Photography is the art of capturing light. With our eyes we deem the moment worthy of capture. With our camera we preserve it. Understanding how cameras work allows us to do this much more effectively.
In this article, I model the camera as composed of four basic parts and explain how the control of each part affects the final photo quality. I focus (no pun intended) on five basic settings: focus, zoom, aperture, shutter speed, and ISO.
A camera is composed of four basic parts:
- Lens: actually, a series of lenses, which are curved pieces of glass that bend light from the outside world to focus an image onto the sensor. In the subsequent discussion, I will refer to the glass lenses and their housing collectively as “lens.”
- Sensor: a miniature screen containing millions of pixels, or microscopic “buckets,” onto which a light pattern (a.k.a. image) is focused by the lens. Each bucket records the amount of light falling on it, collectively forming the photograph.
- Aperture: a fancy word for “hole”, the aperture is an adjustable opening to control the amount of light passing through it and falling onto the sensor; it is constructed by a set of blades arranged in a circular pattern.
- Shutter: a covering for the sensor that controls the timeframe during which the sensor begins collecting light. It opens for a short instant when you press the shutter release to take the picture.
In this basic model, light from the outside world enters the camera through the lens. Some of the light passes through the aperture and falls on the shutter. When you press the shutter release, the shutter opens, allowing light to be collected by the sensor.1
In the following sections, we will see how each of these components affects the final picture and how they can be controlled.
The lens controls the focus and magnification of your shot.
Most if not all modern cameras have autofocus. Usually, autofocus is engaged when you hold the shutter release halfway. The motor whirrs as the camera adjusts the lens so that the designated focus area in your picture will be sharp at the sensor. Autofocus is sufficient most of the time, but if you need to adjust the focus manually, you may turn the focus ring located on the lens housing.
Some lenses are also zoom lenses, which means you can adjust the magnification of the image. If you have a DSLR lens, the zoom ring is the twistable part of the lens that has a wide rubber grip and that causes a portion of the lens to extend and retract. If you have a point-and-shoot (P&S), there should be a pair of buttons labeled ‘W’ (wide angle) and ‘T’ (telephoto), that zoom out and in, respectively.
Effects: Zoom on a DSLR is given in the form of a focal length in millimeters (mm). For a given sensor size, a larger number translates to a greater zoom. A small number like 18 mm produces a wide-angle shot, where more of the scene can fit onto the same 2D area. A larger number like 100 mm produces a telephoto shot, and allows you to scale up things in the distance. At wide angles, the distortion associated with projecting a three-dimensional world onto a flat two-dimensional surface becomes more apparent. While this distortion can be stylistic for landscape shots, a portrait typically calls for a longer focal length to prevent distortion of the face.
Another effect of changing the focal length is that the relative sizes of foreground objects to the background changes. At wide angles, the background recedes much more quickly. Imagine you have three apples lined up away from you. If you take a photo at f = 18 mm, the rear apple will appear much smaller than the front apple, compared to the case in which you take the photo with a 100 mm lens. If you want to capture the greatness of Mt. Kilimanjaro as a backdrop to a caravan of elephants on the savannah, it would be better to use a longer focal length lens (i.e., you want the mountain to be large compared to the animals). If, on the other hand, you want to capture the vastness of the wildebeests herds on the Serengeti, using a short focal length is preferable. In the former case, you want to emphasize the size of the background subject by reducing the apparent separation between it and the foreground; in the latter case, you want to convey the sense of scale by the fact that even with a fast-receding background, the herds just go on and on and on…
Shutter, Aperture, and Sensor controls
The remaining three major parameters—shutter speed, aperture size, and sensitivity—control the shutter, aperture, and sensor behavior, respectively. The first two control the exposure of a shot, or the cumulative light falling on the sensor. The longer the shutter stays open, or the wider the aperture is, the more light the sensor receives. The ISO setting, on the other hand, translates the amount of light received in a sensor pixel to a digital value corresponding to the brightness of that pixel.
These three parameters are interrelated as follows.
The shutter is the “gate” to the sensor. The longer it stays open, the more light the sensor receives, and the brighter your picture.
Shutter speed is given in fractions of a second. A 1/200-second exposure receives 10 times as much light as a shot taken at 1/2000 s, because the sensor is exposed for ten times as long.
Effects: Why is this important? In the real world, things are always moving. In photography, we’re trying to capture a still image, usually the clearer, the better. During the time the shutter is open, any change in the scene causes blur. Thus, a fast shutter speed will not only limit the amount of blur caused by your shaking hand, but it can also freeze subjects (such as an athlete) mid-action. (Motion blur can be used stylistically to convey movement. In that case, the shutter speed can be slower.)
You might wonder why we wouldn’t just use the fastest shutter speed possible to eliminate motion blur altogether. We will get to that when we talk about ISO sensitivity.
The aperture is analogous to the pupil of the human eye. The size of the aperture is controlled by the aperture stop, or diaphragm, constructed of overlapping curved blades arranged in a circular pattern. The aperture stop is analogous to the human iris.
Aperture size is given as an “f-number”, and usually appears as f/#, where # ranges in discrete values from 1.4 to 22. The smaller the f-number, the larger the aperture. f/1.4 indicates an aperture twice the diameter as that at f/2.8, and so it lets in 4 times as much light. You can therefore reduce shutter speed by a factor of 4 and achieve the same brightness. The inverse square relationship between the f-number and the amount of light passing through is useful to remember when you’re adjusting shutter speeds and ISOs.
Effects: You may be wondering why the aperture needs to be adjustable at all if you can control exposure using shutter speed. The most important reason for its existence is to control the depth of field, as explained below.
The focal plane of a lens is a virtual plane a certain distance from the lens where in an ideal world, everything in the plane would produce a perfectly sharp image at the sensor; anything in front of or behind it will blur to varying degrees according to its distance from the plane. For example, if you have three apples lined up away from you, focusing your lens on the center apple will blur both the front and rear apples. The larger the aperture, the blurrier the front and rear apples will be.
In portrait photography, a shallow depth of field helps isolate the subject from the backdrop so that the viewer’s eyes are drawn to the subject. If you were shooting landscape shot, especially with people, you would want to use a larger f-number to capture the distant mountains in addition to the people right in front of you.
Sensor pixels convert light into an electrical signal to be interpreted as a brightness value. The ISO sensitivity sets the multiplier for this conversion.
The ISO setting is given as an integer, usually from 100 up to 1600, and even higher in newer cameras. One would generally use a high ISO in dim lighting to make the picture viewable.
Effects: In a perfect world, you would set your aperture to get the desired foreground-background separation. You would then set your shutter to the fastest setting possible to eliminate motion blur, and then you would take the photo. Finally, you would use a high ISO setting to turn up the brightness.
However, the world is not perfect, and the amount of light the sensor actually receives is critical. You cannot arbitrarily increase the ISO setting because of a ubiquitous, physical phenomenon called noise. In photography, noise is generated by the sensor itself (among other electrical components) and appears as a random pattern of colored pixels. It makes photographs look grainy. Sometimes this effect is used stylistically, but most of the time it’s undesirable because the clarity of the picture suffers; fine details such as text become difficult to distinguish.
When there is a lot of light, the noise generated by the sensor is so little in comparison that the graininess is indiscernable (much like how the sun’s brightness makes stars impossible to see during the daytime). However, in a dimly lit environment, the noise starts to be comparable to the amount of light from the scene. The higher you set the ISO, the grainier your picture becomes. This is why you cannot just set an arbitrarily short shutter speed and then boost the ISO to compensate. You’ll end up with a very noisy picture.2
As you can see, shutter speed, aperture, and ISO speed are interrelated. Modern cameras generally include a fully automatic mode, a fully manual mode, an aperture priority mode, and a shutter speed priority mode. The fully automatic and the fully manual modes are self-explanatory.
As for the other two, aperture priority gives you manual control of your aperture size. The camera will automatically set the shutter speed to normalize the exposure to ensure the image is neither washed out nor underexposed. For example, if you want to focus on the center of a flower but blur the petals, you would manually set the aperture wide to f/1.8 or f/2.8, and the camera would automatically set the shutter speed so that the entire photo is at the correct exposure.
In shutter speed priority, you set your shutter speed, and the camera automatically adjusts the aperture to produce an image of appropriate brightness. If you know you’ll be taking shots of cyclists zooming by, for instance, you can force the camera to use a very short shutter speed.
In either priority setting, you would have to manually set the ISO appropriate to the lighting available in your environment.
Generally, when you need to shorten your exposure, you compensate by increasing ISO. At what ISO the graininess of your camera becomes unacceptable more or less defines your camera’s performance envelope. If you have image stabilization technology, you can decrease the shutter speed by several factors, which means your ISO setting can corresponding go down. On top of that, you can keep the camera steadier by holding the camera with two hands close to your head/body and by avoiding caffeinated beverages the day of your photo shoot. Other techniques include using a tripod, placing the camera on a solid surface and using the self-timer, or bracing yourself against a stable object. As they say, practice makes perfect.
To help you get started, I’ll provide some initial settings for various scenarios. Keep in mind I have an entry-level DSLR, so the sensor size (APS-C) is about 2/3 the linear dimensions of a full-frame sensor. which means it has more noise than a full-frame sensor. I also don’t drink coffee, avoid using flash, and generally shoot hand-held. I almost always have image stabilization turned on.
I usually shoot in aperture priority mode. I start by gauging how much light there is in the environment. If it’s bright, I set ISO to be low (daytime: 100, dawn/dusk: 200-400. If I’m indoors, I try taking a few pictures at 800 if there’s good lighting. Otherwise, I move up to 1600, where I try to stay most of the time. I then set my desired aperture, and then look at what the camera tells me it will use for the shutter speed. If I deem it to be too slow (less than 1/15 s) I adjust either the aperture or the ISO to compensate. If you don’t have image stabilization, a good rule of thumb is to use the reciprocal of your focal length as the slower limit.
The best lighting conditions occurs in early morning and late evening (known as the “golden hour”), as the sunlight gives off a very warm glow. However, there is less light as well, so I’d begin with an ISO of 200 and work my way up if it proves insufficient. Because you get a larger depth of field the farther away your focal plane is, you don’t necessarily have to set the f-number to be very high. If you decrease the aperture past a certain point (f/8 or f/11, according to this article), diffraction effects will noticeably soften your picture, so I wouldn’t go above f/11. I may widen the aperture to bring in more light, depending on what my camera indicates is my projected shutter speed. If shooting handheld, I wouldn’t feel comfortable shooting slower than 1/15 s, so with some margin of error, I would settle for about 1/30 s.
At this point, it’s a tradeoff between how much noise I can tolerate and how steady I can keep the camera. I usually prefer noise over motion blur, because it is easier to remove noise than to reduce camera movement, unless I have a tripod or can set my camera on a stable surface.
I would want to separate a person’s face from the wallpaper, but I wouldn’t want say the nose to be significantly sharper than the ears, so I don’t want to open the aperture too much. F/4 or so is good. I would use a lens of perhaps 70 mm or so to reduce distortion. As before, I set the ISO as low as I can without making the shutter speed too slow. If I’m outdoors and there’s ample light, I’d aim for ISO to be 100 or 200. If my subject is indoors but is next to a window, I’d try 200 to 800. If using artificial lighting, I’d try 400 to 1600, depending on its brightness.
Indoor or Night
I usually use ISO 1600, but if I can get away with it (say in a brightly lit room), I’d try ISO 400 or 800. If I’m shooting from afar, I would use a telephoto lens. At this point, I’m not receiving much light at all, and so I need to open the aperture all the way. I usually try to brace the camera against a solid surface. This allows me to use a slower shutter speed.
Usually the subject is a bit distant from me, so I use my telephoto lens and zoom in. I keep the aperture open as wide as possible to capture as much light as I can. At large distances, you’ll have no problem keeping the entire subject in focus, and a large aperture will help separate the subject from the background. I then need to increase ISO until the shutter speed readout is in the 1/200 s to 1/1000 s range (at high zoom, such as at a focal length of 200 mm, even a little shaking becomes a big problem).
The only way to gain a reliable physical intuition of the tradeoffs between aperture, shutter speed, and ISO sensitivity is to use it in the field. Only then will you know how shaky your hands are and how much noise is tolerable.
Be bold and creative! Vary your shots, choose unconventional angles, play around with grain, try following a moving subject and shooting. But most of all, have fun!
1 I’ve omitted a lot of the details, such as the mirror, viewfinder, and pentaprism present in most cameras. Their effects can be ignored to first order.
2 Using a low ISO and then adjusting the brightness and contrast on the computer doesn’t fix the problem, as it is to first order the same as turning up the ISO (to first order because adjusting ISO can help avoid clipping, and amplifies the signal earlier in the process, before more noise enters the system through other channels); the noise undergoes the same brightness/contrast transformation. They key metric is the signal to noise ratio; turning down the ISO decreases noise but it also decreases signal.