| It's fair to say that most are brilliant, the best are designed to be used with specific lenses.
The greater the extension the more compromises and the larger the drop in quality, x3 were once available but the majority were x2 with a smaller number of x1.4 mainly from the major camera/lens manufacturers.
One major problem is they work best with the lens fitted stopped down, but because they cut light transmission that often means you need to work at wider apertures if your shooting sports events so it's a Catch 22 situation.
They can be useful though, and may make the difference between getting a decent image or none at all, many sports photographers use the 300mm f2.8 Canon lens and the x1.4 or x2 converters but this is an optimal combinations. Converters are not as good with Zoom lenses.
For optimum quality a decent lens is always going to be better than using a converter. I've used a good 2x converter to shoot Cricket & Rugby in the past and while the images are acceptable there is a drop in quality.
Ian |