Back in 2008, I sold off my trusty 50mm f/1.8D AF prime lens, with the idea that I will upgrade to the faster version of the lens. I even placed the order for it on Amazon. For some reason, Amazon had difficulty in fulfilling the order and kept postponing the delivery. I got fed up and cancelled the order and got the 105mm VR macro lens instead.
It was always in my schedule to eventually get the f/1.4 lens, and when Nikon introduced their AF-S version of the 50mm f/1.4, I thought that is the lens I should go for - but the steep price of $440 kept me at bay.
So when I saw an ad for a used 50mm f/1.4 for around $200, I was really really tempted even though I was not sure whether I could afford it. The decision became much easier when my wife urged me to go ahead and pick it up. All photographers should have a wife like mine.
The f/1.4 lens is 2/3rd stops brighter than the f/1.8 counterpart (i.e. 66% brighter). That makes a big difference when it comes to shooting in low-light situations.
This picture of my daughter Erika was taken at f/1.4.
BTW, it is a ritual of mine to first take a picture of Erika with any new lens that I purchase.
The f/1.4 also makes it possible to completely blur out the background because of its extremely shallow DOF (depth-of-field). This is easy to see in the picture above. Erika was standing in a mall when I took the picture - but the whole background is completely blurred out.
The shallow DOF makes it very tricky to get the right focus. Even a slight mistake can render the picture out-of-focus so nailing the focus is extremely important when shooting wide open with this lens.
This picture really demonstrates the wafer-thin DOF of this lens. In this picture (shot wide open at f/1.4), just 2 or 3 lines of text are in focus, while the rest of the book (both front and rear) are out of focus. This can best be demonstrated by a 100% crop of the above image.
Traditionally, on the film cameras, 50mm was considered the "normal" lens because the area of coverage provided by a 50mm is similar to the coverage that we get with our naked eyes. And this principle still applies if you have a FX sensor camera such as a Nikon D700, Nikon D3S, Nikon D3X, etc. However, when you mount this lens on a DX sensor camera such as the D90, D300s, the camera gives you a field-of-view of 75mm because these sensors have a crop factor of 1.5 (so 50mm x 1.5 = 75mm).
Since 75mm is an ideal range for portraits, this becomes an ideal portrait lens when mounted on DX sensor cameras.
Conclusion: If you love low-light photography or portrait photography, you owe it to yourself to invest in a 50mm f/1.4.