TPF Noob!
Dec 21, 2004
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I was looking at a tamron 2X teleconverter or even one of the Nikon ones. I would like one with auto focus as I am using it for taking pictures of birds. I will be putting it on a sigma 70-300mm macro lens. Should I expect a lot of chromatic aberration with this set up? If so what would you suggest for getting out to the 500mm range with out breaking the bank? Thanks for the help.
You'll find that what's "acceptable" is really what pleases you. That setup isn't going to give the best quality in the world, but if there's plenty of light, it may well give a good result.

It's my understanding with the TC units that they need two things to work with AF: One, electrical coupling to the lens (i.e. it must be an active TC). Two, aperture wider than f5.6 available (this means your lens needs to be capable of one or two stops wider than 5.6 at full zoom or it won't be able to focus)

One option may be a brand name 600mm f8 mirror lens. Optically they aren't great, but they'll be long and if you don't mind the bokeh (semi-circular blobs), then it may well be a low cost option.

Good luck!

The Tamron teleconverter isnt bad at all. Its exactly the same piece as the Kenko unit. These are really your only options since the Nikon AF teleconverters only work with selected Nikon AF-S lenses.

Be aware that you will lose 2 stops when using a 2x converter. This means if you were using a lens with a max apeture of f/4 you now have an f/8. Most AF cameras will not AF very well above f/5.6 so f/8 is really pushing it. However, I have used my 300mm f/4 with a Kenko 2x teleconverter and the AF works fine as long as the available light is pretty good. It will definitely start to hunt on cloudy days or in the shade.

Also, the lose of 2 stops will not show up on the camera display. If you set the apeture for f/4 with the converter on you will actually be at f/8 but the camera will read f/4. You will also find your DOF will be cut in half when using a teleconverter so you will need to stop down a bit.

You will also lose some sharpness when using a teleconveter. This is no matter who made it.
Teleconverters are a cheap way of increasing your focal length without huge expense. Rob is right about the aperture - I have a Jessops 2x TC and AF only works with apertures wider than f3.5 (from memory). I bought it around 10 years ago when i started on the SLR route.

However i hardly ever use it now. I have a Canon 20D and my main lens is 24-70mm so with a around a ÂŁ1000 in the camera and another ÂŁ1000 in the lens sticking a ÂŁ65 TC between them immediately reduces any quality in my expsensive gear.

I also feel that the extra focal length isn't that huge even though it is 2x and I'm serioously considering selling it.

Try borrowing one before you buy - you may be disappointed

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