1.It's very sharp.
I'm no expert on lens construction but there can't be much going on in that little lens body so the light has to pass through very fews pieces of glass on it's way to the camera. This makes for some really sharp images.
2. The price is amazingly cheap.
In the UK you can get the Canon f1.8 version for £79 or in the US for $115. The Nikon version is £140 in the UK or $216 in the US. (Although this wasn't an exhaustive search so you might find it cheaper still.) That's a lot of quality light-capture for not very much money. This is one of the reasons the 50mm is the lens I recommend to anyone looking to expand their photography kit and buy their first 'proper' lens, that is, it's not a kit lens with zoom and variable apertures.
3. They have fast apertures.
Even the cheapest Canon has a very fast aperture at F1.8. Not only does this let in a lot of light, it also creates some great bokeh, or shallow depth of field, when used wide open at F1.8. If you want to spend some money on the 50mm Both Nikon and Canon both do two other versions, the F1.4 and F1.2. These faster lenses aren't so cheap, however the build quality goes up. The Canon 50mm f1.2 is from their L-series range and is around £1000 in the UK. It's a huge lens too. That's a lot of money for a bit more light in to the camera.
4. It's like the human eye.
The 50mm supposedly has an angle of view very similar to the human eye. For me it's not too wide nor too narrow.
5. Up close and personal.
It's my lens of choice when I want to get close to a subject with an extension tube. I've taken many shots using this combination. This opens up a whole world of interesting things to photograph. Like Snails!
6. It's very easy on your shoulders.
It's very light and adds little weight to your camera bag. The small size makes for a small profile to your camera so attracts a little less attention than a camera with a larger zoom fitted. Consider how big a 24-70 zoom is compared to a 50mm. Physically bigger and heavier.
7. Filters are cheap!
Because the front element is so small, perhaps the smallest of any lens you can buy, screw-on filters aren't expensive.
8. It makes an ideal portrait lens. You can take advantage of that wide maximum aperture to create some great bokeh and really highlight your subject. A blurred background makes your subject 'pop' out of the frame. The wider the aperture you use, the more blur you create and the greater the effect. Remember to focus on the eyes because everything else can be out of focus as long as the eyes are sharp. Pictures where the subject's eyes are blurred just don't work. Take a look at the portrait of the cyclist, his eyes are sharp but the depth-of-field is so shallow, his ears are out-of-focus and the background is just a big blur. Lovely.
9. Your low-light friend. That fast aperture gathers a lot of light so works really well in low-light situations. When you're shooting in low-light, you want as much light-gathering capability as possible so that wide-aperture gives you that. Just be very careful with the focus as it's easy to miss it when using a wide aperture.
10. The thinking photographer's choice. That fixed focal length means you have to think about the composition. Zoom with your feet, not your hand.
11. Wide aperture, easier focus.
Because there's more light passing through to the camera, it's easier to focus the lens manually.
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