How do lens extenders work?

[rgrebby]

Im getting a Sigma AF 70-300mm f/4-5.6 APO DG macro and im wondering if I should look at a lens extender thing, I see they come in 1.5x or 2x.

Is there anything else I should know, what should I get, any problems, what would they do exactly? I guess that they magnify the mm by 1.5 or 2 so it would make the 300mm into a 600mm respectively.

What kind of price should I be looking at and are they worth it?

 

[Steveo_Hants]

They do extend the focal length (mm) of the lens but they also decrease the maximum aperture (f-number). a 1.5xTC will decrease the aperture by 1 stop and the 2xTC will decrease the aperture by 2 stops. They also affect the IQ. To get good results with TC you really need a fast lens to start with, something like a 70-200 f2.8. Otherwise the aperture just become too small and with average light IQ will be very poor.

 

[pxl8]

The Sigma 70-300 is not so bad with a 1.4 or 1.5x TC but the image quality suffers badly with a 2x. At 300mm with a 2x the widest aperture would effectively be f/8 which the AF will stuggle with (if it works at all) and manual focusing will be difficult in all but the brightest light.

 

[W.Smith]

There's also such a thing as extender rings, usually in a set of 3 or 4 different lengths, without any optics, for macro applications. Like a converter, you stick one or more between your lens* and your body to increase the macro magnification. They function like bellows do.
They're not expensive but require considerable skill to apply effectively. The results can be breathtakingly brilliant, though.

*usually high-quality, fast 50 or 85mm's.

Have fun!