List of Canon parafocal zooms (anywhere?)

[David_Pennybaker]

As long as you don't touch the zoom ring, it should. I can't say I've ever tried it.

[Charles_Beasley]

I have a 1994 Canon Tech Doc which discusses this subject and the term used isn't parfocal or parafocal, but varifocal. I touts the advent of the SIC system which means that you can zoom in (longest focal legth, focus, and zoom out, maintaining proper focus) and lists none of the fixed maiximum lenses such as the the 70~200 2.8L or the 28~70 2.8L (the 24~70 2.8 L wasn't out then). It lists 5 lenses of variable aperture as being of the SIC system.

From the 24~70 2.8L manual page E-6, with asn exclamation point beside it:

"Be sure to finish zooming before focusing.
Changing the zoom ring after focusing can affect the focus."



So, Chuck, semantics aside (parfocal,parafocal, varifocal) can one zoom in (longest focal lentgth), focus, and then zoom out (shorter focal length) and maintain proper focus with the fixed maximum apeture lenses such as the 24~70 2.8L and/or the 70~200 2.8L?

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I have a 1994 Canon Tech Doc which discusses this subject and the term used isn't parfocal or parafocal, but varifocal.

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As far as I know: "Varifocal" is the opposite of "parafocal" - focus *VARI*es when zooming.

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From the 24~70 2.8L manual page E-6, with asn exclamation point beside it: "Be sure to finish zooming before focusing. Changing the zoom ring after focusing can affect the focus."

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I found exactly the same statement in my 16-35mm/f2.8 L manual. Aside from specs and recommensations on paper, I simply "tested" my 16-35mm and 70-200/f4 and came to the conclusion that both lenses are varifocal, the 70-200/f4 veeeeery much so. The 16-35mm is much less parafocal, but I guess that this is more related to DoF at such wide angles than to vari-/parafocal charateristics, no?

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So, Chuck...

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An answer fom him would definitly help.

[Charles_Beasley]

That's why I said 'semantics aside."

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That's why I said 'semantics aside."

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...noticed my 'or'?...

[ChuckWestfall]

Hi, Charles:

I'm quite familiar with the Tech Info document in question, having edited the English version of it. The SIC (Super Inner Cam) system mentioned in the article refers to EF zoom lenses with rear-focusing systems. The 70-200/2.8L isn't on the list because it's an inner-focusing zoom, not a rear-focusing one. The 70-200/2.8L IS and 70-200/4L are also inner-focusing zooms. The 24-70/2.8L like the 28-70/2.8L before it is a rear-focusing zoom with SIC. For more details on the differences between rear-focusing, inner-focusing and front-focusing zooms, you can refer to my message to David Willoughby in another current thread here. There's also a comparison of rear vs. inner focusing on the Canon Camera Museum web site:

http://www.canon.com/camera-museum/t...m/f_index.html

As you say, the point of the SIC system is to maintain accurate focus when zooming with rear-focusing zoom lenses. It does this with a reasonable degree of precision with all EF lenses so equipped, but there's no question that within the category of rear-focusing zooms in the EF lens line-up, the fixed-aperture L-series zooms have the highest precision for this particular feature.

>>From the 24~70 2.8L manual page E-6, with asn exclamation point beside it:

"Be sure to finish zooming before focusing.
Changing the zoom ring after focusing can affect the focus." <<

That's what it says, but the reason it's in the manual is to cover the case of the user focusing at the wide setting of the zoom and then expecting the focus to hold when the lens is zoomed towards telephoto. As you and I both know, that rarely occurs.

>>So, Chuck, semantics aside (parfocal,parafocal, varifocal) can one zoom in (longest focal lentgth), focus, and then zoom out (shorter focal length) and maintain proper focus with the fixed maximum apeture lenses such as the 24~70 2.8L and/or the 70~200 2.8L? <<

As long as the lens is operating according to its specifications, the answer is absolutely, unequivocally, YES. Try it yourself and let me know what you think.


PS: It sounds like David's lens is in need of adjustment.

[Charles_Beasley]

Thank you Chuck for your very concise answer and to Canon for such a useful feature.

And thanks in general for participating here again. I hope the members will not abuse your generosity, as I felt was the case some time ago.