OCD FOR DOF
Lets face it. A sharp photograph with a shallow depth of field often has wow factor to many people. This can be regardless of whether or not its actually a good image. The layperson who is not used to such effects on their point-and-shoot camera or cellphone immediately associates these two attributes as being solely responsible in making a great image. Of course, those of us who've been at it for a while know different.
The same bamboozling is true with newcomers to photography. After playing with their new toy for a few months, the beginner develops the taste for something a bit more professional. This often manifests in the purchasing of the 50mm f/1.8 - and for good reason. Optically it's a great lens. Sharp, shallow depth of field, lovely bokeh. It's cheap, quick to focus too. What more could you ask for in a lens? Unfortunately, because it's a wide aperture lens, therein lies the temptation to use it as such - constantly. I mean, what's the point buying it otherwise, right? (yes I'm being facetious)
While experimenting with a shallow depth of field is great to help you learn about the effects of aperture on both depth of field, sharpness and shutter speed, the amateur often feels the need to photograph the entire world in this manner. This obsession can often result in potential problems, such as overexposure or out of focus images. The first is caused on bright days where the camera's maximum shutter speed still isn't fast enough to get correct exposure. The second is due to the depth of field being so shallow that focus lands on noses, shoulders, hair or ears, rendering eyes somewhat blurred.
In reality, unless you're shooting in very dark conditions and need to get a fast enough shutter speed to avoid motion blur, or are purposely trying to isolate the eyes or a certain feature of a subject, an extremely shallow depth of field just isn't needed. Often it makes faces fall out of focus too quickly and if your subject is a child that tends to move about a lot, you're probably going to end up with a fuzzy photo.
I learned this lesson very early on myself. I cut my teeth photographing my nephews, who were all toddlers at the time. At that age, they never sit still for more than a few moments and even when they do park their bum on the floor, they always seem to be rocking or jiggling about in one way or another. Even on the APS-C sensor of the Nikon D90, f/1.8 was too shallow when trying to get critical focus. For this reason I never opted to use the lens below f/4.
Happily, this actually corresponded with the "sweet spot" of the lens in terms of optimal performance. My resulting images were razor sharp. The eye(s) were in focus and the backgrounds sufficiently blurred to my liking. I got much more "keepers" as a result. Even when I did try shooting with a wider aperture and nailed focus, unless the camera was square on to the face, things just were too soft, at least for my tastes anyway.
What beginners don't realize, is that achieving a blurry background is more to do with focal length and distance to subject (both camera and background), than it is the aperture. Use the 50mm wide open at f/1.8 filling the frame with your subject. Then get yourself a 200mm lens. Fill the frame the same and look what happens to the background. It's even more creamy and buttery at f/5.6 than the 50mm is three and a third stops wider at f/1.8!
Below is an example of what a long focal length does to depth of field. The image was shot at 210mm with an aperture of f/8. Even with such a moderate f-stop, the depth of field still falls off very quickly. It's a very pleasing effect as the eyes, nose and mouth are all in focus but the image begins to soften about 2cm behind the eye and falls into blur quickly beyond that. The background which was only about 2ft behind the subject was very soft.
The fact of the matter is that in nominal light conditions, fast aperture glass is frequently too shallow for a lot of practical applications. Just because a lens goes to f/2.8 or f/1.4 or even f/1.2, it doesn't mean you should always use it at that aperture. Approach each shot/shoot with both technical requirements and aesthetic ones in mind. Are you doing a couple's portrait? What depth of field are you going to require to get both subjects in focus? Do you want to isolate a model from the background or would an environment portrait make for a stronger image?
The moral of the story is to concentrate more on your subject and composition than being obsessed with the effect of shallow depth of field. At first it's a novelty but if you solely rely on that for the "wow factor" in our photographs, you're going to get a rude awakening at some point. Primarily a fast aperture is there to let in more light to get your shutter speed up to a sufficient level to avoid blur and to fascilitate focus. The depth of field is kind of a happy side effect.
While it has its applications, in many ways though it works against you. When shooting wide open it's harder to nail critical focus and the lens is at its softest too. Personally, I'd recommend you use a lens more towards its sweet spot for best image quality but, more importantly for the task at hand. Cover your technical requirements. Apply your artistic slant and get your shot. If you're aim is separation between your subject from their surroundings aim to use a longer lens over a faster aperture. For people, the compression effects are much more flattering, you eliminate problem areas due to the narrower angle of view and the backgrounds become soft and creamy, much more so than a wide aperture 50mm lens.