Just to clarify… This is dealing with normal focal length lenses, say between 35mm and say 85mm. When you get to the extremes with ultra-wide angle lenses or super-telephoto lenses. The equation changes quite a bit. With that note out of the way, why?

It’s pretty straightforward actually. When you use a mode on your camera like P (Program, not Professional), the camera will work to get your aperture to f/5.6 as soon as it gets a safe handholding speed and then it will start working the shutter speed up. Of course, your camera may behave a bit differently in its programming, but the concept is to get to safety, shutter speed first, then aperture. Therefore f/5.6 is “safe”.

So what is bad with safe?

Two reasons. First, it’s not enough depth of field (DOF) to get very much of the scene in focus. If you’re shooting landscape or architecture, you probably should be shooting in the f/8-f/16 range to get the most scenery in focus. Even studio portraits are shot in the f/11 range. At f/5.6 it’s hard to get enough in focus. Even with ultra-wide angle lenses that give you tons of DOF, photographers, who are wanting a considerable amount of foreground in focus will still do focus stacking (shooting multiple photos at different focus depths) to get even more DOF.

In the other direction, the problem is too much DOF. If you’re shooting natural light portraiture, many people want to blur the background. This is frequently misnamed as “Bokeh” (Bokeh is actually the quality of the out-of-focus highlights, or more abstract the quality of the out-of-focus areas). Photographers work to get defocused backgrounds to separate subjects from the background. Sports photographers like to shoot at f/2.8 when they can even with telephoto lenses to separate players from the background. At f/5.6 you just don’t get that separation.

So f/5.6 with mid-range focal lengths is, well, just “Blah”.

Now, many people will read this and just take away that f/5.6 is bad. But it’s possible to get the creamiest “bokeh” ever at f/5.6. You just have to shoot at 500mm. You can get really defocused backgrounds at 50mm and f/5.6, but to do so, your subject needs to be close to you and the background far away.

How much your background blurs is made up of four components:

1. Focal length — the longer the focal length the less DOF you will have
2. Aperture — the wider the aperture (smaller f/ number) the less DOF you will have
3. Camera to subject distance — The closer your subject is to the camera the less DOF you will have
4. Subject to background distance — The further the subject, the faster the DOF fades making seperation easier

Many photographers spend a lot of money on fast, wide-aperture lenses to help make this process easier. It’s important to understand that even if you don’t have a 50mm f/1.2, you can still get defocused backgrounds. Even if you have an f/4-f/6.3 lens that will likely be f/5.6 at 50mm, you can still get defocused backgrounds by controlling the distances involved. Of course, you may not always be able to move a subject far enough away (you might fall off a cliff or end up in a busy street) and you may not be able to get the composition you want if you have to bring your subject to close. This is why it’s much harder to do, and well why you should avoid f/5.6 with normal lenses if you can.