I got a Sony camera for Christmas. Lately, I've been looking at getting a fisheye lens. I really want to buy one sooner or later. The problem is the price. I found a fair price on a fisheye lens from Nikon, but I have a Sony camera. I wanted to know if the lens needs to be the same brand as the camera?
I have one of those big cameras that come with the lens, which is not removable. So I'd need a lens converter of some sort, right?
From your question, it sounds like you have a Sony camera, and that camera does not have a removable lens. If that's the case, you cannot use a Nikon fisheye lens with your camera. A fisheye lens from Nikon is designed to be the only lens on the camera!! It's not an add-on accessory that you can just slip on to the end of an existing lens, or anything like that. While lens converters can allow you to put a lens on a mount that it's not designed for, a lens converter goes between the base of a removable lens and the camera body -- not on top of the lens.
Based on what I understand of your goals, you might be better served by selling your Sony camera, and purchasing a new camera body with interchangeable lenses. The Micro Four-Thirds system should be of particular interest to you; the system's design goal is to provide affordable high-quality digital cameras that you can swap the lenses on, so a variety of lenses are available new and used from a variety of manufacturers.
Even new, you can find plenty of affordable fisheye lenses for the Micro Four-Thirds system like this 9mm/f8.0 for only $100, or, for a real bargain, this kit of three different lenses from Lomography including a fisheye for $90. These won't be good lenses, but with fisheyes you're really going more for special-effect than Pristine Optical Quality. Of course, if you really want quality, you can get this lovely Zeiss 15-30mm f/2.9 zoom lens for the low low price of $23,900 (!!!!! $$$$$) Micro four-thirds is a very flexible system. :P
(KEH.com has good listings of used camera bodies and camera lenses, with meaningful quality ratings, though it helps to know what you're looking for first because they don't do a good job of explaining their listings. If you are concerned about affordability, buying used lenses is a very good deal.)
To mount a lens on a camera, the lens and camera must have a matching mount. Otherwise it will not connect without something called a lens-mount adapter. Not all mount adapters exist because of something called the flange distance which is basically how far a lens is designed to be from the camera sensor.
Brands like Nikon and Sony make lenses for their own mounts obviously but you will find third-party manufacturers like Sigma and Tokina make lenses in multiple mounts. They are the same models but have to be the right one for your camera.
In your case, it turns out there are mount adapters to put Nikon lenses on a Sony camera since the Nikon F-mount has a greater flange distance than the Sony A-mount. Beware that such adapters most often lose functionality and it is unlikely you would be able to autofocus with a Nikon F-mount lens on a Sony body.
To save money, it is more practical to look at used A-mount lenses from Sony or Konica-Minolta since these mounts are the same.
If you're talking about a lens, one that mounts bayonet-style directly onto a camera body, then, no. Not if your camera has a fixed lens. In a fixed lens situation, yes, a cheap screw-on converter could work; they're typically advertised with a 0.#x conversion factor, like 0.4x or 0.2x to say how much wider they will make the lens. But the quality won't be particularly great.
The main question here is what Sony camera are you trying to add a fisheye converter onto? Most fixed-lens cameras don't have filter threads of any kind. Even some of the "bridge" cameras (which is what it sounds like you may have). The HX300, for example, can't take a filter. But the RX-10 has a 62mm filter size.
So if your Sony camera has a filter thread or an accessory tube that adds one, then if you're seeing a filter-threaded converter that's advertised as being "for Nikon", that would work as long as the filter thread diameter is the same size. If the thread on the camera is smaller than the converter, you might be able to get the parts to match using a step-up ring. But if the lens is bigger than the converter, you may not want to go there, because you're likely to get vignetting.
But this presupposes your camera accepts filters, and that's unlikely.
