> Did Pentax make a high performance K mount 2X teleconvertor for
> long lenses? I want one to try on my SMCK 1000mm F8.
The best Pentax TC's for long lenses (but only for lenses that can
fit their "snouts" into the K-mount end of the lens) are the "L"
models (A 1.4X-L and A 2X-L). I don't know offhand if either of
these would fit into your K 1000/8, JCO.
The "S" models (A 1.4X-S and A 2X-S) are also good TC's, if the "L"
models won't fit. However, I really don't know how well they would
work with a 1000mm lens - I've not used either with any lens over
300mm.
In general, the 1.4X models (both "L" and "S") have a greater chance
of causing vignetting, but only with certain lenses.
The 6-element T6-2X is (in my tests) just as good a TC as the
7-element A 2X-S, but (again), I've never used it with a 1000mm lens
(and Pentax, in the user manual, doesn't seem to suggest its use
with lenses over 300mm).
In the event that the "L" models might fit, I'm going to append a
little treatise I posted to the PDML back in 1999, with the
measurements of the "L" TC's "snouts", below. (I apologize for its
length, but I tend to be verbose, and at least it's not OT - <g>.)
Fred
+++++
Safe "X" (safe use of 2X-L and 1.4X-L with non-Pentax lenses)
As far as I know, there have been 6 Pentax rear converter
(teleconverter) models, and each has been designed primarily for
use with certain lenses, or for certain ranges of lenses. [Note:
In the original post I did not include the Takumar-A 2X TC, which
is not SMC and may not actually even be made by Pentax.]
The (oldest) T6-2X is specified for K-mount prime lenses between
135mm and 300mm. While it can be safely mechanically coupled to
shorter or longer lenses, or used with various zooms, Pentax does
not recommend this (presumably for optical reasons).
The (newest) AF Adapter 1.7X is specified for most manual focus K,
M, and A lenses having a maximum aperture of (usually) f/2.8 or
wider (for sufficient light for AF use).
The 1.4X-S and 2X-S ("S" for "short") designs seem to have the
least restrictions in recommendations, being safely usable with
most Pentax lenses from 300mm down to, say, 20mm or so. They can
be safely coupled with longer lenses, but apparently corner
vignetting will generally result.
The most "dangerous" rear converters are the 1.4X-L and the 2X-L
("L" for "long") models, because their front elements project
forward beyond their mounting flanges into the rears of any lenses
mounted to them. Obviously, only certain lenses, which usually are
the lenses longer than 300mm, can safely mechanically couple to
these teleconverters (and apparently vignetting is reduced by the
use of these converters, as compared to the other models).
However, what about the use of all these various rear converters on
non-Pentax lenses?
It would seem to me that, in general, all rear converters except
for the "dangerous" 1.4X-L and the 2X-L converters could be used on
most of the various 3rd-party K-mount lenses that are analogous in
specifications to the recommended Pentax lenses. (However, I would
be careful in the case of any lens with a protruding rear element.)
But how about the 1.4X-L and the 2X-L?
Intuitively (and my own intuition has ~never~ steered me wrong
before - <g>), it would seem that any of the longer 3rd-party
lenses could benefit from the avoidance of vignetting that the
1.4X-L and the 2X-L can provide, if only (and this is a big "if")
the protruding front teleconverter element does not contact the
rear element of the lens, or otherwise interfere with coupling.
For example, say you have your basic Tamkigma (?) after-market
400/4.5 lens - can you risk using one of the "L" rear converters
with it, or should you "wimp out" and settle for one of the "S"
designs? Well, you could go to the trouble of obtaining one of the
"L" models, and carefully (!!!) measure and test to see if it will
couple without damage, and then sell it when you find that it
won't. (Or, maybe you can hold onto it until your long-lost rich
uncle dies and bequeaths his A* 400/2.8 ED [IF] monster to you -
<g>.)
Or, instead, you can rely in advance on the measurements taken by
"yours truly", who wondered the very same questions about using the
two Pentax "L" rear converters with his Tokina AT-X 150-500/5.6
zoom, and who obtained the answers of "yes" and "no". (In brief, I
found that the 2X-L converter will couple safely to the Tokina
150-500/5.6, but that the 1.4X-L will not.)
The 2X-L's protruding front element extends 17.0 mm beyond the
mounting flange, and has a diameter of 31.5 mm for most of its
length, but widens to 33.0 mm for the final 4.0 mm portion closest
to the plane of the flange. These dimensions allow the 2X-L to
fairly comfortably fit inside the rear of the Tokina 150-500/5.6
lens (though there's not a whole lot of room to spare).
On the other hand, the otherwise smaller 1.4X-L has a larger
protrusion, which extends 21.0 mm beyond the plane of the mounting
flange, and which has a width of 35.5 mm throughout. The immediate
problem with these dimensions for me is that the inner diameter of
the Tokina lens is just a wee bit narrower than the 35.5 mm width,
preventing the coupling from possibly occurring. (The fact that
there is about 26 mm between the Tokina's lens mounting flange and
the surface of its rear element, which would theoretically allow
for about 5 mm of "breathing room" between opposing lens surfaces,
becomes then a moot point.) With an internal diameter of about 34
mm, the lens simply "balks" at the insertion of the 35.5 mm wide
rear converter front element.
If you want to judge whether either of the two "L" converters could
be safely used with any one of ~your~ 3rd-party telephoto lenses, I
would suggest carefully making a paper (or very thin cardboard)
cutout with the exact dimensions (above) of the particular
teleconverter protrusion, and then gently test to determine whether
it can be inserted comfortably into the rear of the lens in
question.
Of course, you would have to test this throughout the entire focus
range of the lens. And, if the lens is a zoom, you would also have
to test this throughout the focus range at all different focal
lengths, as well. In some lenses the rear element (and/or possibly
portions of the mounting area) might move while focusing, or while
zooming, or while doing both. A certain amount of movement might
be OK, as long as NO contact could EVER occur under ANY combination
of focal length and focus distance. (Fortunately for me, the rear
element of the Tokina AT-X 150-500/5.6 does not move at all under
any circumstance.)
I am offering the above suggested experiment with CAUTION. Be VERY
careful with trying ANY of these suggestions. The "good news" is
that, in both the 1.4X-L and the 2X-L, the edges of the protruding
front elements are wisely encased with rubber by Pentax. The "bad
news" is that you could probably STILL do serious damage IF you are
not VERY careful.
By the way, Pentax indirectly implies, in its coupling
recommendations, a difference in the dimensions of the front
element protrusions: For almost all suitable lenses, safe coupling
can be accomplished with either the 2X-L or the 1.4X-L. However,
apparently like my Tokina, the 135-600/6.7 zoom is approved for use
with the 2X-L, but not for use with the 1.4X-L.
Fred
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