# Extender for Sigma 150-500?



## EricBrian (Jul 16, 2008)

So, I finally found a Sigma 150-500mm f/5-6.3 AF APO DG OS HSM Telephoto Zoom Lens for Nikon Digital SLR Cameras on Amazon for sale and am getting it on Friday. 

I want to buy a 2x extender from Sigma for it. But am not quite sure of the implications. I've read elsewhere that one looses AF capabilities. Is this so?  

Thanks
Eric


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## tirediron (Jul 16, 2008)

I don't know about the auto-focus ability; I wouldn't think you'd lose it with a Sigma tele-converter and a Sigma lens, what you will lose is maxium apeture.  Your lens will go from a 150-500mm f/5-6.3 to a 300-1000mm f10-12.5 (or thereabouts).  This is going to be a VERY slow lens.  Do you really need a 2x teleconverter on a 500mm lens?


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## cecilc (Jul 17, 2008)

EricBrian said:


> I want to buy a 2x extender from Sigma for it. But am not quite sure of the implications. I've read elsewhere that one looses AF capabilities. Is this so?



I'm not sure about losing the autofocus, either - although I would *plan* on losing it ... on most systems you lose autofocus with extenders on any lens where the maximum aperture is already at 5.6 .... 

The things that I would be *more* concerned with would be image quality AND the speed of your lens with a 2x extender on it. 2x extenders are notorious for robbing you of image quality and you'll lose another 2 stops on an already slowish lens ... the f10-f13 or so seems about right .... and that is ssssllllllooooooowwwwww .....


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## Big Mike (Jul 17, 2008)

The reason you loose AF with teleconverters, is a lack of light...and it's a limitation of the camera's AF sensor.

With most cameras, you need at least F5.6 worth of light, for the AF to work properly.  That's one reason why most cheap zoom lenses have a max aperture of F5.6 (usually at the long end of the zoom).  

A teleconverter robs light...a 2X will steal about 2 stops worth of light from getting to the AF sensor (and the image sensor).  So if you use a lens with a native max aperture of F5 to F6.3...and then to take away two more stops...you won't have enough light reaching the AF sensor for it to work.  

This is one reason why Canon Teleconverters are made to be compatible with only their fast professional lenses...because the should really only be used with fast lenses anyway.

That being said, you can still shoot...you just won't have AF and the viewfinder will be fairly dark, making manual focus pretty hard.

Not to mention that the loss of two stops will require you to use a fairly long shutter speed in less than ideal light.


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