I have been told that it is difficult to focus with an aperture of f/1.4.
So, in which kind of scenes does it actually make sense to use the f/1.4?
In portraits they say that it is important to keep focus on both eyes (ruling off f/1.4).
In moving subjects (street photography) it s simply not possible to focus with f/1.4.
In food photos it will then focus only a particle of food.
What are the PRACTICAL uses of f/1.4?
I use wide open apertures like this very often. It all depends on your style of shooting. You specifically said that portraits might not work well with this aperture. I disagree. Some examples of when I use it include:
Keep in mind, f/1.4 is perfectly useable at a safe distance. If you are right on top of your subject, yes the DOF will be razor thin, but from a distance it is less difficult to be right on with focus.
There's several practical cases here. Your depth of field isn't only a function of the aperture so if other factors suite, then its still fine.
If you're sufficiently far away from your subject, then using f/1.4 would result the majority of your subject being in focus.
If you have a high performance AF system (something like the 7D perhaps), then you're more likely to keep the point of focus exactly where you expect.
If the scene is so dark, you may need to shoot at a wide open 1.4 in order to get enough light - you're willing to trade extreme subject isolation and possibly not get the exact point of focus for noise and proper exposure.
You do see it in portraits some. Its a very trendy thing to do to. The eyes should be roughly on the same focal plane, so you should be able to get them in focus and the ears will be blurred out.
Frankly though, I don't - nor do most people I know - use f/1.4 all that often.
"it is difficult to focus with aperture 1.4."
First, this is not necessarily true without additional qualifiers. And actually, it isn't necessarily difficult to focus, but it can be difficult to get your subject in good focus, particularly when the camera-to-subject distance is short. This is because depth-of-field depends on the camera-to-subject distance: when you are close to your subject, the depth-of-field will be shallower than when you are further away. If your subject is inches away and you're using f/1.4, the depth of field is going to be extremely shallow and can result in the one-eye-out-of-focus situation.
As to your actual question, "in which kind of scenes does it actually make sense to use the aperture 1.4?", there are several:
low-light situations where the use of flash or other supplemental lighting is undesirable, impractical, or impossible and you risk missing a shot. Basically: you don't have any other choice.
any lighting situation where the camera-to-subject distance results in a depth-of-field that is suitable for getting your subject in proper focus. There are numerous depth-of-field calculators that can help you with this, as it depends on sensor size, focal length, and the distance.
I do photojournalism and portraits, it's often needed where flash isn't allowed or you don't want the look of flash. The quality of the 1.4 lenses are also better than the 1.8/2 versions, in general. Better build, better glass, better sharpness, better color saturation, more aperture blades, etc. etc. Consider them the 2.8 zooms of the prime world...
Lenses are all best when stopped down a little. An f1.4 lens will be better at 2.0 than 1.4, and perhaps better at f2.8 than 2.0; it may be "best" at f2.8. Similarly, an f1.8 lens will be better at f2.8 than 1.8; it may be best at f4. So, if I expect to shoot at f1.8-f2, I would likely be better served by an f1.4 lens than an f1.8 lens.
At any aperture I use, the depth of field is controlled by two things: the aperture and subject distance. So if I want to take a photo at f1.4, lets say of a person, and they are close to me I may have trouble keeping focus on both eyes because of how shallow the DOF is. But if I back up and refocus, that same f1.4 aperture will be able to keep focus in both eyes, out to the nose and back to the ear. Increasing subject distance increased depth of field at the same aperture.
So, it's entirely possible to do street photography and have focus on both eyes at f1.4, so long as the subject distance is adequate. (In fact, I have a friend whose street photography is shot almost exclusively at f1.8.) Food photography -- where the distance to the subject is very small -- emphasizes the thin DOF that shooting at f1.4 offers.
At any rate, I can think of two clear scenarios where shooting at f1.4 might be preferable:
Personally, I use it whenever I can mainly for the purposes to get a thin DOF and a very blurred bokeh. Whenever I take portraits of my kids I do it wide open or for shots that are almost macro but not quite; flowers, leafs, machine parts. I keep it steady, focus with care and BREATH properly my my elbows braced and I get very much rewarded when the "paper thin" DOF falls just where I intended. If I can't hold it I stop down to 1.8. The 50mm f1.4 is my staple lens. It was on Pentax and it is still on Canon. In fact I enjoy taking photos of very thin DOF so much I am considering the Canon 1.2F L sometime in the future.
