Extender for Canon with Tamron lens

[msmaslady]

I have read that I cannot use the Canon extender with a Tamron lens. Can someone please recommend an extender for a Canon XSi with a Tamron 28-300 lens.

Thank you.

 

[Big Mike]

Do you want a teleconverter or extension tubes?

If you are looking for a teleconverter, I think Tamron makes one or more. I've got an older 2X TC that was made by Tamron (or maybe Sigma).

Take note however, that using a Teleconverter with that lens will incapacitate your autofocus.

 

[msmaslady]

It is a teleconverter I am asking about, sorry for the confusion....I am new at this.

Why would the autofocus not work, is this always the case with teleconverters or is it something to do with the 28-300 lens?

 

[Overread19 7]

Teleconverters won't work on a lens with a max apeture of smaller than f5.6 unless you are using a 1D series camera body - and then you can use a lens with a max aperture of f8. Remember that aperture numbers go backwards - so the max aperture is the smallest f number that the lens can reach.
When you attach your current lens your max aperture is f5.6 (at the long end) and when you add a 1.4Teleconverter you lose one stop of light - so your lens goes down one stop (to f8 I think is the next stop down) and so you lose your abilty to use AF.
This is a feature put in because even if you could use the AF at that aperture the camera would have too little light getting through, so most times that AF speed would be slow and it would be very likley to spend a lot of time hunting and not getting a lock.

Note that some 3rd party lenses do have max apertures of 6.3 without a teleconverter, but they get round this by not reporting their aperture to the camera - so it never knows.
Also when using a teleconverter any errors/problems with the lens get magnified and you get a drop in overall image quality - so even if you were to use a 3rdparty teleconverter which would fit your lens chances are the image quality would drop enough that it would not prove a viable option most of the time.
On a good zoom (like a 70-200mm L) a 1.4TC can give almost no image quality loss and on prime lenses its almost unnoticable. But the cheaper zooms just don't have the base quality to take them well.

 

[Big Mike]

Why would the autofocus not work, is this always the case with teleconverters or is it something to do with the 28-300 lens?

Both...and the camera as well.

Most cameras need a maximum aperture of at least F5.6 (give or take) in in order to get enough light for the AF to work properly. That's one reason why many zoom lenses have a max aperture of F5.6. The lens is question has a max aperture of F3.5 to 6.3 doesn't it? I'd guess that it would sometimes have trouble with AF when zoomed all the way out.

So, now that we know what the camera needs, we have to consider that a teleconverter steals light. A 2X teleconverter steals about two stops of light. So if you put one behind your F3.5 - 6.3 lens....the camera is only going to get as much light as a F7.1 to F13...which is probably not enough light for the AF system to function.

This is one reason why Canon's teleconverters are made to work only with their top end telephoto lenses. They just won't work well with cheaper lenses.

Another thing to consider is that a teleconverter will degrade the image quality. Of course, some are better than others...but at some point, you would be making a compromise between getting more reach and degrading the image. With a cheaper lens like this, it might not be a great idea to make the image quality any worse.

Another option would be to just do the best you can to get sharp shots with your lens (use a fast shutter speed, use a tripod etc) and then crop the image.