[sharik]

From my experience with DA*60-250 and 1.4x HD TC: it is a usable combination, but if you want to work at the sweet spot of the lens (best sharpness), you effectively lose about 1.5 stops. With DA*60-250 alone, you get the best sharpness by stopping down to f4.5, using smaller apertures between 5.0 and 8-10 brings almost no further improvement, then the diffraction takes over. With the TC, the images at f5.6 are soft and barely usable, unless you downsample to a very low resolution. To get a decent sharpness, you need to stop down to f7.1 or f8.

---------- Post added 10-30-15 at 02:50 PM ----------

The actual degradation caused by the Pentax 1.4 TC has been measured at 3%

Norm, could you please provide a reference to these measurements?

[KevinR]

Apologies in advance for the dumb question

If I use a good quality 1.4x tele-converter (like Pentax's own), I appreciate I'll lose around one stop of light. However, I'm not clear on how it will affect general lens performance.

Regarding CAs (fringing, etc.), will it *magnify* them at any given aperture, or will the lens perform as if it was stopped down by one stop (ie. CAs reduced)? My assumption is that, regardless of the light loss, the converter will increase the effect of CAs...

Regarding resolution, will the resolution be reduced in terms of what reaches the sensor? My assumption is that it will...

Hence, before I start using a tele-converter, I really need to be working with a lens that has very acceptable CA and resolution performance, right?

Thanks in advance

Most points already covered by others, but in essence your presumptions are correct. Good TC magnifies lens defects including CA. Good len good result. And you do need a slightly faster lens to work with AF1.4x TC, say not more that F/4.5.

Some results and observations from my testing here: https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/267012-some...nce-tests.html

[BigMackCam]

Thanks, Kevin - this is helpful.