One for the tech geeks. A f1.2 + adapter on a Full Frame camera.

[Cage]

I am currently selling a rather nice 'K' series SMC Pentax 1:1.2 50Mm.

Yep, the beautifully engineered and built one with the big hole in it. I think it would work superbly with the excellent live view on the D600.

It is such a nice bit of glass that I'm having mixed thoughts about parting with it, although the $'s will come in handy to build my Nikon kit.

I am toying with the idea of keeping it and getting something like the Fotodiox adapter for use with the D600. This particular adapter has a built-in 1.4x Multi-Coated Focusing Correction Lens which is removable for Macro Photography.

But, and ain't there always a 'but', assuming the adapter is 10-20mm thick I'm figuring that I'll finish up with a cropped 60-70mm lens and lose at least 1/2 stop of light.

If my assumptions are correct, it sort of defeats the purpose of the exercise as my 50mm f1.2 is now a 60-70mm f1.4 or thereabouts.

Maybe I'd be better off with a Nikkor AIS 50mm f1.2 instead.

[ameerat42]

Have been going mad looking up refs for this...
I had a formula for "effective aperture using extension tubes" on a sheet that came with a set. (No longer have sheet.)

When reading your post I had a doubt that it would be that much of a change, but one ref I read (http://scubageek.com/articles/tubes.pdf) had this little rider
at the very end...

This result shows that for lenses with small reproduction ratios, the effective f-stop is
obtained by using an “effective” focal length, which is the sum of the lens’ actual focal
length and the length of the extension tube.

I take "lenses with small reproduction ratios" to mean fairly wide (like 50mm) lenses with no macro.

Other refs indicate (at least to me) that with the lens set to "infinity" the above is certainly the case.

If all of this has some relation to fact, and givven your widest aperture is 41.66 mm, then taking your 10-20mm range...

for 10mm extension, 60/41.66 = "f/1.4"

for 20mm extension, 70/41.66 = "f/1.7" (close enough)

Hmm! You could be right. But what weighs against being right?
Am.

PS Google "effective aperture with extension tubes"

[Cage]

Thanks Am.

Will check it out.

Cheers

Kevin