Depth of Field

How Aperture Affects the Focus in your Photographs

by f-stop fitzgerald

Posted: August 31, 2011

Notice how in this image, the front edge is in focus, while the back edge softens, and the greenery behind it is entirely out of focus.

When I first started out in photography there were two things that really took some time to sink in and eventually become intuitive.  The first was the range of f-stop numbers:  why is f16 smaller than f4, when the number is bigger? Eventually I just got used to the contradiction.

The second difficult-to-grasp concept was depth of field.  Simply put, this is the area in front of and behind the specific point upon which you have focused, which is also in focus.  We have all seen photographs where it seems that only an eyelash on the eye of a close-up portrait seems to be in focus.  This is an example of shallow depth of field. Then again we have all seen an image of a flower in the foreground and a seemingly limitless landscape from that flower to infinity, which seems in-focus.  This is an example of a deep depth of field.

There are a few things which affect depth of field:

• The point upon which you focus
• The aperture
• The lens

Here, we are primarily going to discuss the focal point and the aperture.  Regarding how lenses affect depth of field, the wide-angle lenses, like 18mm-35mm, will generally present greater depth of field than a normal lens would.  On the opposite side of the spectrum, long lenses, like the range from 80mm to 300mm, offer less and less depth of field.

Apertures

The smaller the f-stop number (i.e. f4)--which corresponds to the larger opening on the lens, which allows more light to hit the sensor--the shallower the depth of field will be.  So, a setting of f2 has an extremely shallow depth of field; maybe only be a few millimeters, for instance the eye of  a butterfly.

On the opposite side, the higher the f-stop number (i.e. f32)--which corresponds to a smaller lens opening and which allows less light to hit the sensor--the greater the depth of field.  This would be the limitless landscape example for instance, where everything, from the flower in the foreground to the setting sun, is in focus.

In this image the focus is on the front corner of the first fence post; notice how, as we proceed further away from that spot, the fence posts become increasingly softer and at the very back almost blur out entirely. This shows how shallow depth of field can be used to visually soften an element within the photo.

In between f4 and f32 is a complete range of depth of field.  Let's look at a couple of photographs.

Notice how the image of the fence has only the front element in focus and the rest gradually drop out of focus.  As we maintain the same exposure but increase the aperture from wide open (f4) towards stopped down (f32), we see more and more come into focus behind the specific point we originally focused on.  This is showing us the range of increasing depth of field.

In this image, we have focused on the front rock and are using a shallow depth field.  Clearly the front rock is in focus, and the receding fence and rocks become softer, the further away we get from the front focal point.

Creative Use of Depth of Field

Why would we be concerned with depth of field?  It is a part of the photographer’s toolbox for us to use to create an image.  One reason is to decide which things we want in focus and which things we want out of focus.  Obviously, the items in focus are the ones we want the eye to notice.  And if there are things in the photo that we do not want the eye to notice, we can select an aperture that blurs them or makes them so out of focus they are not even noticed.

Here the rock in back is in focus, using a modest amount of depth of field the middle rock and fence posts show a range of focus, or depth of field, of 2-3 posts.

Focal Plane

As we look at the fence again, notice the three rocks on top; these are to show you how the depth of field changes with the aperture.  But they are also used to show how the point of focus can be important.  What happens when we focus on the front rock, what happens behind it?  Then again, what happens when we focus on the third rock, what happens in front of it?  Finally, what about focusing on the middle rock.  What happens in front and behind it?

So, if we wanted to isolate the rock in front, we would focus on that, and use an aperture that had a shallow depth of field (f4 for instance.)  And if we wanted to have all three rocks in focus we would focus on the middle rock and use an aperture that had a large depth of field (f32 for instance.)  Notice how the visual feeling changes between these examples.

Conventional wisdom suggests that in a case like our fence, where you want all three rocks to be in focus, you should actually pick a point approximately one-third of the way in (our example with the rocks is halfway in) for maximum sharpness and a visually pleasing depth of field.

Preview Button

There is something we can do besides just shoot and review to see the depth of field in our images.  Most DSLRs have a depth of field preview button.  It is generally on the right side on the front of the body, located conveniently near where the lens meets the body.  For me, with my right index finger as the 'trigger finger' on the shutter release button, I use my right hand middle finger to press this preview button. 

If you press down the preview button and hold it, you will see the image darken - slightly or radically.  This takes some practice to get used to.  The darkening view shows what will be in focus - in the case of a small aperture number like f4, you may only see a slight darkening.  For a large aperture number like f32, you will see what seems like everything darken, suggesting everything will be in focus.  Experiment with this depth of field preview so that you understand how it works.  Then focus on a fixed object and see how the darkening changes through the range of f-stops from f4 through f32.

Next time out, experiment with depth of field.  Try shooting a row of items like my fence, and use a range of apertures.  Use the preview button.  Notice how the depth of field changes in the final image. Notice how it changes the feel of the image.  There - another tool in your toolbox mastered!

Follow the light…f-stop