[fwcetus]

[...] and NOT modify the K mount lens (I am speaking of Canon users here).

For example -

[Wild Mark]

oohhhhhh nasty sight - damn those canonians - destructive careless mob

[PJ1]

the non-SMC Takumar Bayonet 135/"2.5" is also actually an f/2.8 lens (having only 52mm filter threads).

I have tried the Takumar Bayonet 135/f2.5 and my FA 135/f2.8. The Takumar Bayonet gives me half a stop faster wide open. On the crude maths it is f2.59 (135/52). Then again, maybe it is not quite 135mm. 130/52 gives f2.5 Or maybe the FA135/f2.8 is not actually f2.8?

[fwcetus]

On the crude maths it is f2.59 (135/52). Then again, maybe it is not quite 135mm. 130/52 gives f2.5

However, with a filter thread of 52mm, the actual clear aperture of the front element cannot be more than about 48mm or 49mm, or perhaps 50mm (at the absolute maximum), right? So, 135/50 would be f/2.7, while 135/49 would be f/2.76, etc.

OTOH, with the K 135/2.5, the filter threads are 58mm, so that the clear aperture could possibly be 54mm or 55mm, so that 135/54 might be f/2.5, and 135/55 might be f/2.45, etc.

Or, maybe I'm totally missing some important concept... (It wouldn't be the first time.)

[Dlanor Sekao]

there are actually 3 different versions of the m42 takumars in 135mm 2.5 configuration.......the one everyone is after has a different optical formula and either 2 more or 2 less blades.......there is a different number also ending in 12 on the back of the A/M switch than the regular version. Oh.....it DOES make a difference !

[GarageBoy]

I didn't realize Pentax had the equivalent to Minolta Celtic or Nikon series e lenses...
Is the K 3.5 better than the M version?

[fwcetus]

Is the K 3.5 better than the M version?

I don't know which is better, but I do know that they are different lenses (K has 4 elements in 4 groups; M has 5 in 5).

[yusuf]

The Takumar Bayonet uses a completely different optical formula than the M42 versions (and there were several different formulas for the M42s). Pentax used the same formula as the Tak Bayonet 2.5 for the A 135/2.8, the primary reason it gets bad reviews...

bad reviews are often viral, in my experience bayonet is as sharp as K135 wide open if not sharper

[Lowell Goudge]

I have tried the Takumar Bayonet 135/f2.5 and my FA 135/f2.8. The Takumar Bayonet gives me half a stop faster wide open. On the crude maths it is f2.59 (135/52). Then again, maybe it is not quite 135mm. 130/52 gives f2.5 Or maybe the FA135/f2.8 is not actually f2.8?

Nnote that filter thread is NOT equal to objective diameter

But then again close enough

---------- Post added 05-25-2016 at 03:07 PM ----------

bad reviews are often viral, in my experience bayonet is as sharp as K135 wide open if not sharper

The true SMC pentax 135/2.5 is absolutely the best 135 out there ' but not cheap

[fwcetus]

Nnote that filter thread is NOT equal to objective diameter
But then again close enough

I subtracted a bit from the thread diameters to more closely estimate the "objective diameter" (which I referred to as "clear aperture") when I did my quick calculations of :

However, with a filter thread of 52mm, the actual clear aperture of the front element cannot be more than about 48mm or 49mm, or perhaps 50mm (at the absolute maximum), right? So, 135/50 would be f/2.7, while 135/49 would be f/2.76, etc.
OTOH, with the K 135/2.5, the filter threads are 58mm, so that the clear aperture could possibly be 54mm or 55mm, so that 135/54 might be f/2.5, and 135/55 might be f/2.45, etc.

---

The true SMC pentax 135/2.5 is absolutely the best 135 out there ' but not cheap

Well, of the several "f / two-point-something" 135's, I would agree -- I suspect you are not actually including the A* 135/1.8 as an "also ran", eh?

[Lowell Goudge]

I

Well, of the several "f / two-point-something" 135's, I would agree -- I suspect you are not actually including the A* 135/1.8 as an "also ran", eh?

Nope, it is in a class of its own

[yusuf]

Nnote that filter thread is NOT equal to objective diameter

But then again close enough

---------- Post added 05-25-2016 at 03:07 PM ----------



The true SMC pentax 135/2.5 is absolutely the best 135 out there ' but not cheap

It's indeed good but not the best - https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/256885-da-5...mparision.html

[Digitalis]

with a filter thread of 52mm, the actual clear aperture of the front element cannot be more than about 48mm or 49mm, or perhaps 50mm (at the absolute maximum), right? So, 135/50 would be f/2.7, while 135/49 would be f/2.76

you're running under the assumption that the pentax 135mm lenses are 135mm exactly - they aren't, a margin of 10~5mm +/- should be expected. In the case of the FA 135mm f/2.8, being an internally focusing lens means that it gets wider in focal length as you approach MFD - and the effective transmission of the lens [T-stop] is reduced.

Nnote that filter thread is NOT equal to objective diameter

Correct, there are some instances where lenses have oversized filter rings due to issues with vignetting.

[Lowell Goudge]

It's indeed good but not the best - https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/256885-da-5...mparision.html

Considering the "in focus" subject is off axis and focus point clearly not the same for the three sample images a single shot comparison is not really definitive,

Similarly, comparing lenses at different apertures where depth of field is important in the overall perception of sharpness is highly dependant on depth of field is misleading.

However, the comment really was pertaining to the original post about Kand M42 135 mm lenses, not for canon, and / or computer generated designs 50 years newer.

But if you want to see just how sharp a K135/2.5 is, put it on a Q! The pixel density is equal to about a 170MP. APS-C sensor