Hi Andy, I'm going to pick some of your questions:
> 1. By using the teleconverter, will I loose one F stop or does it > remains the same? Your f-stop number of a given lens is the ratio of its "diameter" related on to its focal length!! So if you have a 50mm lens with 25mm maximum diameter of the aperture, its f-stop number is 50mm/25mm = 1:2 You need a much larger diameter for a f/1,2 lens: 50mm/42mm = 1:1,2. This is quite simplified, but I want to point out: You "add" real focal length by some optical trick (not cropping, like DSLRs do!!!) done by the convertor, but your physical diameter of your lens and aperture stays the same!!! Ergo: Your f-stop number decreases the same way your focal length increases. Take a 2x converter: 135/2.8 becomes a 270/5.6, right? 85/1.4 becomes 170/2.8 Or take an 1.4x converter: 85/1.4 becomes circa 120/2.0 (119/1.96) 300/2.8 becomes 420/4.0 and so on... Or in short, you will lose 1 stop with 1,4x 1.5 stops with 1.7x and 2 stops with 2x converter. > 2. If I use the A series teleconverters on an AF body, do I still have > the AF ability or do I go MF? You will lose the AF-ability. It's a pity, Pentax doesn't have these converters. Others have: Cakon, Ninon, Tamron, Sigma, Kenko to name a few. I have an AF 1.4x from Sigma (ebay - 50$), its "okay" with my F*4.5/300 - I wouldn't be surprised if the Pentax-A 1.4x was way better, but I could never get one to compare both. In general I like 1.4x converters most because I only lose one f-stop. yours, Thomas
