"J. C. O'Connell" wrote:
> What's a little odd is that the SMCT 120 2.8 is a 49mm filter thread
> and the SMCP 120 2.8 is a 52mm filter thread even though they are
> same optically. BUT when the SMC-M 120 2.8 came out they reduced
> it down to 49mm filter thread again but changed the optical design
> to either save weight and/or cost and/or size. ( or to be positive,
> maybe to increase performance??? ).
For whatever reason, many of the K lenses use a 52mm thread
diameter, as you observed on the SMCT and K comparison, even though
they have the same optical formula (an interesting exception that
comes to mind is the SMCT 85/1.8 and the K85/1.8, which used a 58mm
thread and a 52mm thread respectively). If, however, you look
carefully, you'll note that the difference is in the diameter of the
barrel, and nothing more. In fact, the hoods from the 49mm Takumars
work perfectly on the K lenses with a step down ring. There's no
vignetting or any problems with interference.
While I can't be sure, it would seem that Pentax enlarged the barrel
diameter of the first K-mount lenses along with the diameter of the
mount, which seems reasonable since the glass and optics were the
same as the earlier lenses. But, when they designed the M series,
they were designing not so much for optical superiority as they were
for small size. The M series represented the first "all new" lenses
from Pentax in many, many years, and so optics and ergonomics were
able to be designed to work together, unlike the ergonomics of the K
series which was designed around existing optics.
--
Shel Belinkoff
mailto:[EMAIL PROTECTED]
There are no rules for good photographs,
there are only good photographs.
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