I keep learning new things about my D3200, and while playing with button settings in the menu came across another which it seems is not well documented and understood.
As we are no doubt aware, the D3200 and others of its ilk are always in what is known as "focus priority" mode. That is, if you cannot lock onto AF, the shutter will not operate. In darkness or odd circumstances one must either focus elsewhere and recompose, or go into MF.
I was looking into the possibility of doing what is called "focus trapping", whereby one puts the camera in focus priority mode, with a single focus point containing no focusable item. Holding the shutter button down, the camera fires only when something comes into focus. So, for example, you could aim at a tunnel, and the camera would fire when the train comes out, or aim at a basket, and it would fire when the ball goes through it. Alas, the D3200 cannot do this or at least not well, and in finding this out I found out something else.
In the menu, under the "Buttons" heading, one can assign the AE-L/AF-L button in various ways. The last menu option here is "AF-ON". This is an odd choice of terms because what it does is to disable the usual AF operation by the shutter button. Now, AF occurs ONLY when you push the AEL/AFL button. What is not noted anywhere, as far as I can see, is that along with this, it switches the camera to "shutter priority (aka release priority)" mode. The shutter will fire whether or not focus has been found, and if you are not focused, your picture will be blurry.
I am guessing that the D5xxx menus are similar if not the same. So, if you need shutter priority, this is the only way to get it without disabling AF. If you are having problems with AF, this is one setting to check in case somehow it got changed.
Edit to add: It seems that the D5xxx does have a separate setting for focus versus release priority. I do not know, however, how the AF ON button influences it.
Edit to add: You can still almost focus trap with this setup. If you use the AF-ON function to preset a focal point, you can then wait until the action comes toward your preset focal point and make multiple shots. The camera will fire without refocusing, so if a subject is in your sweet spot you will catch it. The arrival of the subject will not trigger the shutter. You'll have to do that yourself. It's not much of a step beyond going manual.