While I was in the smokies a few weeks back I took the opportunity to take a few photos for educational purposes.
When people start photography, it seems Depth of Field is a concept that many struggle with. Then, once they have it figured out, they get a macro lens and that frustration comes back to the forefront again.
Depth of field is, of course, determined by your F stop, but another factor that is in the equation is the distance from your subject. This becomes extremely evident in macro work because you are only inches away from your subject. The other problem that comes in with macro work is that true macro lenses are of longer focal lengths. Our focal length is another factor that helps determine DOF.
Let's take this photo for instance. This is a shot of some cone flowers. I used an F stop of F/11, which when shot with a lens that is equivalent to a 35mm is more than plenty to give us a nice depth of field throughout the image. My favorite macro lens is the Tamron 90mm. So, again, we are very close to our subject and we are shooting at a longer focal length.
Notice those red out of focus dots in the center of the image? Those are aphids. They are probably less than 2 inches away from the cone flowers. You can see here F/11 was no where close to giving us enough DOF if we want both the flowers and aphids in focus.
Here's another shot of the aphids in focus. See how quickly we lose the flowers? Same F stop here, F/11.
The only thing that changed here was my focus point. You can see F/11 is almost nothing in macro work. In the macro world, if you want a wide DOF you are going to find yourself shooting in the F/32 club a lot.
Hopefully this illustration helps you understand DOF when working with macro.
BTW...there is dirt on my sensor here...Don't judge ;)