Thanks for everyones replies, i think i'll aim for pentax ones, but if
another brand comes up at a reasnoble price then i'll grab them.
Thanks
----- Original Message -----
From: "Provencher, Paul M." <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Wednesday, May 23, 2001 10:18 AM
Subject: RE: Extension Tubes and M135/3.5
> There are quality differences, mainly with how well the mount is affixed
to
> the tube. If you are not going to use them frequently, this will not be a
> big issue. I have noticed that cheaper ones use cardboard as light
baffles
> inside as opposed to metal or plastic. Again, not a big deal if you don't
> plan to use them a lot, or in harsh environments.
>
> Not being familiar with K-Mount tubes, I can't say what options are
> available in terms of meter and aperture coupling but you would probably
> want at a minimum to get meter and aperture coupling if possible - this
> makes everything easier and preserves TTL metering.
>
> Paul M. Provencher
> (ppro)
>
>
> -----Original Message-----
> From: Peifer, William [OCDUS] [mailto:[EMAIL PROTECTED]]
> Sent: Wednesday, May 23, 2001 12:48 PM
> To: '[EMAIL PROTECTED]'
> Subject: RE: Extension Tubes and M135/3.5
>
>
> Paul Jones wrote:
> > I am looking at buying some k-mount extension tubes. I am wondering
> > if there is any difference in the performance of different brands? As if
> > there like the screw mount ones i have seen then there is no glass in
> > the tube.
>
> > Are there any other cons of extension tubes?
>
> > Also what sort of size entension tube would need to get an M135/3.5
> > down to say 1:2 or 1:1? if its possible.
>
> Hi Paul,
>
> You're right -- just like the screw-mount tubes, the K-mount extension
tubes
> are just metal spacers with no optic elements inside. I recently bought a
> used set of three Vivitar K-mount auto extension tubes (the kind with the
> aperture linkage to allow open aperture metering) in like-new condition,
and
> I paid $54 plus shipping. The equivalent Pentax tubes would have cost
> considerably more. I'm quite satisfied with the performance of the
Vivitar
> set.
>
> As far as the length of extension necessary to get a particular
> magnification, you can use the simple formulas for thin lenses to get an
> approximation. The ratio of image size to object size is equal to the
ratio
> of image distance to object distance. Thus, for 1:1 magnification, the
> distance from the center of a hypothetical "thin" lens to your focal plane
> will be equal to the distance from that same lens to your object. Also,
the
> sum of the reciprocals of these two distances will equal the reciprocal of
> the focal length of this hypothetical "thin" lens. Thus, you'd need a
> lens-to-film distance of 270 mm, and a lens-to-object distance of 270 mm,
to
> get 1:1 magnification with a "thin" 135 mm lens. Your SMC-M 135/3.5 is by
> no means "thin" -- it's a complex group of several lenses rather than a
> single thin lens. However, you can get a good approximation by focusing
the
> lens to infinity, then measuring 135 from the focal plane to some location
> on the lens barrel. Assume that this point on the lens barrel is the
> location of a single "thin" 135 mm lens element, then do any further
> calculations by measuring from that point. Follow me so far?
>
> Hope this is reasonably clear, and that it helps a little.
>
> Bill Peifer
> Rochester, NY
>
>
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