Lens extenders

How do the lens extenders work? Do they just multiply the focal length to the extender factor. For example, if I had a 1.4 extender and I was shooting with a 50 mm lens, would my new focal length be 50 * 1.4 = 70 mm? Is there a loss in the picture quality?

Thanks,
Chaks

Yes, your math is correct. There is *some* level of image quality loss, but this will vary based on the lens, extender, body and photographer preference. Mostly, when used for the super telephotos they're tweaked to perform best. They can be used with a number of the shorter lenses, as well, but results tend to vary much more.

chakrade wrote in post #17874848
.....Is there a loss in the picture quality?

Thanks,
Chaks

It's also important to note that it works by magnifying the image, in essence, making the image circle bigger, therefore, not only do you get a loss of quality because of the additional glass elements in the tele-converter, but you also loose some quality by increasing the magnification of the image as well as about 1 stop of light with the 1.4 t-con and about 2 stops with a 2.0 t-con.

chakrade wrote in post #17874848
How do the lens extenders work? Do they just multiply the focal length to the extender factor. For example, if I had a 1.4 extender and I was shooting with a 50 mm lens, would my new focal length be 50 * 1.4 = 70 mm? Is there a loss in the picture quality?

Thanks,
Chaks

A few things to note. Although your math is correct there are other considerations. Canon extenders won't mount on many lenses so make sure the lens you want to use is compatible. You also lose the ability to gather light, a 1.4 teleconvertor costs you 1 stop, a 2x will cost 2 stops. Depending on the max aperture of your lens that might cause problems. For instance a 2x converter will turn an f4 lens into an f8 lens and the loss of autofocus on many canon bodies. Even if you get past those issues using a converter may cause your lens to focus slower and you will lose some quality. The amount of loss depends on the lens TC combo. They work best on fast primes and not as well on low end zooms.

The point about loss of light is a major one that I missed; it isn't just amount of light, either. It affects the maximum aperture of the lens, so if you have an f/2.8 lens, adding a 1.4 converter drops your max aperture to f/4, etc. This critical to note when your addition of a converter would cause your max aperture to drop to f/8 or smaller since most bodies will only AF properly at a max of f/5.6; a few will do so to f/8 in higher end cameras.

chakrade wrote in post #17874848
How do the lens extenders work? Do they just multiply the focal length to the extender factor. For example, if I had a 1.4 extender and I was shooting with a 50 mm lens, would my new focal length be 50 * 1.4 = 70 mm? Is there a loss in the picture quality?

Thanks,
Chaks

Telextenders/teleconer​tors work by effectively increasing the FL of the lens, with no change in the max diameter of the aperture. So 200mm f/4 lens (with 50mm diameter aperture) mounted to 1.4X teleconvertor is effectively 280mm FL and 50mm diameter aperture, therefore f/5.6 maximum.

Really good optics are somewhat minimally impacted in IQ by teleconvertor, as proven by photozone.de measurements that showed -10% loss of MTF values for Canon 70-200mm lenses. But other lens designs seem to take a greater hit to IQ. Even more true of the 2X teleconvertor use.