View Full Version : Don question about Teleconverters

02-10-2008, 08:42 PM
Would I be wasting my money on a Teleconverters for my 70-300 Quantary lens?
I was going to buy a 1.4x. So I would only ( and I use the word only loosely ) loose one f-stop. I would use it outside 99 percent of the time where the loss of the f-stop won't hurt me as much. Or does it affect your pictures in other ways.


02-10-2008, 09:50 PM
First ... a T/C can reduce your image quality (IQ)
Second ... your autofocus at 180mm-300mm will begin erratic response, because you have now changed the base aperture from f/5.6 to f/8 ... which it the failure point for AF. What this means is that when you go to get that long focal length shot (300mm x 1.4x = 520mm) ... you will have to manually focus to get it.

... and I can understand your wanting to increase your reach, but the best way is to purchase a lens designed to do just that ... unless you are just playing. The small amount of additional telephoto and loss of light is rather significant and does change the way you will shoot.

I have mounted this combination (300mm x 1.4x T/C) on my A700 ... and at 180mm ... autofocus just drifts back and forth past the proper focal point. That can really tick you off when you trying to quickly snap a distant nature shot.

Experience has shown me that you will be a lot happier with a naturally longer lens, such as the TAMRON SP AF 200-500mm f/5-6.3 Di LD ... and you should not use this lens with a T/C either ... as it screws up the "infinity" focus and that defeats the whole purpose of having a long lens. It does not work like a telescope. These lenses are designed to work on their own ... based on distance to the sensor ... and you cannot interfere with that, as you corrupt the optical dynamics involved. The result, inaccurate focus.

So ... you can usually get away with a T/C on some lenses ... but not the 70-300 f/4-5.6 or the 200-500mm f/5-6.3 (just to name a couple). Don't even think about using one on an 18-200 or 18-250 f/3.5-6.3. You'll be manually focusing for nearly 70% of time.