Hi Dawid, C.H. and all,
> From: Dawid Loubser <dawidl@xxxxxxxxxxx>
> Ken, my experience does not agree with you on this point -
> I have used about a dozen Canon EF-series zoom lenses, and every
> single last one of them, from the cheap kit lenses, to the monstrously
> expensive 28-300mm L IS USM, maintained a constant focus distance
> regardless of focal length.
>
I remember reading on a book on Optics something about zoom lenses being
unable to keep constant focus when zooming... IIRC, they swing aroung an
"ideal" point so many times as moving groups in the zooming mechanism. But I
haven't any experience to verify or deny this.
> Any of the zoom lenses with a focusing scale inside a window
> on the lens is guaranteed to be - is the term "parfocal" ?
>
I think it's parfocal, yes... but the focusing scale in AF lenses is
intended as "orientative" only. Maybe the focus shifts are small enough to
be negligible in some cases.
If the lens in question did not maintain focus when it zoomed, the
> focus distance scale would move when zooming. I have not seen a single
> Canon lens do this.
>
I don't think any maker nowadays would bother with such "complex"
mechanism...
From: "C.H.Ling" <ch_photo@xxxxxxxxxxxxx>
> For AF lenses, if you never tried to use MF and zoom without re-adjust the
> focus you will never know whether your lens can maintain focus or not since
> every time you 'zoom and shoot' the AF re-focus again.
>
Yes, as long as the lens is able to focus to infinity, those focus shifts
aren't an issue with AF, "nobody" would notice...
> The last one I bought was having very
> uneven performance from left to right but I sold it at no loss. This kind
> of
> problem is seldom seen with MF lenses.
This also happens to my EF-S 18-55... maybe is related to the poorer
construction of (some) modern lenses?
Cheers,
--
Carlos J. Santisteban Salinas
IES Turaniana (Roquetas de Mar, Almeria)
<http://cjss.sytes.net/>
--
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