Controlling your aperture, and understanding it’s relationship with the exposure trifecta can be one of the most profound tools to manipulating the look of your photographs.  As we discussed in the trifecta post, the aperture is a hole which allows light to pass through the lens and onto the sensor or film.  By controlling the size of this hole, you control the amount of light passing through the lens.  This is it’s primary function.  Bright day, smaller hole.  Dark cave, larger hole to allow as much light in as possible.  The other feature of the aperture is its responsibility for your depth of field (DOF) or area in focus.  If you look at the picture above, you can see on the bar top, that it is blurry in the foreground, sharp at the first wine glass, and then fades quickly back into the out of focus area.  This was achieved by setting a large aperture, which when also getting the camera physically close to the subject in focus, it narrowed my DOF.

Simply put, the larger the aperture (smaller f/number) the shallower your DOF.  The smaller the aperture (larger f/number) the deeper the DOF.  Usually, larger apertures (f/1, f/1.4, f/2, f/2.8 and f/4) are used to isolate subjects by way of shallow DOF.  The other factors in isolating your subject with a large aperture is A) Focal Length,  B) Distance between the camera and closest point of focus, and C) Sensor Size.  The wider the focal length, the inherently deeper your DOF by way of optical physics, while the longer the focal length, the shallower the DOF will be all other things being equal.  The closer to the subject in focus, the shallower the DOF, while the further away, the deeper the DOF.  If you shoot with a compact camera, the sensor is tiny which is good if you want deep DOF.  The larger the sensor, the shallower the DOF at the same aperture setting vs a smaller sensor.  I will post on DOF and hyperfocal distance later on.  “Medium” apertures (between f/5.6 and f/11 or so) are good for group shots, or sharp portraits, or general everyday snapshots (for me anyway).  Smaller apertures (f/16 and smaller) will allow for deeper DOF essentially allowing an entire scene to be in focus.  You will see smaller apertures used in many landscape and architectural applications where it benefits having everything in focus.

As discussed in the Trifecta post, the other two factors directly affecting exposure are ISO and shutter speed.  For this example, I set my camera up on a tripod, kept the ISO setting the same (ISO 100) and adjusted the aperture by way of the “Aperture Priority” setting on the mode dial to show the difference between a large and small aperture.  By setting my camera to “Aperture Priority” it changed the Shutter Speed automatically to properly expose the scene based on the Aperture setting I had set.

Shallow DOF achieved by a large aperture.  Settings: f/1.7,  1/6sec,  ISO-100

Deep DOF achieved by a small aperture.  Settings: f/16,  13 seconds,  ISO-100

While I kept the same ISO setting, you can see that my shutter speed needed to adjust for the same exposure to be achieved with a smaller aperture (from 1/6 of a second to 13 seconds).  Even with a large aperture setting, I needed to shoot at 1/6th of a second which, if I were hand holding the camera, would have come out blurry (more than likely).  In the event that I would have been hand holding the camera, I could have increased my ISO setting to achieve a faster shutter speed.

Exercise Time!

Spin the mode dial on your camera to it’s aperture priority setting (“A”, “Av”, “Ap” or the like).  This takes your camera out of the auto modes and allows you to adjust the aperture.  While you adjust the aperture, the camera will adjust your shutter speed (and any other setting you have set to be automatically adjusted like ISO, White Balance, etc) so that it exposes the scene in front of it properly.  Depending on the lens you are using, your maximum aperture may be anywhere from f/1.2 (if you’re lucky and a big spender) to f/3.5 – f/4 or so.  Set the aperture to it’s widest setting (lowest number) by way of the click wheel, thumb wheel or directional buttons (each camera will be a little different, see your manual if you’re not sure).  Set something static as close to you as your lens will allow you to focus on based on it’s minimum focusing distance (usually printed somewhere on the lens itself) with as much room in the background as possible to allow your subject to be separated from it and take a shot.  If the entire shot comes out blurry, increase your ISO setting one stop at a time until you get it sharp.  Notice how the background isn’t in sharp focus?  Now, stop down your aperture (set it to a larger number like, say f/11 or so) and try to take the same shot.  Two things should become apparent, firstly the shutter speed is probably WAY too slow to hand hold, and secondly, once you find something to brace your camera on, the background is more in focus assuming the camera is still enough to get the shot.  Now, the whole scene being in focus can be good, or distracting depending on your desired outcome, but now you know how to choose which way to go when a shot calls for one or the other.