The Fundamentals of Exposure

Aperture

The aperture is the opening in the lens that lets light in. The size of the opening is controlled by a control wheel on the camera or by a ring on the lens.  The size of the opening is called the "F-stop", "Aperture Value" (Av), or for short just "Aperture" and is indicated by a number prefixed with "F" or "f/" (the two notations express the same thing) . Examples are: F4, F5.6, F22.

Tip: if you are using a DSLR and your lens has an aperture ring with an "A", setting always leave it set to A and use the camera to control the camera.

Counter-intuitively, the larger the number, the smaller the aperture!

Worse, the scale is not linear. F4 lets in twice as much light as F5.6, which in turn lets in twice as much light as F8. The scale in full F-stops runs like this, where each successive number lets in half the light (1 EV less) of its neighbor to the left:

F1, F1.4, F2, F2.8, F4, F5.6, F8, F11, F16, F22, F32, F45

The scale in half stops runs like this - here each number lets in three-quarters of the light of its neighbor to the left:

F1, F1.2, F1.4, F1.7, F2, F2.4, F2.8, F3.5, F4, F4.8, F5.6, F6.7,
F8, F9.5, F11, F13, F16, F19, F22, F27, F32, F38, F45

Watch the animated GIF above to familiarize yourself with the concept of the F-stop numbers running the wrong way.

Tip: When you unpack your new Pentax DSLR it is set to a scale of 1/3rd F-stops. You may want to change that to the more convenient half stop scale in the custom function menu (this will cut down the number of possible settings and save you some scrolling with the e-dial).  For beginners, half-stops are less confusing than third-stops.

No lens has all the F-stops shown above; any given lens covers a smaller range of the scale. As an example, the HD Pentax-DA 35mm Macro goes from F2.8 to F22.

Note: The range of many lenses do not match the full F-stop scale, but starts out or ends at "half stop values" or even at some other value. On many zoom lenses the aperture range even shifts with the focal length set! Fortunately, the light meter in our camera takes this into account and we need not be concerned about it in practice.  Modern metering systems are in fact "stepless" under the hood.


The Effect of Aperture

So what does the aperture do for our images?

Enabling Low-light Photography

The aperture controls how much light to let in. The leftmost image below was shot in the middle of the day and the rightmost image at dusk. At dusk there is less light going around so we opened up the aperture to let enough light in to achieve a proper exposure (as determined by the light meter in the camera):

F22 F4.5
Middle of the day: F-stop = 22
(1/30s, ISO 100)
Dusk: F-stop = 4
(1/30s, ISO 100)

Think about light as a stream of particles. The larger the opening in the lens the more particles get through in a given time. The darker it is the lower the density of light particles and we must use a larger aperture (smaller F-stop number) in order to let enough particles in to get a correct exposure. 

Controlling Depth of Field

The aperture actually does more than controlling how much light to let in. We're finally getting to the point where we can reveal why we need to be in charge of setting the aperture rather than letting the camera do it for us: The aperture controls the Depth of Field (DoF): the range from near to far in a scene that is rendered sharp (in focus) in our capture. Let's study an example shot with the smc Pentax-D FA 100mm F2.8 Macro and where we have run through the entire range of apertures from the largest (F2.8) to the smallest (F32). Click on the first thumbnail in order to browse through a larger version of the images.  The lens was focused on the closest flower:

F2.8 F4
F5.6 F8
F11 F16
F22 F32

These examples show how depth of field is quite narrow at the largest aperture F2.8 and gradually increases as we stop down toward F32. At the smallest aperture, F32, the background is quite defined and we can identify what's there. Which image is best; in other words, which F-stop to use on a subject like this? That's for you to decide depending on how you want the image to look. If your primary goal was to capture the flower, then we would say that F4 and F5.6 have the best balance between keeping the main subject sharp and blurring the background. At F32 the background is definitely too busy and distracting.

Let's say you decided to use F2.8. You proceed as follows: first, set the aperture to F2.8. Then adjust the shutter speed (and/or ISO) until the meter readout is centered. In this example we adjusted the shutter speed from 1/180 of a second through 1/90 s to 1/45 s and left the ISO at 160:

Tip: If you only are concerned with depth of field for a given scene then the Av semi-automatic exposure mode is the logical choice. In Av mode you set the aperture and the camera will set a matching shutter speed (and also adjust ISO if in auto-ISO mode).

We have seen how the aperture affects depth of field. The depth of field also depends on the distance to the subject and the focal length oh the lens. This means that of F5.6 was the F-stop you'd chose for the scene above you would need a larger F-stop to achieve the same narrow depth of field if you were shooting from farther away.

Maximizing Resolution and Contrast

The widest aperture settings on any lens are not as sharp as slightly narrower settings.  Similarly, the narrowest settings such as F22 are not as sharp due to a phoenomenon known as diffraction. Many lenses deliver peak sharpness between F5.6 and F8.  

If your objective is to render as much detail as possible with no regard to depth of field you should pick an F-stop where the lens performs its best in terms of resolution and contrast. This could be relevant in connection with product photography for example. Most lenses reach peak performance when stopped down around two stops, but this varies for each lens (refer to our in-depth lens reviews for details on each Pentax lens). Many Pentax DSLRs have an "MTF" program mode that will set the optimal aperture for you based on information encoded in the lens. This program works for Pentax autofocus lenses only.

Bottom Line on Aperture

  1. The creative use of the aperture is to control how much of the depth of the scene is rendered sharp. If we want everything rendered sharp from near to far (landscape photography for example) we must set a small aperture (F11, F16, or F22) whereas if we want to isolate a subject and blur the background (portrait or macro work) we must use a suitable large aperture, typically in the range of F2 to F5.6 depending on the look we want.
  2. The secondary use is to enable handheld shooting in low light. By using a large aperture (F1.4, F1.7, F2) we let more light in and can use a shorter and hand-holdable shutter speed for the same EV.  Prime lenses are therefore your go-to choices for nighttime photography, as there is only one zoom lens currently in existence with an aperture wider than F2.8.
  3. Finally, if our concern is to maximize resolution and contrast, then we will set the aperture to the value where the lens performs its best optically. This is typically two or three stops below the largest aperture.

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