I don't have an answer for this but have a question kinda bout same thing. I have the cannon rebel t3 with the 300x lens. It does really great but I'm looking to get a little closer for football games etc. What off brand lens is recommended for getting closer. Also what's the difference between the mirror lens and regular as far as quality.
Thanks
Which lens do you have...a 300mm? Maybe a 75-300mm zoom?
Anyway, to get closer, you need a longer focal length, so something longer than 300mm. It sounds like you've been looking at those 500mm 'mirror' lenses. Yes, there certainly is a difference in the quality between a 'regular' lens and a mirror lens. Probably a more accurate way to put it, is that the mirror lenses you see for sale, are just all around cheap...so their quality is going to be shoddy anyway.
On of the main disadvantages is that they (I think) have a maximum (and fixed) aperture around F8. That means the amount of light they can gather, is limited. That, in turn, means that it will be harder for you to get a good exposure without having to use a slower shutter speed. A slower shutter speed will mean blurry photos.
They aren't useless...they are a way for you to get a lot of reach, for very little money (a normal 500mm lens is very expensive)...but if you are expecting to get nice photos out of it...don't hold your breath.
Another option might be to use a teleconverter. That is something that mounts behind a regular lens and magnifies the image. They usually come in strengths like 1.4 or 2.0. So with a 2X TC, you could turn your 300mm lens into a 600mm lens. That sounds good, but there is a catch. The TC eats some light and you end up with a smaller maximum aperture...which means less light, which means slower shutter speeds. Also, there is an image quality loss...depending on the quality of the TC.
Lastly, it's likely that neither of those options will allow your camera to autofocus. The cheap mirror lens is probably manual focus only...but even though you can get AF compatible TeleConverters, your camera needs a max aperture of F5.6 for the AF to work. SO if you use something with a smaller max (higher F number), the camera won't have enough light to AF.
So really, my suggestion would be to just try to get closer, rather than using a longer lens or a silly lens. Or, just get as close as you can and try to get very sharp photos, then crop them later.