Tele-Converter Question

[firemedic0135]

Hey guys another newbie question for you.I was looking at these teleconverters and had a question about the reaction to aperture size while using on.They said it decreases av by 2 stops..What would that be with a 1.7,or 4.0?
any help is appreciated as always.:thumbup:

 

[tpe]

If that is from 1.7 then 1 stop would be 2.4 then the second would take it to 4.8? I think starting at 1.7 is a little odd as normally you would start from 1.4 hence the odd f stops.

tim

 

[ksmattfish]

Doubling the f/# = 2 stops, so 2 stops less than f/1.7 would be f/3.4.

Teleconverters effectively increase focal length. f/# = focal length divided by the actual size of the aperture. A x1.4 teleconverter reduces aperture 1 stop, and a x2 teleconverter 2 stops, no matter what aperture setting you are using.

SLR lenses stay at their max aperture for focusing and framing, and then it closes down to the set aperture right before exposure. Many AF systems begin to fail when the max aperture gets to f/8 or smaller (higher number). This is why teleconverters affect the AF with zooms with a max aperture of f/5.6. It turns them into f/8 or f/11 zooms.