Moose thus grunted:
> Undoubtedly someone has gone to the trouble of designing an internal
> focusing prime with close to constant effective focal length, but it's
> got to be tricky and expensive to do. OK, maybe nobody has built one; I
> just want to cover my bases.
>
Please, somebody, tell me who makes that lens! I'll get one today! (well,
maybe not today, but you get the point).
In any case, most of them do change focal length as focused closer. That
> was one factor in my choice of the Tamron 90/2.8 macro, which uses a
> conventional long helix to focus. The Canon 100/2.8 macro is IF. So as
> one focuses closer, the working distance gets even shorter than would be
> expected with conventional design.
>
Exactly. This is one of those reasons why I've been such a fan of my lowly
100/2.8 with extension tubes. I get fantastic macro quality as well as
serious working distance. Extending the 300/4.5 gives me stupidly long
working distances.
> With any lens at 1:1, the the focal plane to subject distance is 4x the
> focal length. The Canon 100 mm, instead of 400 mm, has a close focus of
> 1:1 @ 310mm, for an effective FL of 77 mm.
>
Moose, I agree with your calculations. However, I am remembering something
that might alter that equation. I hope that somebody on this list has one to
confirm or deny my allegation.
Vivitar made a 2X Macro/Teleconverter. This was a brilliant piece of
engineering that included an extending section of the tube which moved the
lens farther away from the film-plane which yielded close focusing and then
the 2X optics further magnified the image. If you placed a 50mm lens on the
Vivitar, you could achieve an effective coverage of a 100mm lens, but the
working distance remained about that of a 50mm lens. Also, the bokeh
characterists remained that of a 50mm lens--just blown up bigger.
It all had to do with the position of the 2X optics themselves. If you
placed a normal 2X teleconverter on the back of a 50mm lens and then
extended the pair from the film plane, it acts like a 100mm lens in working
distances, etc., but when you extend the 50mm lens first and then multiply,
it acts pretty much like a 50mm lens. Anybody with a 2X and some extension
tubes can easily experiment with this and see for themselves.
The reason why I mention all this is because certain modern
lenses--especially the 14-54 and 12-60 act like shorter lenses with
teleconverters built in. The working distance of the 14-54 in macro is
stunningly close. Like REALLY close. As in even at the 54mm setting the
lens hood is shading the subject. The 14-54 really does, to MY eye, look
like it is being internally extended (focused closer) giving a bit of
shorter focal length to begin with, and then multiplied by the second lens
group at the back of the lens.
AG (it's all smoke and mirrors) Schnozz
--
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