Originally posted by: rudder
Are those 2X magnifiers worth it? I have a 75-300mm lens for my 35mm eos which will work on my new 20D. I like to take pictures of my daughters playing sports and sometimes the 300mm is not enough.
Is it accurate to say the the 300mm with a eos rebel will actually be around 400mm with a digital camera?
2X teleconverters are only worth it if you have an already excellent lens to mount them on and have a body that can still focus with the resultant loss of two stops of light, provided you care about autofocus.
2X teleconverters will reduce the light reaching your camera by two stops. For example, a f/2.8 lens will now effectively be an f/5.6 lens. That lens stopped down to f/4 will effectively be at f/8. This can impact your camera's ability to autofocus.
Autofocus:
For example, EOS 20D, XT, and 30D bodies can retain autofocus down to f/5.6. Beyond that, like f/6.3, f/8, they will not be able to autofocus or they will have extreme trouble autofocusing because there is not enough available light. 1D and 1Ds bodies can autofocus down to f/8.
So if you've got an f/4 lens that autofocuses just fine on your Rebel XT, when you slap on a 2X teleconverter if effectively because an f/8 lens, and it will no longer be able to autofocus on the XT. If you had a 1Ds series camera you would be fine.
Brightness:
In addition to autofocus issues, you're also obviously making the lens darker, meaning you'll need to shoot with longer shutter speeds/have a steadier grip/have a more stationary subject. The image through your viewfinder will also be dimmer, which can make manual focusing difficult.
Quality:
If you buy a pro-level teleconverter with excellent glass, the extra glass by itself that you add to the light path will not degrade your image too much. What really degrades your images is the lens that you stack the t-con onto. What t-cons do is effectively magnify the image that's already coming from your master lens, optical impurities and all. This means that if your master lens does not produce sharp images to begin with, the t-con will only be magnifying those unsharp images further, resulting in really unsharp images.
Thus it is only advisable to use 2x t-cons on excellent primes or excellent zooms. Used on a 75-300mm in your case would produce dissappointing results, as well as disable autofocus and require much longer shutter speeds. This is bad for sports
A 300mm will have the equivalent field of view of a 300mm x 1.6 = 480mm lens. The 1.6x comes from the Rebel's 1.6x sensor crop factor.
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