The auto-focus capability of today’s DSLR cameras easily meets the needs of the average user. Along with such things as pre-programmed exposure modes, the auto-focus function will deliver an acceptable result most of the time. However, reliance on this feature leaves much to be desired when critical work demands full control of every aspect of the photographic recording process.
Older film based cameras, particularly those designed from commercial use featured such things as lens barrels engraved with accurate focusing and hyperfocal distance information, split screen or similarly assisted viewfinders and readily accessible diaphragm stop-down levers or buttons. Today many DSLR’s are supplied with zoom lenses rather than fixed focal length optics and these zoom lenses don’t have depth of field (DOF) markings or hyperfocal distance pointers, so if you want maximum control of focus, you’re on your own.
This quick article explains the concept of DOF and how to consistently apply it to your photography. There are also two more articles following this one covering do-it-yourself depth of field testing with your lens and camera and the concept of hyperfocal distance focus. Understanding what auto focus does is critical to knowing when to turn it off and go full manual. Trust me, you can’t critically judge DOF by looking through the viewfinder or by peeping at the captured image on the preview screen, no matter how much you magnify it. The physics are just not in your favor. You can however use a simple set of principals and data tables to setup your focus so it’s bang-on every time without tearing your hair out. In the rest of this article, I’m going to give you the industrial-strength tools to understand the process and consistently guarantee your results. So lets dive in.
For any given point (really a plane) focused on by a lens, there is an area in front of and behind that point that will be acceptably sharp-this is known as the “DOF.” As an example, if we critically focus on an object that is 18 feet from the camera and the overall DOF is 9 feet, objects from 15 to 24 feet will be in acceptable focus. DOF is a fixed optical principal and applies to ALL lenses at ALL TIMES regardless of lens design or what the salesman in the camera store tells you.
Further, the rule of thumb is that DOF is divided into thirds. For any given point focused on, one third of the DOF will be in front of the critical focus point and two-thirds will be behind. That is why in the example cited above we know that the area of focus is from 15 to 24 feet; a third of the field depth is in front and two-thirds behind the object focused on. So, to wrap up this concept, if you are shooting a portrait and the camera is indicating focus on the nose, 1/3rd of the field depth will be in front of the plane of the nose and 2/3rd behind. Assuming that the overall DOF is adequate your head shot will be in focus from front to back. More on this in a bit.
Next we need to understand that DOF is determined by lens focal length, f-stop and camera to subject distance. We can calculate the DOF for any combination of these three factors, or more conveniently we can consult tables prepared for our lenses. I highly recommend that you at least look at the DOF tables featured on www.dofmaster.com . Upon visiting this site, you may input your camera type, the focal length of the lens and quickly output a finished table. Obviously, for zoom lenses, input each of the prime marked settings on the lens barrel such as 17, 35, 50, 85, 100mm and print a table for each. I have a printed set for my lenses and keep a copy in my camera bag along with a 25’ tape measure. The tables and tape measure are the bread and butter of focus control!
Reviewing these tables you will quickly come to understand the following general optical principals:
1. Short focal length lenses give greater DOF than longer lenses.
2. Increasing the camera to subject distance increases the DOF.
3. Stopping down the lens gives greater DOF.
4. 1/3rd of the field depth is in front of the object focused on, 2/3rds is behind.
As an example, consulting a table prepared for my Canon 30D and a 50mm lens, we find that when an object is focused on at 10’ our DOF at f 2.8 is from 9’-5” to 10’-8” but when we stop down to f-16 we are in focus from 7’-4” to 15’-9”. Quite a difference! By understanding and applying the focus tables, you take full control of the focus process to the extent that you can know the DOF and where it will fall. This gives you real certainty in visualizing how your finished photos will look without guessing and allows you to manipulate the focus process to serve your creative ends.
The next article in this series is called “Depth of Field Testing DSLRs” and details how you can run a simple series of tests with your own camera to show how depth actually works.
There is one more condition known as hyperfocal distance that I will deal with in the third article: “DSLRs and Hyperfocal Distance.”
Finally many thanks to Don Fleming for putting up and maintaining www.dofmaster.com a wonderful site that has all sorts of free info and goodies related to DOF and related subjects.