A lot of people shun using converters complaining they degrade image quality. But there is one very important thing that converters CAN do. That is, magnify the image by significant amounts. Sure converters may mean a little loss of image quality, but at the end of the day, being the keen wildlife photographer that I am, I choose to use them and especially at times I get to be very close to a subject to be able to extract some tight detail or crop. I rather use a converter than crop an image. Period.
The most important thing about converters is that you should always buy the BEST that you can afford, preferably the same brand as the glass with which you’re shooting. I choose Canon glass, so my L series telephoto lenses are best suited to Canon’s L series converters, the 1.4x and 2x converters (both MkII versions).
The below images clearly demonstrate WHAT can be done from the SAME standpoint with no converter, a 1.4x then 2x converters. I was in exactly the same position while shooting one night and wanted to show the magnification changes. I will put the equivalent field of view in brackets (Eg 1,000mm on a 1D body is 1,300mm FOV). All three images are full frame straight out of camera with minimal post processing in ACR and Photoshop.