Yup, that is exactly what they are for. There is a chappy on the manual focus forum who specialises in these things who has this lens. While it is rare and unusual, "photo arsenal" have a bit of a rep for way OTT prices. I think I'd rather have that A* 400/f2.8...

I thought I could see it but turns out to be dust on my screen...

Clear here but impossible to expose for both at the same time. Moon was at 125th, f/16, 100 ISO, Mars was at 1 sec, f/16 @ iso 200! Using the same settngs for the moon but zoomed into just Mars resukted in pure blackness. Got some potentially nice shots of the moon though!

K.

I think I have already spent next months budget!

I agree.

Hi Steve, as Adqam says, a price and some pictures would be helpful.

Thanks, Kris.

You need one of these. Various versions on the market.

K.

The definition does say "effective" aperture. Like most things, clever design gets around various problems like the reverse telephoto (I cannot remember the correct phrase) effect which allows a 12mm wide angle to be kept away from the inner workings of the camera. One day I'll go and read up on optics properly, probably when I retire.

K.

We were thinking of the original K series 2.5 which has an SMC coating and a front element of 58mm. The Takumar version has a filter size of 52mm which makes it seem impossible to be a f/2.5.

The K/2.5 version is a 6 6 design weighing 500g, 58mm.
The K/3,5 version is a 4 4 design weighing 365g, 52mm.
The M/3.5 version is a 5 5 design weighing 270g, 49mm.
The Tak bayonet 2.5 version is a 4 4 design weighing 395g, 52mm.

The K 2.5 cannot be too far off otherwise there would be complaints when the metering was off!

K.