At 11:20 AM -0700 8/15/02, Kotsinadelis, Peter (Peter) wrote:
>DelRosario, Tom wrote:
>
>I use the 500D and the reason I chose it over extension tubes is that I
>use it with a zoom lens. I was told that zooming with an extension tube
>completely messes up focus, while zooming with a diopter changes focus a
>little, but not much, and mainly changes the magnification.�
>-------------------------------------------------------------------
>
>I use extention tubes with zoom lenses often. I think whoever told you this
>wanted to sell you a 500D.
>The one thing they did not tell you is that the added glass degrades (albeit
>minor) the image since it adds 2 more glass to air surfaces while the
>extention tube does not.
I have found that the 2 element achromats tend to do a better job
within the range that they work with zooms and strong telephoto
designs, and extension tubes are generally better with fixed focal
lenght lenses when those are close to symmetrical construction. The
extension tubes naturally allow a much greater range of
magnifications, but create smaller effective apertures. With present
day coatings the 2 additional glass/air surface isn't a noticeable
factor; just count the number of transition surfaces in the Canon L
series of teles and zooms. The IS has increased the number of groups
significantly, yet the new lenses tend to be better than the old.
Just keep things reasonably clean.
Another effect is that close-up 'filters', whether single element or
multi-element, work by changing the focal length of a lens, while
extension tubes maintain the focal length. Therefore you get greater
working distances with the extension tubes, and this is also the
reason for the effective aperture reduction.
I use a 500D on the 100-400 and it gives excellent results, but the
magnification range is limited compared with my 100 macro. If you
don't shoot flat objects and the far corners aren't too important,
the 100-400 with 500D can hold its own against the 100 macro.
--
* Henning J. Wulff
/|\ Wulff Photography & Design
/###\ mailto:[EMAIL PROTECTED]
|[ ]| http://www.archiphoto.com
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