> > Hi! > > jwc> Summary : My Pentax ZX-5n has a Sigma 70-200mm F2.8 attached to it via a > jwc> Sigma 2X Teleconverter. When the zoom is set to 400mm (w/2x teleconverter) > jwc> and the apeture is opened all the way (F2.8), my Zx-5n reads the apeture at > jwc> an F-stop of 2.8. > > jwc> Can this be right? Shouldn't the F-Stop be 5.6? Do I have a special > jwc> teleconverter? I'm really confused here. > jwc> Any help would be appreciated. > > jwc> ~Alejandro > > I just bought a Vivitar 2X Macro Focusing TC in KA mount that does the > same. I suppose it is actually quite right. > > Let's see. The lens tells to anyone concerned (body or TC <g>) that it > is f/2.8. Now, you set an aperture, say to f/2.8. What converter has > to do with that? It either has to double that number or pass it > through as it is. I suppose Sigma (and Vivitar) chose to pass > through... > > I could care less... After all, I do know what is going on and do you. > > Now, consider it the other way around - say you want to set an > aperture from the body?!
No problems (or, at least, no exposure problems). Thank Pentax for amazing foresight in the way they designed the stop-down mechanism for the lens (and the aperture sensor coupling). Your teleconverter passes the maximum aperture of the lens to the camera body, unchanged. Most teleconverters do this - I was quite surprised when I first came across a teleconverter that applied the aperture correction factor. But the maximum aperture isn't needed (or used) to calculate the correct exposure; all that is needed is the difference between the metering aperture and the exposure aperture. And that's the way the mechanical couplings between the body and A-or-later lenses work; during exposure the body tells the lens to stop down by some number of stops. It doesn't care whether you're stopping an f2.8 lens down to f4.5, or an f5.6 lens down to f9 - the difference is still a little over one stop. The only time you'd really care whether setting the aperture to f8 on the body was setting the lens to f8 or f4 would be for DOF issues, and it's easy enough to just stop the lens down and check; I rather doubt many photographers work out DOF using numeric calculations as opposed to empirical tests.
