"Faster" lenses are those with larger apertures (smaller f-stop numbers), and are described as such because a larger opening gives you more light, which in turn translates to a faster shutter speed -- great for freezing moving subjects, sports, action, and low light situations. They usually cost much more due to the lens design, as well as the quality of glass. Prime lenses (lenses with fixed focal lengths) generally tend to be faster than zooms, but more lenses are needed to cover the range of one zoom lens. You can buy a zoom lens on a budget, but you may find it comes with a variable f-stop. What this means is your largest aperture changes depending on the focal length set. For example, if it's a 70-200mm f/3.5-4.5, this means at 70mm the largest opening is f/3.5, but zoom in to 200mm and your largest aperture is f/4.5 -- this not only affects depth of field, but exposure as well, through light loss resulting from a change to a smaller aperture. Higher-priced zooms have fixed apertures regardless of focal length -- the largest aperture remains constant as you zoom in and out, so there's no exposure change or difference as you zoom.
Depth of field is the first of two characteristics that aperture controls, affected by the size of your aperture, the lens you choose, the distance from your subject and the size of your image sensor. The larger your aperture (the lower the f-stop number), the less depth of field you have. The smaller your aperture (the higher the f-stop number), the more depth of field retained. The parameters also change among different focal lengths -- wide-angle lenses can obtain greater depth of field, while telephoto lenses often have a limited amount of depth of field, even when both lenses are set at the same f-stop. The same applies to different focal lengths on the same zoom lens, as well.
One example of this is f/22 on a 24mm lens - you may get everything from two feet to infinity in focus, where as f/22 on a 200mm lens will only produce a depth of field from 25 feet to infinity. In contrast, telephoto lenses tend to compact a scene by drawing the eye to a single spot, and since the distance between focal points are much farther than on shorter or wide-angle lenses, the opposite occurs with depth of field, as proven in figure 1-1. The closer you are to your subject, the less depth of field you have to work with. A similar effect occurs with macro lenses; as you get closer and closer to your subject, depth of field drops significantly, from feet to inches to millimeters.
Image sensors also play a role in depth of field. The smaller the image sensor, the more depth of field you get -- that's why photos captured with your smartphone are sharp throughout. On the other side of the coin, larger sensors yield less depth of field (in general depth of field is inversely proportional to format size), giving you more control over the look of your photos. This difference is evident when you test a compact camera next to a full frame DSLR -- compacts have such a large depth of field, making it less possible to get a blurred background, even when you zoom in.
Here's an example of a large aperture (on the left) bringing little into focus, and a smaller aperture (on the right) obtaining more depth of field.
It's a numbers game
When in comes to numbers and photography, figures have a tendency to read backwards or contradict one another -- one of the big reasons exposure can be so confusing. Such is the case for f-stops. Slightly different than maximum aperture (which is the largest opening), maximum depth of field is obtained through a smaller opening or a higher number f-stop. Minimum depth of field is created by doing the opposite -- using a larger opening or smaller f-stop number. More on this later.
Picking the right aperture can help direct the viewer, give the image a certain feel or mood, or creatively edit elements in your composition. Minimum depth of field can help eliminate backgrounds or foregrounds through the blurring of an area, creating a less distracting image through the use of bokeh (the visual or aesthetic quality of the out-of-focus areas of a photograph as rendered by a particular lens). This is nice to utilize when you want to lessen the impact of specific elements within a scene. Reducing depth of field can also be used to direct the viewer's eye to a specific place within the frame, because our eyes are typically first drawn to the parts of an image in focus. Depth and dimension through the perception of distance also occurs as objects drop out of focus. Maximum depth of field can bring more into focus, creating sharp, detailed scenes, which works well for subjects like landscapes. In addition, a smaller aperture creates less dimension, producing a flatter-appearing scene by blending subject matter in the foreground and background.
One example of using minimal depth of field with a low number f-stop can be seen in the shot of a Belding ground squirrel below. By using a large aperture with a long lens, I was able to soften the foreground and background -- blades of grass just millimeters from its head -- drawing focus to the animal and creating a slight three-dimensional effect in a two-dimensional medium.
A second example of maximum depth of field can be seen in this shot captured along the California coast. Set to f/22, this wide-angle lens held edge-to-edge sharpness, from the closest rocks and incoming waves to the distant outcropping. The scene feels flatter since the foreground and background comes into focus, therefore dimension is obtained through the placement of these elements, the angle chosen, and the direction of light. My boots and legs got soaked too, but the final rendition was worth the bath.
When looking through your viewfinder, you always see depth of field through your camera at largest aperture setting of the lens, regardless of the lens or f-stop used. Try adjusting your f-stop from f/4 to f/16 and the view will look the same before the shutter is fired. This is set up by manufacturers to allow you to see through your camera with the most available light. In order to review your f-stop setting, you must use a depth of field preview button on your camera (if your camera has one). When you pull the preview button back, the scene through your viewfinder may get dark, depending on your f-stop, due to the decrease in the amount of light entering the camera. However, this depth of field button helps you determine which elements will be in focus after you capture an image.
The purpose of hyperfocal distance is to maximize your depth of field, guaranteeing sharpness throughout a scene by adjusting both the f-stop and focus distance. In theory, it can be used with any lens and at any f-stop, but is mostly applied to wide-angle lenses since they offer the most depth of field coverage. Landscape photographers often use hyperfocal distance with wide-angle lenses since it's one way to guarantee focus throughout a scene.
On almost all SLR lenses, the focusing ring shows you the distance at which your lens is focused. If you are shooting a scene while focused at infinity (basically referring to the farthest thing away such as a distant mountain, the horizon, the Moon), the furthest spot will be in focus, but the closest distance that's sharp might only be five feet – even when shooting at f/22. Set your focal point to two feet, possibly on a foreground flower, and at f/22 everything from just below two feet to 20 feet will be in focus (losing sharpness on your distant mountain). To attain hyperfocal distance, instead of focusing at the furthest distance of infinity or the closest distance (the minimal focal point of the lens) set your lens to a distance where the depth of field extends from half of that distance to infinity. This means your focus may not be on your main subject at first, but once hyperfocal distance is applied, that subject will come into focus in the final image.
Hyperfocal distance / lens comparison for 35mm camera systems
Scale is in feet, with a position to focus which provides half that distance, to infinity, in focus.
How do you determine the hyperfocal distance of a particular lens? Table 1-4 illustrates the optimal distances to use for a range of focal lengths at specific f-stop settings. It also proves that shorter focal lengths offer more depth of field than longer ones. In the past, lenses showed the range of depth-of-field on the outside of the focusing ring, making it easier to determine hyperfocal distance, but that went away for some reason. What's up with that, camera manufacturers? Today, either you have to guesstimate from experience, carry a chart, or download an easy-to-use $1.99 app like OptimumCS to get a hyperfocal distance/ depth of field chart at your fingertips. There are a number of apps in this category, and as with most, there are often free versions or pro versions available for a nominal fee.
OptimumCS iPhone app
Documenting the California coast near La Jolla at sunrise, I set my hyperfocal distance on my 20mm lens to 2.5 feet in order to maximize my depth of field and insure sharpness from the closest to the furthest object.
Now having an understanding of what aperture is, it is important to note this just one of three main components of exposure; shutter speed and ISO being the other two. As with any aspect of exposure, there are trade-offs associated with adjusting your f-stop, so the one you chose not only plays a huge part in metering, but your decision also affects the message you may be attempting to convey with exposure.
By controlling the amount of light entering the camera, your aperture affects your shutter speed, and visa versa. This can control part of your decision-making depending on the amount of available light, in combination with the subject you are capturing. Questions arise, like "am I dealing with a moving subject?" or "do I need to use a tripod?" and so on. Choose a larger aperture (f/2.8, f/4, etc) for minimal depth of field and more light filters into the lens, causing your shutter speed to increase. This can be to your benefit, but on occasion you may not prefer a fast shutter speed, hence the trade-off and decision you must make when exposing the scene. If you need a fast shutter speed, deciding on more depth of field may not be an option.
Obtaining more depth of field requires a smaller aperture (higher f-stop number, f/16, f/22, etc.), but also limits the amount of light traveling through the lens. Consequently, an f-stop like f/16 causes you to use a slower shutter speed; something to consider when shooting subjects that may move during your exposure, or, when your camera isn't mounted on a tripod. And then there's ISO. Some feel they can simply bump their ISOs up sky high to obtain depth of field and
a fast shutter speed, but the trade-off here is tons of digital noise. So the key with every scene is to choose the most optimal settings for the conditions, keeping in mind that the aperture you desire may not be the best to dial in.
A comparison of a large ("wide open") aperture (on the left) allowing more light in but less depth of field, and a small ("closed down") aperture (on the right) forcing less light but producing more depth of field.
Back to the numbers
For photographers thinking in mathematical as well as metering terms, as numbers increase (f/8, f/11, f/16), the aperture diameter gets smaller, thus allowing in less light. Yet the first inclination of a photographer learning exposures is to increase the number for more light, given that the term "increase" is usually synonymous with the word "more." However, this makes your aperture smaller, causing a darker exposure. The other problem with apertures is that they are described as larger and smaller (going from more light to less light), yet when you talk about f-stop numbers it's smaller to larger (again going from more light to less light). The same happens with shutter speed – a higher fraction number implies more light, but it's actually capturing less. Whether an error is made with the shutter speed or aperture, both are common mistakes made by beginning or even experienced photographers.
A better way to think about settings when adjusting f-stops and shutter speeds is to imagine what actually occurs when you alter one or the other. For f-stops, you can either remove your lens from the camera and dial the f-stop higher watching the aperture get smaller (as in figure 1-4), or you can simply remember that as the f-stop number increases, less light enters your camera.
Since aperture plays a part in exposure, it's important to understand "stops of light," a measurement of the amount of light entering the camera. If we assume you have the correct exposure setting for a given scene, and you wish to change the aperture, you must also make a change to the shutter speed or ISO to compensate for the increase or loss of light. This is measured in stops. Similar to shutter speeds and ISOs, f-stops run in 1-stop, 1/2-stop, or 1/3-stop increments, often depending on what EV steps are set in your camera's menu. EV steps set your aperture increments and coordinate with your shutter speed and ISO increments, so if you set it for one, it is usually set for all three. Not all camera systems offer all three increments -- some may offer 1/2-stop and 1/3-stop increments only, others only 1-stop increments depending on the component.
One stop of light is usually defined as halving or doubling the amount of light entering the camera, depending on the shutter speed, aperture, or ISO changes made. With shutter speeds and ISO, the change is easy to remember since the number usually halves or doubles every one stop (from 1/1000 to 1/500 second or from ISO 200 to 400 is doubling the amount of light, for example). Apertures are more confusing since the actual f-stop numbers halve or double every 2 stops; f/4 to f/8 is 2 stops less light, f/32 to f/16 is 2 stops more. However, the amount of light halves or doubles every 1 stop -- f/4 to f/5.6 is 1 stop less light, f/32 to f/22 is 1 stop more -- so memorizing the 1-stop increments with f-stops can help in this process.
A common mistake many people make is to assume that one click of their aperture or shutter dial is one stop, but again, if your EV steps are set to 1/3-stop increments, then it's going to be three clicks for one stop. Stops of light at first seems confusing and not as important to those who don't understand exposure, but if you ever plan to become a master at photography, learning the terminology is key.
Aperture Priority (Av)
As an automatic exposure mode, aperture priority allows you to set your f-stop while the camera calculates the appropriate shutter speed to expose the image correctly. Usually abbreviated with Av or A (and not to be confused with full auto), it's a nice option when depth of field is key to the photograph -- great for a hurried situation or fast moving event that does not allow time to manually meter. This helps you to specify aperture alone, giving you the freedom to concentrate on the scene at hand. But with freedom comes a price, and whenever you entrust your camera with metering a scene using an automatic exposure mode, you have little say as far as the final exposure. Nevertheless, it's a great mode to use when learning more about how to use f-stops and depth of field. Try setting your camera to Aperture Priority for a day or even a week -- when you eliminate more variables and are able to concentrate on one aspect of your camera at a time, it's much more easier to learn about that feature.
Changing your plane of focus
Another aspect of aperture involving depth of field is plane of focus. Determined by the angle of the film plane or image sensor as compared to the lens, most cameras' plane of focus moves out in an imaginary invisible wall, parallel to the back of the camera -- but this can be altered in a number of ways, from the bellows movement of a large format camera, to the funky effects of a Lensbaby. By simply tilting the front lens element on a Lensbaby, or the back of a large format camera, your plane of focus is shifted depending on the angle you have moved it to.
How to cheat with depth of field
An additional difference between lenses is the perception of depth of field. When using a wide-angle lens, you can get away with setting a low number f-stop (large aperture) without giving up the appearance of decent depth of field. This is due to the shorter distance the focusing ring has to travel from its closest to its farthest focal point. As seen in the image below, most of my subject matter was more than 10 feet away, allowing me to use a larger aperture to let in more light for a faster shutter speed, yet the entire scene has the appearance of being in focus. However, because of this, if you decide to blur the background to give the appearance of less depth of field, your subject has to be extremely close to your wide-angle lens.
The difference between technical and practical
When it comes to the optimal quality of a lens, the general rule of thumb is that any lens is usually sharpest at f/8. In past workshops, students have asked if using f/8 is then a better way to retain sharp images, but my answer is always the same. I make the analogy of driving a stick shift -- a certain gear may be best as far as gas mileage or torque, but you can't drive a car in one gear, nor can you only use one aperture. Throughout my career, in reviewing hundreds of thousands of images, I have yet to see an image suffer due to the fact it was captured using anything other than f/8. We have many choices when it comes to f-stops, and they are there to be used.
Apertures have remained fairly stagnant since the first cameras were built in the early 1800s. However, earlier this year a company called Lytro
announced a new way to photograph -- its light field camera allows you to determine focus in post-capture editing. Built from technology perfected at Stanford University back in the mid-90s, the camera captures the color, intensity, and direction of all the light, giving you the capability to focus and re-focus after the moment, anywhere in the composition -- whether in the foreground, middle area or background.
I imagine this trend will continue with cameras or software, maybe even offering the chance to alter depth of field after the initial capture. Clearly, post-capture editing time will increase with these new options, but the technology will open more creative doors on how you document any given scene.
So there you have it. Who knew holes would be so fun to discuss. Time to grab your camera and go to town, putting your newfound knowledge to the test, understanding how the choice of the aperture used could make or break your shot. And when you find yourself struggling to become the next Henri Cartier-Bresson or Richard Avedon, just remember, if it were easy it wouldn't be as fun.
Stay tuned for more in this series -- we're just getting started!
[Image credit: OptimumCS
: The Optimum Camera Settings Calculator]
Sean is a commercial photographer, author of The Complete Guide to Nature Photography, photo expert, and all around nice dude.