A look at depth of field

Photographers seem to constantly talk about depth of field as if it’s some talisman that magically converts so-so pictures into works of art.  Or that explains why the photograph you thought was in focus suddenly comes out all blurry – of course it was an artistic application of depth of field…  Actually, there’s not much mystery to the concept nor its application once you realize how a camera lens operates and how our brain reacts to photographs.

When you focus a camera lens you are making a sharp image on the film or digital sensor, but it is sharp only along a pretty narrow plane that is parallel to your film or sensor.  For example, imagine you take a picture of the side of a barn, standing so that you are exactly 90 degrees to the side of the barn.  When you focus on the barn the side you focus on is a plane that is parallel to your film or sensor and it will be in focus.  If you could look closely enough at the image coming through the lens you would find that most of what is between you and the side of the barn is not in focus.  Because we need some amount of out-of-focus before we actually notice it, there is an area in front of and behind that “plane of focus” that we perceive as still being in focus.  That is the depth of field.

So if depth of field is the distance from the camera where everything appears to be in focus, why do some pictures have lots of blurry stuff and others don’t?  Turns out you can control depth of field by adjusting the aperture of your lens – that’s the f/stop thing photographers keep talking about.  Small f/stops like f/2 or f/4 will show narrower depths of field than large f/stops like f/16 or f/22.  Simply by changing the aperture you can give your photographs a different look, just don’t forget to adjust the shutter speed as well to get the proper exposure.

Buy why would you want a narrow or wide depth of field?  Research has shown when people look at photographs their brain zeros in on objects that are in focus first, then scans around looking at other patterns, and then returns to the object in focus.  Makes sense when you think of our evolutionary heritage.  Wouldn’t you want to immediately notice the lion on the savanna that’s about to run you down and eat you?  As compared to paying more attention to the waving grass the lion is hiding in?

Some examples are in order.  This first picture has a narrow depth of field.  Notice the trees at the front of the line are in focus and then get progressively blurry as your eye travels back along the line.

ISO100, 200mm, 1/320 sec., f/3.5

The next photograph is the same image with the same settings only this time the focus is on the rock at the back of the line of trees.  Bet you didn’t even notice that rock in the first picture, did you?

ISO100, 200mm, 1/320 sec., f/3.5

As you hopefully notice as you glance from one image to the other,  your eye goes right to the place that is most sharply focused.  You almost can’t help yourself, as if your eye is not under your control.  To a great degree, it’s not.  It’s under the control of the photographer, who has decided where you should look first and most often, and put that part of the photograph in sharp focus.  And hiding elements you shouldn’t pay attention to by putting them outside the depth of field – remember the rock you didn’t see in the first image?

Besides using it as a visual sleight of hand to lure unsuspecting viewers into disreputable parts of a photograph, selective use of depth of field is an artistic device to bring order to visual chaos.  For example:

ISO100, 200mm, 1/8 sec., f/22

Here I’ve switched to a larger f/stop number (went from f/3.5 to f/22) and increased the depth of field in the image.  I focused on one of the trees in the middle of the line.  Now observe how your brain reacts.  You are probably looking all through the image with your eye wandering here and there but not really finding a place to land.  I’ll bet compared to the earlier images you find this one less pleasing, even to the point of thinking “there’s too much stuff in the picture” when there’s same amount of stuff as the earlier images.

Sometimes this “everything in focus” approach works – think of your favorite grand landscape.  You get that you-are-there sense because almost all the elements in the photograph are in the distance and look like they would if you were standing next to the photographer looking out over the scene.  But most of the time a judicious application of a shorter depth of field is a better approach.  Think of portraits, or pictures of people on a busy street, or flowers that seem to jump out from the background.  Good examples of that are not accidents – they are intended to look that way to you by the photographer.

One of my classmates from photo school makes images almost exclusively with a narrow depth of field.  She uses it very well, giving the viewer a clear sense of the subject of her photographs and where she wants you to look first.  You can see examples of how she uses this photography principle at Hailey King Photography.

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