1.7 converter problem!

Could anyone throw any light on why ,when fitted to a Sigma 50/150 2.8 lens,the camera gives a 9.5 reading, when you only drop 2 stops with Pentax lenses!

There's no guarantee that other manufacturers' lenses will report correctly to the camera.

Very often the readings from converters don't make sense
I normally just take a couple of test shots and look at the histogram

You can actually read the LV afterwards with PhotoMe I normally take
the Centre reading..... see if you can make anything of it... comparing
it with a reading with no converter?

Generally Sigma converters lose a little bit less than the Pentax
ones, possibly because they are usually 4 element. The 1.7x should
only lose 1.7 stops.