Roger Key wrote :
> And regarding the B-300, can I combine it with my 85-250/f5 to give a
> 425mm/f5, or will I get problems (apart from the length!!)?
To which Jeffrey Keller responded :
> I believe C.H. made the observation that you are unlikely to have an
aperture
> better than FL/Diameter where the diameter of the B300 is about 80mm.
I can confirm this from experience, and 80 mm is the the outer diameter of the
B-300 lens, not the front lens element diameter.
The iS-Series page on the eSIF claims that "these converters have virtually no
effect on the overall lens speed" but my tests with an OM-4 with a 135 mm f2.8
Zuiko and the B-300 prove the contrary, almost exactly 1 stop less speed.
As CH Ling already pointed out, it would be physically impossible to achieve a
2.8 speed at 230 mm focal length with a front lens diameter of barely 70 mm.
Focal length / aperture = minimum front lens diameter.
You would need 230 / 2.8 = 82 mm diameter of the front lens element at least.
Instead I got 230 mm at f4, which would relate to an effective front lens
diameter of only 57 mm.
The situation may be different with the IS-3000, being 180 mm f5.6.
With the B-300 you can reach 300 mm and 300 / 5.6 = 53 mm, which is certainly
less than the front lens diameter.
With the 85-250/f5 at 250 mm plus the B-300 you have 425 / 5 = 85 mm.
It looks like you will loose some speed at the longer focal lenghts.
How much ? 425 mm focal length / 57 mm effective diameter is f7.45 :-(
Zoomed to 200 mm, things get a little better : 340 / 5 = 68mm, for an estimated
speed of f5.9.
Zoomed to 165 mm, you might get even : 280 / 5 = 56 mm.
I have the impression that the focal length / lens speed of the original
lens must be less than 57 / 1.7 (which is about 33), in order not to loose
any lens speed.
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Wim Verheyen, Houthalen, Belgium, wimwim@xxxxxxxxxxxxxxxx
The Aemit modular analog home, The Kurzweil K2000/K2500 Atomium
http://gallery.uunet.be/wimwim/
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