
>
> Wiliam Wagenaar,
> Middelburg NL
>
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