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05-08-2022, 07:36 PM   #1
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Interesting experiment with push-pull zooms and extension tubes

Get an old push-pull style zoom lens and put an extension tube on the back. The lens no longer retains focus during a zoom in or zoom out like it does without the tube.

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05-08-2022, 09:27 PM   #2
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If you use a TC I would expect the focus to not change, but with adding extension the zoom is likely so far from normal operating distance, it likely will no longer hold same focus. Most zooms are likely designed to hold approximate same focus but likely not under extreme close up.
05-08-2022, 11:59 PM   #3
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Not quite sure what the OP is achieving … surely switching the camera from af to mf performs the same function?
05-09-2022, 02:22 AM - 1 Like   #4
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QuoteOriginally posted by kypfer Quote
Not quite sure what the OP is achieving
I believe he is pointing out that, having manually focussed (as he must with that lens anyway), when you change the zoom setting it loses focus. It is an interesting curiousity.

People take for granted the fact that zoom lenses normally keep focus as they zoom. It was not always so. The first such lenses did not, and technically are called "variable focal length" lenses*. The requirement to keep focus while zooming came from the movie industry and is not so necessary for stills photography. The ability of zoom lenses to keep focus while zooming is a technical marvel really, but as the OP found, they do have limits.

* Here is an example of one : https://www.pentaxforums.com/accessoryreviews/jessop-zoom-slide-duplicator.html


Last edited by Lord Lucan; 05-09-2022 at 02:24 AM. Reason: Added footnote
05-09-2022, 04:35 AM - 3 Likes   #5
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With extension tubes, the focus distance and magnification depend on the focal length. With a short focal length lens on a set of tubes, the focus distance will be extremely close and the magnification will be high. With a long focal length lens on the same set of tubes, the focus distance will be further away and the magnification will be less.

Zoom lenses only keep their focus when zooming because they adjust internally adjust the extension as the focal length change. But with a set of fixed external extension tubes, the zoom mechanism cannot adjust or control that extension to correct for the interaction between extension, focal length and focus distance. Thus, the focus distance changes with extension tubes on zoom lenses.
05-09-2022, 07:33 AM - 1 Like   #6
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Two types of zoom lenses exist. The first is the parfocal type which is designed to keep focus throughout its zoom range (the type the OP has). Interestingly, these are "tuned in" to track focus by fine adjusting the back focus spacing between the lens, and the film or sensor (in the design and manufacturing process). This is what the OP did away with by adding an extension tube, and therefore the lens will no longer track focus as it's zoomed. As mentioned, adding a teleconverter shouldn't affect a parfocal's ability to hold focus, but it can happen with flaws in the design of a teleconverter because the back focus spacing of the main zoom is so critical to focus tracking.

The second type of zoom is what is usually seen today in still camera zooms, and it is a vari-focus type which has to be refocused whenever the zoom is adjusted. The latter doesn't relate to the OP's issue but as a side note, it finds use today because of auto-focusing cameras which will adjust the focus each time they take a shot.

You still see parfocal zooms ($$$) used extensively in cinema production because they need to track as zooming is adjusted. Otherwise the focus puller would have an incredibly difficult job keeping them in focus throughout a zoom shot.

Last edited by Bob 256; 05-09-2022 at 07:45 AM.
05-09-2022, 10:56 AM   #7
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QuoteOriginally posted by Lord Lucan Quote
I believe he is pointing out that, having manually focussed (as he must with that lens anyway), when you change the zoom setting it loses focus. It is an interesting curiousity.

People take for granted the fact that zoom lenses normally keep focus as they zoom. It was not always so. The first such lenses did not, and technically are called "variable focal length" lenses*. The requirement to keep focus while zooming came from the movie industry and is not so necessary for stills photography. The ability of zoom lenses to keep focus while zooming is a technical marvel really, but as the OP found, they do have limits.


Got it thank you
I remember when Tamron launched their original 28-200mm Adaptall "super-zoom", the #71A, they were at pains to point out that the focus would change if the focal length was changed they managed to work around that issue (and make a few other improvements) with the second release, the #171A owned them both

05-09-2022, 04:51 PM   #8
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The op and everyone else has missed a fundamental point of optics.

When you zoom changing the focal length of the lens changes where you need to have the subject relative of the lens to achieve focus,
05-09-2022, 05:59 PM - 1 Like   #9
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Look how much excitement can be had with a 40-year old zoom and a tube.
05-11-2022, 12:18 PM   #10
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QuoteOriginally posted by dkpentax Quote
Look how much excitement can be had with a 40-year old zoom and a tube.
LOL! It's never a dull moment with a motley assortment vintage optical tidbits.

At least half the fun of vintage equipment is in seeing just is possible and how to make interesting use of the features and character of the fruits of LBA.


(After all, people have been making great photographs since the invention of photography despite using what we now consider to be obsolete equipment.)
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