We may want the subject to stand out from the background, but the ideal is not going to be achieved if the subject itself is out of focus.
Let’s say we are shooting with a full-frame camera – in other words, a film camera or a digital camera with a sensor the size of which is the same area as 35mm film.
Let’s say I am using a 50mm lens and I am two metres (about six feet) from the subject.
And let’s say I have the aperture set nice and wide at f2.8
The distance from front to back that will be acceptably sharp is about 30cm (about twelve inches).
What I mean by’from front to back’ is this. Imagine a person facing me and I am taking their picture. Imagine that I focus on their left eye.
With the set-up I just described, there will be a depth of about 12 inches – say 4 inches in front of that eye and eight inches behind the eye – that will be in focus.
As long as the person doesn’t move more than eight inches back or four inches forward, then their eye will be in focus.
That means there is plenty of latitude in case my focusing is off or they move.
Now try it with a more typical portrait lens – let’s say a 135mm lens.
The distance that will be sharp from front to back is about 4cm (about an inch and a half).
In other words, if you focus on the eye of the subject, and the camera’s focusing mechanism is a bit off – and it often is – or the person moves just a couple of inches – then chances are that the eye will not be in focus.
With an APC-sized sensor that one finds in a typical consumer dSLR, the front to back sharp region is smaller – just 3cm or just over one inch.
And with a micro 4/3 camera, the region that is sharp is less than 2cm (about three-quarters of an inch).
With a typical point-and-shoot camera it is less than 1cm (about 1/3 of an inch) – and there is practically no chance of being sure that the eyes are in focus.
So don’t get hooked into ‘open apertures at all costs’, because sometimes the cost is too high.
Here is a link to Bob Atkins’ Depth Of Field Calculator.