Hours of Idleness-A Photographer's Journey in St. Louis



A beginner’s guide to aperture.

What is aperture?

When the photographer aims the camera at something and takes a picture, he or she is making an exposure.  An exposure is the net result of a combination of several mechanical, chemical or electronic factors working together in unison.

An exposure fixes an image in time, and can be considered “proper”, “under-“ (meaning too dark), or “over-“ (meaning too bright).

Aperture, shutter speed and ISO are the three primary adjustments that affect exposure.  In a proper exposure, a change to any single one of these will necessitate an equal and opposite change in at least one of the others.  This truth is known as equivalency.

The simplest definition of aperture is that it is the opening in an otherwise light-tight box (known as a camera) that allows in light to enable exposure.

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The size of the aperture, a generally adjustable diaphragm, controls the amount of light entering, and is measured by f-stops in still photography.  The smaller the f-number, the more light is allowed.

For most ILC shooters, these f-numbers commonly range from about f/1.4 to f/32, and follow a sequence of stops in-between (a geometric formula equating to one-half or twice the amount of light entering, depending on which direction one moves on the sequence).  Individually, each f-number represents a ratio between the diameter of the aperture and the focal length, so that at f/2, the diameter of the aperture is 1:2 or 1/2 that of the focal length.

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What is very interesting about this is that, between lenses of different focal lengths, the total amount of light transmitted to the digital sensor or film plane remains the same at similar f-stops (or close enough).  Though maybe a bit difficult to exactly comprehend, this formula allows photographers to think universally about f-stops in terms of their effect on exposure, even as they change between lenses of drastically different focal lengths.

What does aperture do?

We have established that aperture governs the amount of light entering a camera, which affects the brightness or darkness of an exposure if all other adjustments remain the same, but by changing the diameter of the opening of the camera, aperture also impacts what is known as depth of field.

Depth of field is the area of critical focus within an image.  As f-numbers increase, and the diameter of the aperture decreases, the in-focus area of a photograph expands around the point of sharpest focus.

It is a good rule of thumb to consider depth of field as extending 1/3 of its area in front of the point of sharpest focus and 2/3 behind it.  This is very useful to understand when photographing more than one subject at different distances from the camera.  For instance, to better ensure that both subjects are in focus, the photographer should place his or her focus point on the subject closer to the camera or at a point 1/3 of the distance between the two, and select an aperture appropriate to the depth of field required.

Although we have determined that any given aperture between lenses of different focal lengths produces the same amount of light, it does not render the same depth of field.  If the distance to the subject remains fixed, longer lenses will always produce narrower depths of field when compared to shorter lenses at any f-stop used.

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Depth of field at f/11; note sharpness in grain of wood floor.

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Depth of field at f/1.8; note how floor has gone out of focus.

Another impact of aperture is on flash exposure.  When a photographer introduces artificial light to a scene in the form of a strobe or on-camera flash, aperture tends to control the impact of this light on exposure more so than shutter speed (which governs ambient light).

This truth is easiest to observe as light from ambient sources increases, which is why the fact is extraordinarily useful to photographers employing fill (“fill-in”) flash for environmental portraits.

It is fairly easy to understand why this works the way it does: because flashes emit a maximum intensity of light in a matter of milliseconds, the duration of an exposure (shutter speed) matters less than the amount of light allowed to enter the camera (aperture).

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How should the beginner use aperture?

Most photographers who are new to photography leave their cameras in a fully automatic setting, and there is nothing wrong with doing that.  However, the first and often most important step these photographers make is into Aperture Priority.

This partially automated mode allows the photographer to set the aperture while the camera controls the rest, and opens up the full creative potential of depth of field.  The decision of what part of your subject to isolate and what to define using focus alone usually gives new photographers a taste of the photographs they purchased an expensive camera to emulate.

Using exposure compensation (defined on camera as “AV” or “EV”), the photographer can even make the automated exposure brighter or darker, without having to worry about changing shutter speed or ISO.

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For the photographer ready to think about aperture in terms of using the camera manually, it is important to remember that aperture is one of the core adjustments moderating exposure, and that this relationship is reciprocal to shutter speed and ISO.

This means that if you increase the size of the aperture, you must also increase your shutter speed and/or decrease your ISO value in order to maintain exposure.

Knowing this can be handy when shopping for new lenses.  Lenses that have a smaller constant f-number setting (ie. f/1.4, f/1.8, or even f/2.8) are considered “fast” because they allow the photographer to use faster shutter speeds in order to freeze action, especially in lower light scenarios.  Though faster lenses are usually more expensive when compared to their slower counterparts, exceptions exist.  For instance, Canon and Nikon’s 50mm f/1.8 lenses are among the best bargains in photography.

However you choose to use the information provided above, most important to your development as a photographer is to get out and make photographs.  Every moment you spend snapping pictures is a step toward lessoning the barrier between the photograph you envision and the one you actually take.

More lessons on photography here.

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  1. […] Aperture, shutter speed and ISO are the three primary adjustments that affect exposure.  In a proper exposure, a change to any single one of these will necessitate an equal and opposite change in at least one of the others.  This truth is known as equivalency. […]

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