Pentax 645 rear converter... 1.4X or 2X?

[Fast]

HI.
I wondering what the differences are between these two rear lens converters (other than the obvious magnification factor). I'd likely use it most with my 150mm lens (manual focus). Taking a quick look on KEH site, the 1.4X is selling for roughly double the price, (though both are affordable). Any thoughts or personal experience would be welcome.
Thanks,
Doug.

 

[pentaxuser]

I have a P645N but haven't tried a converter. If you don''t get enough responses from users here then the Pentax Forum site is worth a look. No charge to use it. From what I remember from posts there in the past most favoured the 1.4. The drop in max aperture is less of course than a 2x converter and this may account for its extra popularity, hence the price difference. The 1.4 gives you the equivalent of 130mm in 135 format which is an increase from the 135 format equivalent of 90 mm in the 150 which is an extra 40mm which for medium format isn't bad jump with only a little loss in max aperture and certainly acceptable in most situations in my opinion

pentaxuser

 

[AgX]

A factor 1.4 teleconverter yields a speed loss of one stop, a 2.0 converter of two stops.

 

[hsteeves]

general consensus is that the loss in IQ using a 2x is much greater that the 1.4x. If you have a 150, have you considered buying a 300 instead? In my case, I prefer the 150-300 zoom.

 

[AgX]

Is that general consensus?
All I know is that in the old days 3x converters were rejected by general consensus due to their bad built. Regarding the 1.4 and 2.0 version image quality was said to generally depend on number of elements. Some manufacturers offered two versions and even in their designation hinted at the number of elements.
If instead of a converter one thinks of employing a lens of greater FL, one should not overlook that quite some long focal distance lenses suffer from chromatic aberration as typical fault themselves. So one should not look only at the converters but at the image quality of the alternative, non-converter solutions too.

 

[Pasto]

I used the 1.4x with the Pentax 300mm lens and I can report that the sharpness was still excellent as compared to the 300mm alone. In fact, unless you're printing very large I could not tell the difference on anything up to about 16x20. Maybe it's my old eyes I don't know about the 2x though....

 

[pentaxuser]

I used the 1.4x with the Pentax 300mm lens and I can report that the sharpness was still excellent as compared to the 300mm alone. In fact, unless you're printing very large I could not tell the difference on anything up to about 16x20. Maybe it's my old eyes I don't know about the 2x though....

That sounds pretty good to me. Thanks

pentaxuser