Telephoto Converters

The smaller your subject, the longer of a lens you need. Telephoto Converters (TC's) simply add magnification to your existing lens making it's focal length longer by whatever the multiplication factor is (i.e. Nikon's TC 20E III is 2x).

But this relatively inexpensive gain has it's negatives.

First, you are adding additional glass into the optical path and this tends to degrade image quality. The less the magnification increase the less image degradation. The only 2x TC's I've found worth using are the latest versions and only when paired with lenses from the same manufacturer. Some of this loss of IQ can be regained by stopping down which results in less light (slower shutter speeds/higher ISO's).

Secondly, you have just made the lens longer and this change causes a change in the aperture. Aperture is a ratio of focal length to entrance pupil diameter, the "f" in f/2 stands for focal length. So lets say you have a 100mm lens with an effective aperture (entrance pupil) of 50mm, that's an f/2 lens. Now we add a 2x TC so we have a 200mm lens w/ a 50mm effective aperture, it's now an f/4 lens. That means less light gets to the camera sensor and you often need MORE light.

Thirdly, the change in aperture results in a change in the DepthOf Filed (DOF). Because of how Focal Length (FL) and aperture affect DOF (More on DOF ) when you double the FL you actually wind up with 1/2 the DOF. A 600mm at f/5.6 has half the DOF of a 300mm at f/2.8 when used from the same distance. This can be bad if you need DOF and requires you to stop down and therefore you get even less light. But this can also be "good" if you want a shallower DOF in order to blur the image background.

And lastly, all cameras made currently (2013) have their autofocus system degraded by smaller apertures; some worse than others. Currently only a few cameras offer AF functionality at f/8 so you may not even be able to use a 2x TC on an f/4 lens and retain AF. Even the very best cameras are crippled to some degree; here's what happens to my D4/D800 at various maximum apertures. (Note that stoppiing down does not change the maximum aperture)


Even with all of the negatives I still use TC's because they are cost effective. A top quality TC costs between $300-600, but a longer telephoto lens can cost almost $20,000. And if you put them on better quality lenses (f/2.8) the penalties "hurt less." I get VERY similar performance from my 400mm f/2.8 VRII with the TC 20E III as I do from my Sigma 300-800mm f/5.6 with a whole lot less cost and weight.

This was taken with the Nikon 400mm f/2.8 VRII with the TC 20E III on a Nikon D4 at f/5.6 (wide open) and an ISO of 5600 (too high for a lot of cameras).





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