First to be said is not all cameras will accept a reverse ring, as there is no electronic contact between the lens and the camera body. If the camera cannot find the lens, then some will not function. I have tried this method on my Nikon, and it works okay. I have a kit lens that came with the camera, and I wanted to try using this for macro photography.
It is a 18-55mm, and since everything is reversed, I will be able to go closer to the subject, zooming (into) 18mm.
Working with the reverse ring, 50mm that is normal will be a 1:1 ratio. The 18mm will be closer.
You can make good use of your old lenses without auto focus and lenses of any brand will do. Actual an old manual lens, have the advantage that you can set the aperture manually, so you know that this works with a reversed lens. The way to set your aperture on an automatic lens when the lens is mounted in reversed is a little more complicated and could make you wonder if that way could harm the camera or the lens.
But, if your lens suits your camera, then mount the lens as normal. Then, set your aperture at your wanted opening, f.x. f:3.5, then unmount the lens, without turning off the camera. This way the aperture opening will remain because there is no electricity, driving the aperture, and you can use it on your reversed setup.
The idea is that you put a reverse ring on your filter tread of the lens, the other side of the ring, then have to have the right mounting for your camera.
Also, those lenses that normally do not fit your camera can work like this, if your filter tread fits the thread on the reverse ring.
A Nikon with the lens mounted reversed.
What you buy is a ring where one side fits the filter thread (f.x. 52mm), and the other side the mount for your camera. F.x. a Nikon.
This is probably the cheapest way to get into macro photography.
And you can really get in close to your subject.
One thing is though that you will lose a lot of light.
If you combine this, with the bellow, or the set of extension tubes, you will get a real good close-up of your subjects.
You will not be able to use auto focus with this trick, as all connections between the lens and the camera are off. Also the aperture will not work. This is a very cheap way to get to do some macro photography.
And using your cheap kit-lens will suit very fine.
Or you will make good use of your old lenses from another brand, or analog cameras.
I see a ring in a shop for about £2.50
It is just a simple piece of metal, very small and very simple. But you can get a good magnification using a 18-55mm lens.
To get close, any value below 50mm is better (closer). Like an old 28mm or 35mm manual will do really good.
There are most likely many cheap lenses from old style analog cameras to be found second hand.
Notice that you will have to work in manual mode, as again, there is no connection between the camera body and the lens you are using. If you have a flash meter, then you can find what settings to set your camera to. Or again if you work with ambient light you can find old style analog light meter. Notice that these cannot be used with flashes, only ambient light (and therefor also video). But look at ebay here, they are found in plenty.