Nikon F to Canon EF Lens Converter?

Might be a stupid question, but does such a thing exist? Nikon has differently priced lenses and there are after market lenses (Sigma, Tokina) that have an F mount, but i'd like to buy them for my Canon T3i which has an EF lens mount. Is there any kind of converter?

Why not buy a proper Canon EF mount lens? If any converters exist, which I doubt, the full functions of the lens would not be available. Go for an easy life - get lenses designed for your camera, or, if you must have Nikon lenses, get a Nikon camera body.

There are indeed converters to let you use Nikon F mount lenses on a Canon EF mount cameras but, it is a false economy if you think using them will save you money. Novoflex makes the best converter mount available at around $250 but, it still turns $1000 Nikkor AF-s lenses into little more than manual focus lenses when mounted on a Canon body. Even the newer NT version of that mount that provides an "aperture ring" for Nikkor "G" lenses will leave you with a manual focus only lens.

You need to know that Canon and Nikon use entirely different electrical contacts and other circuits to control their lenses. That's part of what makes them proprietary mounts and the same is true for Sony/Minolta, Olympus, or Pentax. It should also be noted that until a few years ago, most Nikon and aftermarket F-mount lenses all depended on a drive motor inside Nikon camera bodies for autofocus operation where Canon has been an all electric mount since 1987. Because the Canon cameras don't have internal motors to drive the focusing mechanisms and the other issues mentioned above, no lens designed for a Nikon camera is going to autofocus on a Canon body.

If you want to use Sigma, Tokina, Tamron, or others, they all make versions of their lenses specifically for the EF mount. There's no need to buy an "F to EF" adapter if you choose lenses from these manufacturers. Having said all that, Canon has never released the design specs for their lens mounts to aftermarket manufacturers. As such, all the aftermarket lens makers who wanted to offer EF mount lenses have been forced to reverse engineer the EF mount and autofocus system to make their lenses work with Canon EF-mount cameras. They don't always work well, particularly with newer model cameras. In some cases, the lens makers offer firmware upgrades to make their lenses work with newer cameras but, that doesn't always solve the problem.

The bottom line is that if you want consistent performance and lenses that work 100% of the time with your Canon camera, you need to stick with Canon lenses. Many other people have done the same thing and learned the same lessons. It's not unreasonable to want to save a little money but, buying adapters to go between your lens and camera is usually not the best way to achieve it.