Getting Depth of Field Correct

Recently, I was with my friend Tom Applegate photographing Downtown San Diego from the neighborhood of Point Loma.  This location, albeit in a residential neighborhood, is very popular.  Beside us, were two photographers and their relationship seemed to be something of a teacher and a student (they were probably friends, but one was instructing the other).

The teacher was explaining to the student that she likes to “use a small f-stop to get depth of field.”  Besides the awkward phrasing, I was astounded because the location in question was down a narrow lane of view and the entire scene as shot by anybody was at infinity.

What she was “teaching” made literally zero sense in the sense of the composition in front of them.

Stop Using The Sunny F/16 Rule

I think this misunderstanding of depth of field begins with one of the beginner photography rules, “Sunny F/16.”  While most of you know this, I will explain if you don’t.  Sunny F/16 means that in bright mid-day light. you can shoot without metering at F/16, ISO100, 1/100th of a second.

People tend to not understand the difference in incident light at noon versus a sunrise or sunset.  There are several stops difference.  The reason F/16 is used at all is to compensate for how bright mid-day light is.  The rule in this case has nothing to do with depth of field.

So What Is Depth Of Field Anyway?

Depth of Field is simply a measure of the relationship between a near-subject and far-subject.  This is most accurately described when you are shooting a distant scene with a close foreground.  Your lens can only see a single focus plane, and that point is minute.  With a larger aperture (opening up), the lens diameter can be far beyond that single point.  As a simplification, think of a large circle and a small pencil point.  That pencil point is your focus, and the rest of the circle’s volume is out of that focus point.

With a smaller aperture (stopping down), the lens diameter can be quite small.  Think of the circle in the example above, and draw a smaller one just around the point.  Now the same scene has less volume out of that focus point.

So Small Aperture Wins, Right?

Absolutely not.  The key to aperture and creating depth of field is use only what you need, and nothing more.

Think of light as Einstein first envisioned it.  Photons are not straight.  They travel in a wave, which is why we have colors (different wavelengths) and infrared/ultraviolet light (non-visible to the human eye).  The difference in these photons is that they have different wavelengths, or frequencies.

Light is also easy to distort.  Stopping down can create a sense of depth of field, but it does not increase the focus point.  It creates larger near-focus.  If your aperture is too small, you introduce a light defect called diffraction.  Remember, light is a wave.  If light travels through too small an aperture, its frequency is increased, and your sensor will lose detail.

Here is an easy way to test diffraction:  squint.  For those of us who are older, we may find ourselves squinting a little bit reading our computer screen.  Squinting does create better near focus, but squint just a little further and you find the text become blurry again.  That is diffraction.  Its an issue for people needing reading glasses to photographers to astronomers looking at galaxies that are billions of light years away.

Understanding The Scene

So why was the teacher in Point Loma providing bad guidance?  In the case of our scene, where we are capturing Shelter Island, San Diego Bay, and Downtown San Diego, the entire composition is at infinity.

People pay attention to the minimum focus distance of their lenses, which is always great to know.  What people should also know is the maximum focus distance of their glass.  This the distance your lens sees at infinity.  Why?  Because it will help you make depth of field decisions.

When a composition is completely at infinity, it doesn’t matter what aperture you are using.  Think about it, at infinity, you have a flat subject.  F/2.8 and F/16 won’t change how flat that subject is.  It literally doesn’t matter.

That isn’t to say that aperture isn’t a concern.  Every lens has a sweet spot that is sharpest, and while that varies, it is usually somewhere in the range of F/8.  There is a famous maxim in journalism photography:  “F/8 and don’t wait.”  That is because it was important to capture the news as it was happening, and shooting in this sweet spot was your best chance at a sharp photograph.  That hasn’t changed in the modern world.

The teacher should have been telling her student to shoot in the sweet spot of her lens, not create a diffraction-limited image for no reason.

Depth of Field As A Careful Dance Between Two Extremes

Depth of Field is a careful decision.  The closer your foreground subject and the further it is from infinity, the bigger the need will be to stop down.  The further your foreground subject, and closer to infinity, the less you will need to use a smaller aperture.

There are depth of field calculators on iOS and Android for a reason!  It is a careful calculation.  As I have developed as a photographer, I have learned that simply using F/16 for all scenes results in images I wish were a tad sharper.  While you can sharpen some of them up in Photoshop or Lightroom, the sharpening process is easy to introduce halos, artifacts, and other issues in your photographs.

These days, I start at F/8 and then stop down if I need to.  I find that it is often all I need.  Sure, there are images where I am really putting in a subject up close, but that is the exception.

Work on understanding this relationship, of the careful dance between your near-subject and your far-subject, and you can make sharper images just by using a better aperture.

Now it’s time for me to have a talk with that teacher.

tmschultze

T.M. Schultze is a San Diego-based photographer, traveller, and writer. He writes, photographs, and draws things of the outdoors that have inspired humans for thousands of years. He co-authored the Photographer’s Guide to Joshua Tree Park which can be purchased here.