From top to bottom and left to right: Minolta 28-70 manual zoom lens, Vivitar 28mm manual prime lens, some brand 28mm manual prime, Canon 18-200mm kit zoom lens, Canon EF-50 f/1.4 automatic prime lens, Canon EF-50 f/1.8 automatic prime lens.
Reverse Mounting Lens On Camera
So now I have the lens, how do I reverse mount it on my camera? It is actually very simple -- get a reverse mount adapter for your lens. A reverse mount adapter has camera mount on one side and male filter thread on the other side. So it is important to purchase a reverse mount adapter for your camera and it should have the same filter thread size as the front filter thread of your lens! Here is an example of it:
Here is what it looks like when it is attached to a lens (in this case my Canon EF 50mm f/1.8 prime lens) which has 52mm female filter thread at front.
And on camera
Here is another one with Canon EF 18-200 kit lens reverse mounted on camera.
Magnification Of Reversed Lens
Now we have a lens reverse mounted on a camera, what kind of magnification can be achieved? Well it depends on the lens you are using. For a 50mm lens on Canon, magnification is about 1x when it is reverse mounted, a 28mm prime can give 1.79x magnification. The Canon 18-200 kit zoom lens can achieve 4x magnification when zoom is set at 18mm, that is a lot of magnification.
So, it depends on the lens. However, general idea is that the shorter the focal length of the lens, the higher the magnification when reversed. With this general idea, I even attempted to make a variable magnification lens using a manual Minolta 28-70zoom lens (why not the kit zoom lens? I will explain this later)
So, what is the point of only get 1 or 2x? Well, we can always add extension tubes. For example, a 50mm prime can achieve 3x magnification when 100mm extension tube is added, as matter of fact, for each 50mm extension tube added, you get 1x additional magnification. Now if a 28mm prime lens is used, then every 28mm extension will get you one additional 1x magnification. So now it is obvious that shorter focal length lens has some advantages -- higher magnification and shorter extension length to achieve same level of magnification.
Manual Lens vs Automatic Lens
As I have indicated that I have attempted to make a variable magnification lens using a manual Minolta 28-70 zoom lens. Why not my Canon kit zoom lens? Is it because of image quality?
The answer is, automatic lenses, such as the Canon 18-200mm kit zoom lens, do not have aperture ring so you can not change aperture. Camera controls the aperture with these automatic lenses via electronic contacts. For Canon automatic lenses, the aperture is kept wide open when it is dismounted from camera and for Nikon automatic lenses, the aperture is kept to minimum. Without aperture control, it is difficult to achieve the result you want. So manual lenses are better because it has aperture ring and you can change it. Another advantage of manual lens is that they are cheap, relative to automatic lenses. There are also abundance of high quality manual lenses that can easily out perform current automatic lenses.
Does this mean you can not use automatic lenses? Well, you still can, at least with Canon lenses. When you shoot with automatic lenses with Canon camera, the camera usually keeps aperture wide open so it is easier to see in view finder. However, almost all Canon camera I know of have this aperture preview button right below the lens mount. Pressing that button will make camera to set aperture to the right size when image is actually captured. So, there is this trick that while pressing the aperture preview button and simultaneously dismount the lens, the aperture will stay at that size. Note, try this at your own risk please.
First of all, almost everyone has a kit lens, be it a prime or a zoom with wide focal range. So it is just a matter of get the right reverse lens adapter, maybe with a set of extension tube or two.
Second of all, you do not have to use the lens of same brand as your camera, as long as you get the right reverse mount adapter for your camera with the right filter thread size, it will work.
Finally, there are a lot of high quality manual lenses out there at budget prices, so cost wise, it is a good start.