Teleconverters. Explain please

[DB83]

I know that Teleconverters increase reach on a lens but I have a few questions about them. I would look to get the Canon EF 1.4x in the future.

1. How does it go onto the camera?
2. Can I mount a EF-S lens on a EF mount on a XSi?
3. Why does the 2x teleconverter reduce autofocus capabilities?
4. why do you give up 1 f-stop by using one?
5. How much would a 55-250mm lens be turned into with a 1.4x?

 

[musicaleCA]

1. It is attached like a lens to the body, and the lens goes on the front of the teleconverter.

2. Yes.

3. Because it's so freaking long, the minimum aperture is significantly reduced, so far less light can reach the AF sensor, reducing AF performance.

4. Because of the optics involved. By how much the minimum aperture is reduced is dependant on the teleconverter's magnification factor.

5. Er...well, it'd be turned into a 55x1.4-250x1.4mm lens. That's 77-350. If you then take into account the crop factor of the sensor on your camera, that's 123-490mm equivalent to a 35mm sensor size.

If you're thinking of slapping a teleconverter on the EF-S 55-250mm f/4.0-5.6 IS USM, I suggest against it. That lens already produces a fair bit of CA and isn't super-sharp (particularly at the long end). If you put a teleconverter in between you're going to see a lot more purple fringing, and there's no guarantee your images will be anywhere near as sharp.

 

[CxThree]

I believe the Canon 2x teleconverter only works with L series lenses.

 

[Dao]

Canon 2x teleconverter
Canon EF 2x II Extender Review


Canon 1.4x teleconverter
Canon EF 1.4x II Extender Review

 

[epp_b]

Teleconverters are useless on all but fast lenses. Don't even bother with your variable-aperture zoom lenses.

The only lenses you can properly use them on are fast primes (ie.: 300mm/2.8) or a fast zooms (ie.: 70-200/2.8).

 

[Big Mike]

I agree with 'epp b'. Unless you have 'fast' (large max aperture) lenses, it's not a great idea to use teleconverters.

3. Why does the 2x teleconverter reduce autofocus capabilities?

Most cameras need a minimum of F5.6 for the AF to work. That's why most cheap zoom lenses have a max aperture of F5.6 at the long end. When you add a TC into the mix, it steals some light. The 2X takes about two stops of light. So when used with a lens that only has a maximum of F5.6, the camera would only be getting light of an F11 aperture (two stops from F5.6)....and that's not nearly enough for the AF to work.

If you have a lens with a max aperture of F2.8, then loosing two stops only brings you down to F5.6...which is still OK.

 

[djacobox372]

Teleconverters don't make much sense on digital IMO--every test I've done/seen has achieved equal results by cropping/scaling instead of using a teleconverter, and if you crop/scale you aren't reducing the speed of the lens. About the only benefit of a teleconverter is the ability to compose the final shot in the viewfinder.

For film teleconverters made more sense, film grain is much more coarse then digital noise--so cropping/scaling was not as good of option as a teleconverter.

 

[DB83]

Hmm. Ok. well, I guess the main reason I was looking at teleconverters is for my future 70-200mm 2.8L. I may have to rethink getting one altogether if it will have significant adverse effects.

 

[Big Mike]

Hmm. Ok. well, I guess the main reason I was looking at teleconverters is for my future 70-200mm 2.8L. I may have to rethink getting one altogether if it will have significant adverse effects.

On a high quality lens like that, a TC isn't as bad of an option as it is with cheaper lenses.

But as someone mentioned, you might be just as well off by getting a sharp image without the TC and just cropping the image.

 

[MrLogic]

Teleconverters don't make much sense on digital IMO--every test I've done/seen has achieved equal results by cropping/scaling instead of using a teleconverter, and if you crop/scale you aren't reducing the speed of the lens. About the only benefit of a teleconverter is the ability to compose the final shot in the viewfinder

Interesting. Some people say this logic is fundamentally flawed, but I haven't seen a test so far. Do you have link?