Depth of Field (DOF)

Proper and creative use of DOF is perhaps one of the single most useful tools for creating and forming your desired composition.  I’ll be spending a lot of time in future columns demonstrating many different uses of DOF.  In this weekly we are merely going to define DOF and discuss how we use the basic five variables to influence DOF to meet our desires during composition.

DOF simply refers to the portion of the scene which is in focus.  If you have a person standing 20 feet in front of you, mountains in the far background, and a small fence in the foreground (ahead of the person) and the person is your main subject, then you will focus and expose on the person.  When photographing any subject with eyes, be it humans, animals, birds, or fish.. you always select the closest (towards the camera) eye as the focal point.  If that closest eye is in focus, then the image will look correct.  Depending on the camera settings (aperture, shutter speed, ISO, focal distance and focal length) the scene will stay in focus from that closest eye outwards from just inches, to miles.  How much of the rest of the scene stays in focus from your main focal point (closest eye) is totally dependent on the photographer and his/her desires via the choice of camera settings.  That portion of the scene that remains in acceptable focus is referred to as the depth of field or DOF.

There is a series of mathematical formulas used to calculate DOF using the above five variables, and there are slide rulers made just for DOF, scales on the older lenses, and all kinds of aids have been created in the past to ‘help’ people understand and calculate DOF, but I find that each time I try to use one of these to help my students understand they get more confused than ever.  Controlling DOF isn’t that hard to understand and I believe the best way simply involved a tiny bit of memorization and lots of experience.  This is how it works in just a single sentence.  Learn it, repeat it, and think about it when choosing your settings.

The wider (more open) the aperture, the closer (more near your subject) the focal distance, the greater (more magnification) your focal length, the LESS your DOF.  This assumes the same size sensor in your camera.  The size of the sensor also affects DOF which is why compact cameras seemingly have an almost unlimited DOF and you can never seem to get them to produce a decent shallow DOF.  For instance you have two DSLR sensor sizes.  APS size (14.9x22.3mm), and Full Frame (24x36mm)  The full frame sensor is bigger which makes a more shallow DOF possible than with a APS size sensor.  So, if you have for instance a Canon 20D DSLR using an APS sensor and you have a 135mm F2 lens mounted, and your settings are F2, 135mm, and you’re ten feet from your subject, and you have a Canon 5d with a full frame sensor with the exact same 135mm F2 lens mounted, and the exact same settings of F2, 135mm, and ten feet from the subject, you will have a significantly greater DOF in the image from the Canon 20D APS (crop) sensor, than from the Canon 5D full frame sensor.  So, you can see that the size of the sensor or film plane directly and significantly affects DOF, but since we normally shoot from the same camera I’ll omit this variable from further discussion.

Lets look at some extreme examples.  Below is an entire frame of an image I captured while down south interviewing the kris master.  At first glance is appears nothing is in focus.

Kris --shallow DOF

But look closely at this small crop of the image.  Do you see the part of the blade that is in focus and very sharp?  The “depth” of field for this image is approximately 1 inch!  Not much at all.  The image, as taken, is in focus!  But at first glance you’d never realize it.  For this image I used a full frame DSLR (sensor size large), a 85mm F1.2 lens (focal length), a F1.2 aperture (aperture), and a working distance of about 2 feet (focal distance) and produced an extremely shallow DOF.

Kris in focus

What about a more practical use?  Take a look at the picture below.  This was taken in a dark room near sunset with just a bit of light coming in through the window.  I set the camera to properly expose his face and the settings were:  DSLR (full frame), 85mm F1.2 lens (focal length), F1.2 aperture (aperture), and a working distance of about 10 feet.  The DOF is still very shallow at roughly 5 inches, but it’s five times more than the above example.  What variable changed in the scene?  Yes, the working distance (focal distance) is the only variable that changed, from 2 feet to 10 feet, five times more.  Do you see how the math works?  You change one variable, and the other variables change the same degree.

85mm, F1.2

 

But was the image sharp at F1.2?  Many experienced photographers will try and tell you that any lens at F1.2 will never produce a sharp image, but the truth is a F1.2 lens is highly specialized and requires a specific skill set to use correctly.  Look at the image below and judge sharpness for yourself at F1.2.

Critical focus

Lets take a look at this last image, a seaside scenic.  Every part of the frame is in focus from the tiny yellow flowers which were actually touching the bottom hood of my lens to the furthest point across the bay.  The settings were:  DSLR (full frame), 17mm F4 lens (focal length), F16 aperture (aperture), and a working distance of at least 200 meters from the closest significant structure (focal distance).  Because of the focal length and aperture, everything on either side of that focal point for miles was sharp and in focus.

A house by the lake - Deep DOF

Once more for review.  This is the one sentence you need to fully understand:  The wider (more open) the aperture, the closer (more near your subject) the focal distance, the greater (more magnification) your focal length, the LESS your DOF.  Using this sentence you can substitute any single or multiple variable and know in which way you’ll affect DOF, the area of the frame which will appear in focus.  You’ll see this material again and again in the future as we go over the many techniques which take advantage of DOF.