Monday, 11 July 2011


So what is depth of field?
Sounds grand, doesn't it. Depth-Of Field or DOF.
Actually it's very simple in it's basic form. This is what it means:
The distance in acceptable focus, from the closest, to the farthest point.

So why does it change? Can I control it? 

OK, at a given distance and at a given aperture, DOF will (without the confusion of focal length for now), be a given value.
That is to say, that there  exist a mathematical formula for depth of field.
I'm not going to even approach it.

Whilst it is vital to understand the basic principles of DOF, it isn't necessary to delve into the science of optics and obtain a certificate!
I shall discuss DOF as though we only had one lens to hand. One lens of a fixed focal length, (known these days as a "prime" lens).
Consider the two images above. The first shows a shallow depth of field of about 9 inches.
The second shows a somewhat deeper depth of field of about 4 feet. The lens in this case, is a 60mm prime on a 1.6x APSC sensor, (Canon). The reason that the two look so different, (apart from my poor framing....Sorry), is that one is shot at an aperture of f2.8, and the other at f8. Now, f2.8 is a fairly wide aperture, that is to say, it's a fairly large hole in the lens.  F8, on the other hand, is moderately small. Which one is which?
The answer is, that the larger aperture, (in this case f2.8), produces the LEAST DOF.
As the aperture, (or hole in the lens), gets smaller, so the DOF, or area showing acceptable focus gets DEEPER.
Got to go now............. More soon.