I have a Nikon D5000 F-mount camera, and I just ordered a Bower 500mm T-mount telephoto with an F-mount adapter. I want to be able to slap on a 2X teleconverter, but I don't know how to tell which ones will fit my setup. Help?
Any Nikon TC should be fine, but I'd strongly not doing it if you have to spend money. You could even try a T mount TC (if you can find one), but it's not going to change the reasons why I'm suggetsing it's a bad idea.
A TC will result in a your aperture being reduced by the factor of the TC. So using a 1.4x TC will loose you one stop of aperture and using a 2x TC will take off two stops !. As the Bower 500mm is probably an f6.3, that means you'll be down to an effective max aperture of about f12.6 ! If it's an f8 lens then you're reduced to an terrible f16 with a 2x TC.
This would mean autofocus will probably be crippled, but with a T-mount lens there's no AF anyway. But it will still make manual focus problematic.
In my own experience a TC will typically not give you much more than very marginal benefit over simply using a decent enlargement algorithm in an editor. Now if you were using a top grade lens and a top grade TC there's some more benefit, but I'm skeptical that the Bower and a TC would be worth the price of doing it.
Note one other thing. Using a 500mm on a crop frame DSLR like the D5000 will need a shutter speed of about 1/750th to avoid shake (and more if you pixel peep). Using a 2x TC as well would mean you'd need 1/1500th. Even with a tripod I'd expect to have some issues with shake and vibration on such a setup.
Now remember that effective aperture with the TC. This means you're trying to shoot at maybe f12.6 and 1/1500th. Even for a "sunny f16" target that means you need to crank up ISO quite a bit, so you'll also get more noise, which won't help detail at all.
So keep these things in mind.
This video might be helpful : https://www.youtube.com/watch?v=_t9lUUu8StI
A lot depends on the lens/Teleconverter pairing. If you have a look at David Ruethers excellent article at http://www.david-ruether-photography.com/slemn.html, you can get an idea of just how much difference which converter you use with which lens can make
Having tried a cheap teleconverter with some good Nikon lenses, I agree with StephenG - The difference between using a generic teleconverter and just enlarging the image is marginal. I tried it for some bird photography, and although there were differences between the results, neither was clearly better than the other.
If you were using it with an AF lens, rather than the presumably manual focus T mount one, you'd also notice that the slower effective aperture tends to make AF performance worse - if it still works at all.
Matched teleconverters - designed to pair with a particular lens - are a different story - they tend to work well. But a generic TC - especially one that's designed for shorter focal length lenses - tends to be disappointing.
In the slide film days, a generic TC may have been worthwhile. But with the current digital systems, that's much less apparent. If you can find a camera shop that will let you try the TC out with your lens, it may be worth shooting some test shots with and without the TC, and see for yourself how the TC compares with enlarging things digitally.