2x Doubler

[Jchafitz]

Bummer.  Have a 1st generatioon 2x that I successfully use on my 70-200 2.8-IS F lens.  Visiting my brother who has a

100 - 500 F series & I couldn't wait to play with his lens & my doubler.  He lives on the country and there's lots of wildlife.  Doesn't auto focus.  Incompatibility issues?  Or am I missing something?

[cicopo]

I'll assume you mean the 100-400 which is f5.6 which is the max for some AF systems but the pro bodies & now the 5D3 can AF at f8.0 so by adding a 2X to an f5.6 you're at f11.0 so NO AF.

[Jchafitz]

100 - 400, of course.  Blame my typing skills, not my misinformation.   Thanx for the answering.  Assuming the newer 2x II or III may work with this lwns?

[Skirball]

You still will not have AF.

[cicopo]

The 2X TC cuts the light by 2 stops and the AF can't work when the target isn't bright enough. This rule applies to ANY 2X TC and a lens that's f5.6 or above. (smaller aperture when wide open).

[Jchafitz]

AHA!  Makes sense now.  Thanks again.

                                           ~never too old to learn~

[TCampbell]

When you use a 2x extender you double both the focal length AND the focal ratio.   

The focal "ratio" is so named because it's the ratio of the diameter of clear aperture divided into the focal length.  If you add a 2x, then you're "doubling" the focal length but you are not also increasing the aperture (that part stays the same size.)  That means the diameter will divide into the length twice as many times as without the double... hence the focal ratio doubles as well.

If you were to use a 35mm manual focus camera with split prism focusing you'd find that at high f-stops, if you can see image through half the prism, the other half is dark.  Move your eye over enough for the other half to show an image and the first half goes dark.  You can't get both halves to show visible light at the same time to focus at high f-stops. 

You can use a 2x extender with f/2.8 lenses (that combination would likely work on any camera body).  You can use a 1.4x extender with f/4 lenses (again, probably any camera body can handle this).  But above those extender/f-stop combinations it will probably not work.  If you have a camera that can focus at f/8 (such as a 5D III or a 1D X) THEN you can use a 2x extender with an f/4 lens and a 1.4x extender with an f/5.6 lens.