This applies especially during low light situation, where one does not have the luxury of having a small apeture opening.

Low Light-> use large apeture -> shallow depth of field -> quite impossible to get entire subject in focus esp in close range

and Thus, it is very easy to get blurry images in low light.
What is the best that we can do ?

Focus on the eye!
Once you got the eye, your picture is still fine !

See the example in this link

This is a 18inch deep tank. the only light source is a pathetic 2 ft FL light.

This picture is taken with film (asa/iso 800) and later the photo is scanned using a flatbed scanner.
what you see is what you get. Only autolevel was done later to compensate for the scanning.
no flash used in this case.
50mm lens wide open at f1.4.
super shallow depth of field.

ANd as you can see, film has a distinct look that digital cannot duplicate.