extenders not working

[BraCas]

I was reading some review on the tamron sp 2x pro extenders. Repeatedly people gave low marks because of focusing issues with 18-55 ans similar slow and "short" lens. I am not a pro when it comes to this but arent the extenders good for 70mm and longer lens and usually fast lens...generally??

 

[Derrel]

Slow-aperture lenses usually are pretty doggy with 2x converters added. And...most 2x converters are pretty so-so when used on zoom lenses. Honestly--I think most 2x converters suck, with a very,very few exceptions. Using any kind of converter on an f/3.5~5.6 kit zoom lens makes almost no sense at all. Many people WILL give low marks to 2x converters when they are used with hundreds of different, haphazardly chosen lenses. Even GOOD 2x converters can be only so-so if the lens being used is not a good match. "some" 2x converters pair up well with "some" lenses; these are typically exotic manufacturer's tele-prime lenses like 300/2.8 lenses, paired with same-maker 2x converters, of the proper generation.

A much better type of converter is the modern 1.4x converter, which costs only one f/stop in light loss, and typically has MUCH better imaging performance, on more lenses, than the 2x units do. Also, when used with "matched generation", high-grade lenses, some of the new 1.4x manufacturer's converters are pretty good! Nikon's 1.4x and Canon's 1.4x are pretty good with their new 70-200 f/2.8 zooms, and their 300/4 and 300/2.8 primes.

Nikon's TC14e or TC14E-II pair up very well with the 300/4 AF-S, and is pretty decent with the 70-200 AF-S models.

 

[robbins.photo]

As Derrel mentioned TC's are sort of a mixed bag really - they really aren't all that useful on slow glass, for a 2x TC your looking at losing 2 Fstops of light. So I your lens starts at 5.6 (which a lot of lenses do when zoomed out) your looking at F8 - which most cameras will struggle with because it's not enough light for them to focus properly.

So really to use a TC you need fast glass - I use them occasionally with my 70-200 mm F2.8, and they work fine. Thing is though that for what you'll spend on a telephoto lens with a TC you can buy a telephoto lens with a longer zoom for about the same - so really if your need something in the 400, 500, 600 mm range in most cases you'll just be better buying a lens that will do it without a TC. A 70-200 mm F2.8 is not cheap, and with a 2x TC your looking at 400 mm @ 5.6 - so really it only makes sense if you need the fast glass for other reasons and the TC is used occasionally when you need more reach

Cheap TC's are not worth it, at all - and if possible you want your TC to be the same brand as the lens, odds are good you'll get the best results that way.

If you want to buy a TC - check KEH and buy used. There are thousands of them out there because a lot of people buy them, use them on slow glass, don't get them to work and sell them after only using them once or twice.

So really unless you've got fast glass already, I don't generally recommend them, they are sort of a limited use item.

 

[Dave442]

Have almost never used a 2x extender on a lens slower than a fixed f/4. Just skip over any reviews that mention a kit lens and poor results with a 2x extender, especially the 18-55mm as they could have just bought the 55-200mm lens for about the same price as the 2x extender.

With 70mm and longer lenses you still need to start with a lens that is fast enough so that when you lose the two stops of light the camera can still autofocus.