Aperture and Aperture Priority Mode – Part II

So why do photographers care about aperture at all?   Can’t you just adjust the brightness later in your photo editor?  To some extent, yes.  BUT, there is an almost magical photography concept called depth of field that makes understanding aperture necessary and worth it.  Depth of field refers to how much in front and in back of your main subject in an image is also in focus. 

Notice how Freeport Middle School photographer, CJ, makes fabulous use of depth of field in the image above by having only a few words in focus with others dropping out of focus as one’s eye moves away.  Put another way, the gradual coming into focus of the words drives the viewer’s eye to the point of significance chosen by the photographer.

ZB's image of ice on the ground also makes effective use of depth of field as the image fades in to a pleasing blur of color as the viewer's eye moves from front to back.

Imagine if all the words in CJ’s piece were in focus.  The image would be too busy without much meaning beyond it being a snapshot of a sign.    With the emphasis falling on the messages of “hard work” and “make your own luck,” however, the photographer makes effective use of depth of field to convey one or more messages through his art. 

The degree to which things are in focus surrounding the main point of interest is largely controlled by aperture setting.  Wider apertures (bigger openings) like f/2.8 limit the amount of image in focus to mostly the main subject.  As the photographer stops down the lens (makes the aperture smaller), more of the scene being captured around the main subject remains in focus.

Examine the side view of two remarkably familiar pirates being chased by an ostrich with a head resembling a character from a well-known movie franchise.  As can be seen, the characters are spaced about every 1.5 to two inches from front to back.

f/1.4 (Focus on Back Character)
f/1.4 (Focus on Front Character)
f/16 (Focus on Front Character)

The first of the three images above involves focusing on the ostrich in the back and using a very wide aperture of f/1.4.  The ostrich and the bush beside it (being the same distance from the lens) are in focus.  The pirates being chased, however, even though only a few inches in front of the ostrich, are almost completely out of focus.  The second picture accomplishes the same thing in reverse by using an aperture of f/1.4 but focusing on the front pirate.  Moving to a very small opening, like f/16, allows the photographer to have the whole scene remain in focus.

If it has not occurred to the reader already, ponder for a moment why the same scene exposed at f/16 is not WAY DARKER than the correctly exposed f/1.4 image. 

Aperture Priority

Most DSLR Cameras possess an “aperture priority” mode set by choosing Av for aperture value on the control dial.  

Aperture Priority Mode allows the photographer to chose the aperture desired to produce a specific effect, and the camera will automatically set the other factors that influence exposure.   Selecting the Av setting serves as a terrific training tool to learn about aperture as well as to achieve a creative effect involving depth of field even when the photographer must act quickly to avoid missing the desired shot.


The chart below provides a hint to  why the shot at f/16 is not way darker than the shot at f/2.8.  Captured using aperture priority mode, the image series reflects a one stop smaller aperture in each consecutive picture.  Each of the images appears to have been exposed correctly and at the same level of brightness.  …  Each shutter speed listed is roughly 2 times longer than the one before it (e.g.,  0.04 x 2 = 0.08).

Not to worry if the point is not yet clear.  It will be.  The point to take away now is that a photographer can choose just about any aperture that the situation may call for and end up with the same level of picture brightness/exposure.  

Aperture and Aperture Priority – Part III covers examples of common uses of depth of field and pitfalls to avoid.

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