On Sun, 1 Dec 2002 13:27:30 +1100, "wsong" wrote:

>I realise, to take portraits, the ideal focal length is at around 100mm with
>shallow DOF.
>So I was thinking, why not just add the doubler to the very bright normal
>lens (f1.4)?
>Is this going to work?
>Would I be compromising the quality of the image by adding the doubler to
>the 50mm?
>BTW, I am currently using Pentax-M 50mm f1.4.

Unless you use stop-down metering, the f1.4 lens will not
meter correctly on a 2X converter (these are generally
"f2" optics, and anything faster on them will pass no
mor light than if they were f2). As for quality, it will
likely be very poor at the widest stops, with improvement
with stopping down, reaching acceptable performance with a
50mm around f4-5.6 (f8-11 effective...), and VG-excellent
performance around f11-16 (f22-32 effective...;-), assuming
the converter is good-quality (many are not...). This
makes 2X converters best for flash macro work at small
stops, unless you use well-matched moderate teles on them,
or use expensive specialized 2X converters on fast long
teles... BTW, 1.4X converters generally perform better,
with 1-stop light loss instead of two - these are more
practical for most uses.