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Parfocal lenses
#1
I'm asking this to promote discussion as much as to learn... Can anyone please explain what changes are necessary to make an ordinary (varifocal) zoom a [url="http://en.wikipedia.org/wiki/Parfocal_lens"]parfocal[/url] zoom? Is it an addition of extra elements or can it be done through a more complex movement of existing elements?



Thanks,

GTW
#2
I assume you've found a darn sharp zoom lens from 70s but it's varifocal...



Lately I was checking [url="http://www.adaptall-2.org/"]adaptall lenses[/url] and on that site there's a short article about zoom lens structure. According to this article, a group called "compensator group" in coordination with the "variator group" keeps the lens in focus when zooming. So other than a solution without changing the structure would be quite interesting to read...



Serkan
#3
[quote name='PuxaVida' timestamp='1298898899' post='6398']

So other than a solution without changing the structure would be quite interesting to read...

[/quote]

Thanks for the link... right after I put my question in to words, it occurred to me that any AF varifocal zoom lens can be made a parfocal lens if the lens (or the body) can read the focal length in real-time and then automatically adjust the focus to the last manually set focal distance. In the case of lenses with a built-in AF motor, the lens itself can do this without needing the body to do anything but just providing power.



In the past I'm sure this was done using additional elements. But still what I said could be mechanically (i.e. not electronically) implemented if the focus variation when zooming is linear or something... maybe.



GTW
#4
[quote name='genotypewriter' timestamp='1298941874' post='6409']

Thanks for the link... right after I put my question in to words, it occurred to me that any AF varifocal zoom lens can be made a parfocal lens if the lens (or the body) can read the focal length in real-time and then automatically adjust the focus to the last manually set focal distance. In the case of lenses with a built-in AF motor, the lens itself can do this without needing the body to do anything but just providing power.



In the past I'm sure this was done using additional elements. But still what I said could be mechanically (i.e. not electronically) implemented if the focus variation when zooming is linear or something... maybe.



GTW

[/quote]

Actually, if AF is adjusted by th ecamera continually, it si technically not a parfocallens.



A parfocal lens keeps focus all by itself when zooming, no further intervention or transmission of data required, i.e., it is mechanical rather than electronic/electrical.



Yes, in the past this has been done with additional elements, but is also a matter of very tight tolerances and very complex moving mechanisms. Actually, normally it is a combination of the two, which is quite logical <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='Big Grin' />.



Kind regards, Wim
Gear: Canon EOS R with 3 primes and 2 zooms, 4 EF-R adapters, Canon EOS 5 (analog), 9 Canon EF primes, a lone Canon EF zoom, 2 extenders, 2 converters, tubes; Olympus OM-D 1 Mk II & Pen F with 12 primes, 6 zooms, and 3 Metabones EF-MFT adapters ....
#5
[quote name='wim' timestamp='1299231771' post='6503']

Actually, if AF is adjusted by th ecamera continually, it si technically not a parfocallens.

[/quote]

Just like if the car changes gears automatically it doesn't mean it's a gearless car <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/smile.gif' class='bbc_emoticon' alt='Smile' /> Hmm I should patent this perhaps... call it auto-parfocal perhaps!



By the way, as soon as Sigma announced that 12-24 DG Mk II, I wanted to ask you whether you were willing to give it chance mk II <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/wink.gif' class='bbc_emoticon' alt='Wink' />



GTW
#6
Not completely. Is a car with continuous variable transmission a car with a gear box or not? I think it is, even if it is stepless.

However, parfocal means that a lens maintains focus unaided when zooming. If it is assisted by the camera AF module with continuous adjustments, it isn't therefore parfocal, because it requires help from outside the lens.



As to the Sigma: No, I won't. <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='Big Grin' />. I've burnt my fingers a few times too often with Sigma now. And I am perfectly happy with the TS-E 17L <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='Big Grin' />. 2 1/2 times the price, but 10X the lens. <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='Big Grin' />



Kind regards, Wim
Gear: Canon EOS R with 3 primes and 2 zooms, 4 EF-R adapters, Canon EOS 5 (analog), 9 Canon EF primes, a lone Canon EF zoom, 2 extenders, 2 converters, tubes; Olympus OM-D 1 Mk II & Pen F with 12 primes, 6 zooms, and 3 Metabones EF-MFT adapters ....
  


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