I thought I understood dof but I'm starting to question my understanding. I know and understand it by definition but does it effect focus. For instance, taking a pic of bird from 1.5 meters at zoom of 40 on 17-55 lens set to apiture 5. It seems that the face is in focus but as you look down (further away) the bird the focus fades slightly. This could very well be something that I'm doing wrong (beginner) but I wanted to rule out the possibility of it being a dof issue (wrong choice). I hope my question make sense. Thanks you for your help.
Two things affect DoF - magnification, and f/number.
There are two aspects to magnification, focal length and shooting distance (also sensor format, but you can't change that).
If you make the image bigger, either by moving closer or using a longer focal length, DoF is reduced. If you use a longer focal length but also move back so that image size is maintained, DoF stays the same. And of course lower f/numbers reduce DoF, and vice versa.
Use the link posted above to DoFmaster and put in a few numbers and see how things change. DoF doesn't change on a linear scale and when you're shooting close, as in your example, DoF is dramatically less than at greater distance. At macro distances, DoF is commonly measured in millimetres! With a bit of practise you get a feel for how DoF changes and know when to adjust settings.
Edit: there is always more DoF behind the subject than in front. When shooting normal distance subjects, like maybe a group of people, the DoF is roughly one third in front and two thirds behind - hence the 'focus one third in' rule. But at greater distances, there is far more DoF behind the point focused on, eg landscape, and in really close working like a very tight head shot or macro, DoF gets closer to 50/50.
