Beginner Tele-converters...

[Forkbeard]

What effect do they have on speed etc? If a lens is at say, f5.6 what would it be at with a tele-converter? If it's 2x would it be twice as slow/dark? This is probably a stupid question!

 

[gramps]

A 2x converter on a lens would add 2 stops to the aperture setting so your f5.6 lens would become f11 - clearly that would have an affect on the light getting to the camera sensor and would affect the AF speed of the lens, also it would affect the image quality depending on the quality of the converter.
If there is plenty of light and the converter is very good quality the amount of degradation of image and AF speed might be minimal but in less light the degradation would become more noticeable.

 

[HoppyUK]

What effect do they have on speed etc? If a lens is at say, f5.6 what would it be at with a tele-converter? If it's 2x would it be twice as slow/dark? This is probably a stupid question!

Two stops of aperture lost with a 2x, one stop with 1.4x, ie a 100mm f/2.8 lens would become either 200mm f/5.6 or 140mm f/4. Minimum focusing distance stays the same; AF speed is slower, but maybe not noticeably; image quality is reduced, though that depends far more on the quality of the mother lens than with a decent quality telecon; they work better with longer lenses like over 100mm.

Other very important points are that not all telecons will fit all lenses (the protruding front won't physically fit), and if the f/number rises above f/5.6 most Canon cameras* won't AF at all; Nikons will try to AF, but may not, or not very well.

* Canon 1DX, 5D3 and 7D2 will AF at up to f/8.