lens extender?

[bournemouthmike]

Hi,
I've recently got the Canon EF 70-300mm f/4.0-5.6 IS USM Lens. I like it but sometimes i'd like some more reach. Am i better to save up for a different lens or would an extender be a good option (on a budget!). I've got a canon 600D. [h=1][/h]

 

[SCraig]

Teleconverters are common and people use them frequently, however they are not without their costs. A 1.4x TC will add 40% to your lens focal length, but it will also cost 1.4 stops of light. If your lens is a f/4.0-5.6 wide open it will become an f/6.3 to f/9 lens. A 1.7x TC adds 1.7 stops and a 2x TC adds 2 full stops. Additionally, inexpensive teleconverters can ruin image quality.

I use a 1.4 from time to time with my Sigma 150-500 when I need more reach. I also have a 2x TC that is not worth the aluminum it's made of.

 

[Overread]

Teleconverters (which is what everyone except Canon calls them, for some reason they call them extenders) allow you to increase the focal length at a cost of aperture and some image quality, with varying amounts of both depending on the power of the magnification the teleconverter gives you.

That said the first important thing to be aware of is that all teleconverters have a front element that protrudes forward into the lens mounting area. This means that they are physically limited as to what lenses they can and cannot fit onto. Canon's own teleconverters have a long protruding front that prevents them being mounted to almost anything that isn't an L series long focal lengthed lens, because it needs a clear back of the lens to fit its element into.

Sigma make good teleconverters with a smaller protrusion and Kenko Pro series teleconverters have the smallest protruding front element of all and are still very good quality.

The thing is a typical 70-300mm lens has a very flush rear element and as such almost none of those lenses will take any teleconverter.


In addition a 70-300mm is typically already suffering from 2 further problems:

1) If you add a teleconverter you reduce the effective maximum aperture. The smallest is by one stop with a 1.4 teleconverter which would take you from f5.6 to f8. This is a problem for Canon setups because barring the 1D series camera bodies auto focus disables if the camera detects that the effective maximum aperture of the lens is smaller than f5.6 (eg f8). Some 3rd party teleconverters won't report to the camera and there is a pin taping trick to hide a teleconverter without removing AF (works on canon telecovnerters). However the restriction is in place because after f5.6 lower level camera bodies struggle with auto focus as there is so little light getting though. You end up with inaccurate and slower AF that is prone to hunt.

**note on most camera bodies with live view you can retain AF up to f8, but only in live view mode; this is because its a different AF sensor assembly, however live-view AF is slower than regular AF and you've still the light loss that will impede its performance.


2) Image clarity will suffer, you'll gain focal length but lose optical quality. On a lens that is already likely going softer at the 300mm end when you magnify that with a teleconverter you end up with a less than ideal result. Of course individual standards will vary here and some people might be ok with the results, but most would consider it record shots only.



In short you can't use one and chances are even if you could use one you would likely suffer noticeable image quality reduction and loss of auto focusing capabilities. The best thing you can do is save up for a better lens - wildlife is sadly one area where you can't easily go cheap.

 

[bournemouthmike]

Thanks for the replies. Looks like a might have to save up for another lens in the future. In the mean time I'll just have to get a bit loser to my subjects