On Effective Aperture

[Boris Liberman]

Hi!

I am still struggling with the choice that I have. Should I buy the 
Vivitar M42 100-300/5 that is coupled with special doubler that would 
give me something like 200-600/8 or so?!

Anyway, today I tried to mount it (without the doubler) on my ZX-L and 
observed the following strange thing - the auto focus wouldn't confirm 
state of focus (for 300 mm wide open). Of course f/5 is not the 
brightest of apertures, but still, the darn body does bring to focus 
Sigma 28-135/3.8-5.6 on the long end. And there the aperture is even 
narrower. 

So here is my confusion -
1. Does it mean that Vivitar has narrower actual aperture? Or probably 
Vivitar is too soft for the sensor to discover the contrast?
2. Does it mean Sigma has wider actual aperture on the long end?
3. Does it mean that aperture numbers of the lens are more of a 
guideline and less of a precise technical data?

Ultimately, I think I will have to pass on that zoom lens though I 
really like to be able to go as far as 600 mm of focal length. I can 
focus it well with my ME Super but I wanted all my lenses to be 
useable on all my bodies. It would not seem to be true for this lens 
and ZX-L.

Cheers!

Boris