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Author Topic: Exactly how does an internal focusing lens work?  (Read 668 times)
Arlen
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« on: August 25, 2013, 04:48:26 PM »
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I thought I had a pretty fair idea how internally focusing lenses work, but it seems I don't completely understand what is happening with light paths and apertures.

It's my understanding that when focused very close, the focal length of an internally focusing lens actually becomes shorter, and it's angle of view is increased; much like the wide end of a zoom lens. Pupil magnification ratios and the effective f-stops also change on some lenses, like my Canon 100mm f/2.8 Macro USM lens.

I would like to find a resource that diagrams the working components of an internally focusing lens like this one, showing the relationships of the component parts, the light paths, etc., and explains how those relationships change during focusing. Can someone direct me to such a resource? Every illustrated explanation I can find about focusing just discusses externally focusing lenses, where the focal length increases at close focus.

Many thanks for any help you can provide.
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NancyP
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« Reply #1 on: August 27, 2013, 11:00:41 AM »
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http://books.google.com/books?id=cuzYl4hx-B8C&pg=PA195&lpg=PA195&dq=internal+focusing+lens+group&source=bl&ots=n0NvATGvOE&sig=yx_9Z0QrYUl7i6zGKOr5jn4-f9c&hl=en&sa=X&ei=ecwcUtD8Io-MyAGojIC4Bg&ved=0CGoQ6AEwBzgU#v=onepage&q=internal%20focusing%20lens%20group&f=false

for a diagram. Only a few central elements have to move in internal focusing lens, so the motor load is lighter and autofocus can be faster. Also, if manual focus, the focus throw is much shorter.
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Ellis Vener
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« Reply #2 on: August 27, 2013, 11:41:23 AM »
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Nancy P: Wow what a great book!

Arlen you have read right but if and by how much the focal length changes  depends on the specific lens. For instance it doesn't appear to happen with the 105mm Micro-Nikkor  but not the 60mm and 200mm Micro-Nikkors! But in general for practical photography it is not something to worry about
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Ellis Vener
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Creating photographs for advertising, corporate and industrial clients since 1984.
SeanBK
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« Reply #3 on: August 27, 2013, 11:52:04 PM »
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Somehow I recall reading someplace the physical length of the lens does not change at different focus, so for Micro Focusing it is very good.
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RFPhotography
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« Reply #4 on: August 28, 2013, 06:14:45 AM »
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It's called 'breathing'.  It's one of the reasons filmmakers often don't like using still camera lenses for video work.  The focal length change while adjusting focus changes the framing of the shot slightly.
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Arlen
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« Reply #5 on: August 28, 2013, 09:59:40 AM »
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Nancy, thanks for the link. The info given there sheds some light on the subject  Smiley.

Ellis, I agree that much of time this doesn't have a practical effect in general photography, but in some cases it does. As Bob mentioned, a reduction in focal length changes the angle of view at close focus. And at least in the case of my Canon 100mm macro lens, the aperture changes too, increasing the f-stop by 2 stops, which at the small apertures typically used in macro work also increases diffraction. Looking through the rear of the lens, I can actually see the diaphragm blades moving and closing down as the focus is changed. But the main reason I'm researching this is not for a practical purpose, rather for my own edification as to what is happening and why.
« Last Edit: August 28, 2013, 10:02:27 AM by Arlen » Logged
NancyP
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« Reply #6 on: August 28, 2013, 07:00:59 PM »
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The compromises involved for still camera lenses often include "focus breathing", which really doesn't matter all that much to us. Design of lenses without focus breathing and with exceptional optical quality - NOT CHEAP - really good cinema lenses seem to be up to 10-fold the price of equivalent still lenses.
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