Finally, teleconverters also reduce AF performance. Except in some high-end pro cameras, DSLR AF systems were designed to function at apertures of ƒ/5.6 and larger. When the aperture is smaller than that, the beam of light transmitted to the AF module isn't wide enough to assure the required focusing accuracy, so most cameras deactivate the AF system when a lens or lens/converter combination slower than ƒ/5.6 is used. This means you'll lose AF when you use a 1.4x converter on a lens slower than ƒ/4 or a 2x converter on a lens slower than ƒ/2.8. Additionally, all AF systems slow down as the light gets dimmer, so using a teleconverter will slow AF performance noticeably from that of the lens alone. That said, AF performance with fast pro supertelephotos and teleconverters is still good enough to do birds in flight.
Effect Of Teleconverters
In these illustrations, you can see how the subject-to-lens distance stays the same with and without a teleconverter. This is one of the main reasons why macro and supermacro photographers use them. You can also see the magnification at the image sensor. Notice that in Figure 2 the subject-to-lens distance is the same as in Figure 1, but the sensor, and, therefore, the photographer, is slightly farther away.
As with everything in photography, there are plusses and minuses associated with teleconverters. Their small size, versatility and relatively low cost make them tools every nature photographer should have in his or her bag.
Teleconverter + eBay = Inexpensive BIG Telephoto
If you're willing to focus manually, you can save some money on your telephoto setup and still get excellent optical quality. A new 300mm ƒ/2.8 supertelephoto lens can cost upward of $6,000. A new 300mm ƒ/4 is around $1,400. But an old, non-AF Nikon 300mm ƒ/4.5 sells for around $200 on eBay. In some cases, you can find an old telephoto lens that mounts directly on your camera (Nikon, for example); in other cases, you may need an adapter.
Lens adapters let you mount lenses for other cameras on yours, such as old Canon FD lenses on EOS bodies, Nikon lenses on Canon bodies and just about any lens for which you can find an adapter onto mirrorless bodies. The big concerns regarding lens adapters are availability for the lens and camera body in question, and flange-back distance (the distance between the lens mount and the image plane). If the camera body plus the adapter's flange-back distance is too great, the lens won't focus all the way to infinity, but you can use it for close-up work. This isn't a problem with mirrorless cameras and their short flange-back distances. If the flange-back distance for the body and adapter is too short, the lens will focus past infinity—not a big a deal, but be aware of it. Many adapters are just connectors with no linkage; they allow you to attach a given lens to a given camera, but focus and exposure are manual (or aperture-priority AE). Some adapters (generally, those from the camera manufacturer) provide metering and even AF capability.
Depending on the lens-teleconverter-adapter combination, you may or may not be able to use all of your auto-exposure functions. There are so many possible combinations available that we can't cover all of the nuances of each here. This can be an excellent solution, but you have to do some homework. Check your camera's manual to determine compatibility or try contacting the manufacturer directly.