When I first started with my SLR the two major resources I used were the website Digital Photography School (DPS) and the book Understanding Exposure by Bryan Peterson. These two sources provided the basis for this post and I highly recommend them (especially Understanding Exposure) to anyone starting out with an SLR.
There are three major elements to a well exposed photograph and they are the ISO, shutter speed and aperture. These three elements are often grouped together into what is referred to as the exposure triangle. Each of the elements are conceptually placed at their own vertices of the triangle. Changing one element of the triangle requires that one or both of the others be changed in order to maintain the correct exposure (the amount of your light source that ultimately reaches and is collected by the film or image sensor of the camera). Each element of the triangle also influences another element of the final photo and thus there are always trade-offs to ensure that you achieve your desired visual effect while maintaining proper exposure.
I will cover the three elements of the triangle from the inside out.
ISO
ISO is a measure of the camera's digital sensor's sensitivity to light (the equivalent to film sensitivity on film cameras). A more sensitive sensor needs less light to produce the same exposure as a less sensitive sensor. Equivalently, a less sensitive sensor needs to be exposed to the same light source for more time to produce the same exposure as a more sensitive sensor. The higher the ISO, the more sensitive the sensor. On my camera (Nikon D90) the ISO ranges from 200 to 3200. It may seem intuitive at first to almost always use a high ISO, however the major issue with a high ISO is that the image becomes grainy. The grain on a digital image presents itself in the form of noise, little specs all over the image. For some applications (i.e. more artsy shots) a grainy photo may be acceptable, however the general goal is to have sharp, clean images so you often want to use the lowest ISO possible for a given situation. Newer and higher end digital cameras often have much better noise reduction capabilities at the higher end of the ISO spectrum and photo editing software is getting better at eliminating this noise as well, but with my Nikon D90 the high ISO's do create a lot of distracting noise so it is best to keep the sensitivity of the sensor as low as possible for given lighting conditions.
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| Notice that the background (the sky and the trees) and the shadowed regions of the leaves appear smooth with little to no noise. This photo was shot at a low ISO of 200, the native ISO of the Nikon D90. (1/800 sec., f/4.5, ISO 200) |
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| Noise is much more evident in this photo taken at ISO the high ISO of 3200. There are some obvious artifacts throughout the photo but they are especially evident in the darker regions such as the reddish brown of the number three die on the left. (1/1000 sec., f/1.8, ISO 3200) |
Shutter Speed
The next element as you move away from the sensor is the shutter speed. The shutter controls how long the image sensor is exposed to light. It is essentially a door that is opened and quickly closed again to allow light in just long enough to see the image, but not too long as you want to prevent over exposure.
The selection of shutter speed is largely based on two factors: subject motion and image sharpness. Subject motion has to do with shooting a moving or stationary subject and, if you are shooting a moving subject, if you want to freeze the motion or allow some motion blur for artistic effect. To freeze the motion you need a faster shutter speed and to allow for motion blur, a slower shutter speed. Image sharpness is also a major factor, especially if you are hand-holding your camera. Generally you want sharp images and if you are shaking while taking the picture you need a relatively faster shutter speed. This also depends on the focal length of the lens you are using (measured in millimetres, mm). If you have a short focal length (are 'zoomed out' on a 'zoom' lens) then the shake is not exaggerated very much and you can use a slower shutter speed if needed. If you have a large focal length lens (are 'zoomed in') then any shake is exaggerated (it's all moment arm stuff, think of a baseball bat being swung: the grip of the bat moves slowly compared to the tip of the bat that is moving very quickly) and you need a faster shutter speed to stop the shake-induced motion blur.
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| An example photo taken with a fast shutter speed. Even though Mark Webber's Formula 1 car was moving fast the fast shutter speed of 1/2500 sec. slows it down enough to create a sharp image of the car. The tires were spinning fast enough that the text on the sidewall is still blurred. (1/2500 sec., f/5.6, ISO 800) |
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| Taken through a dirty window from the Vdara hotel in Las Vegas the Bellagio fountains are a pleasant blur in this 30 sec. exposure. The long exposure also allowed for the buildings to be well exposed despite the dark conditions, narrow aperture (high f/number), and low ISO. (30 sec., f/22, ISO 200) |
Shutter speed is generally measured in fractions of a second or in seconds when using very slow shutter speeds. On my camera only the denominator of the fraction is displayed (i.e. 60, 125, 1250, etc.) or if it is in seconds the 'second quotes' are used (i.e. 1", 2", 10", etc.). The term shutter 'stop' is used to describe doubling or halving the amount of light getting to the sensor. If you decrease the speed by one stop you are doubling the amount of light getting in (and therefore, to maintain the same exposure, must alter one of the other elements of the triangle to halve the light again). The shutter stops are defined as when the shutter opening time is doubled or halved: 1/500 sec., 1/250 sec., 1/60 sec., 1/30 sec., 1/15 sec., etc. Many modern cameras also contain one-third stops in between those numbers to provide for greater control
Aperture
The final element of the exposure triangle to be discussed here is aperture. The aperture setting describes the size of the opening in the lens and is often measured in f-stops (i.e. f/2.8, f/4, f/5.6, f/8, etc.). If you change one f-stop you either double or halve the size of the opening and therefore double or halve the amount of light coming into the camera (passing through the shutter and reaching the sensor). Small f-stop numbers refer to a larger opening (and more light) while larger f-stop numbers refer to small openings (and less light). The aperture stops are when the opening size is doubled or halved: f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32. Many modern cameras also contain one-third stops in between those numbers to provide for greater exposure control. The main impact of aperture on the final photo is its depth of field (DOF). The DOF refers to how much of the photo is in focus. With a low f-stop (and large aperture) you get a shallow DOF, meaning that only objects at the distance you are focused on will be in clear focus. Anything nearer or further from the camera than the focus plane will be not as clear. At the other end of the scale a high f-stop (small aperture) you get a deep DOF and a large range of distances from the camera will be sharp and in focus. Shallow DOFs can be used to draw attention to the objects of interest in a shot while keeping the rest of the image in soft focus. Deep DOFs allow for many objects to be in sharp focus (such as landscapes or group photos).
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| A wide open aperture (small f/number) of f/2.8 provides a shallow depth of field which allows for my one year old and the tree she is using for support to be in sharp focus while the trees in the background are blurred to the point where they do not distract from the main subject. (1/200 sec., f/2.8, ISO 200) |
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| Using a narrow aperture (high f/number) of f/22 a deep depth of field means that everything from the vegetation and rocks in the foreground to the Peggy's Cove lighthouse in the distance and even the clouds in the distant background are in sharp focus. In this case the subject (the lighthouse) is not isolated by blurring the rest of the photo like in the photo above. (1/320 sec., f/22, ISO 500) |
Bringing it All Together
So, one of the keys to good pictures is good exposure. If you are shooting a sporting event and need to freeze the motion you need a fast shutter speed and therefore you may need to increase the aperture size (lower f-stop) and increase the sensitivity (higher ISO). There may be a limit to your aperture size (either your lens only goes so far or your DOF gets too shallow for your needs) and you need to sacrifice and accept some graininess in your photo brought ion by upping the ISO.
It is important to recognize that simply having a well exposed photo is not the end of it. Every correct exposure has three elements: the shutter speed, the aperture, and the ISO, so if you change one, you must change either one or both of the others, leading to a virtually infinite number of possible combinations to get the same correct exposure. Determining the right combination depends on the situation and that is a much more complicated issue. Future posts will be dedicated to the different elements individually but it is always important to remember, understand, and apply the exposure triangle concept when trying to get away from the auto setting on your camera.
Integral to achieving proper balance between these three elements, and therefore proper exposure, is either a keen eye for light levels or, more likely, a light meter. Most cameras these days have a built in light meter that is seen when looking through the viewfinder or at the LCD screen on the back of the camera. Sensibly the meter indicates a mathematically well exposed photo if the meter is in the middle and an over or under exposed photo if the meter is to either side. A discussion of the different metering modes and dealing with special situations is left to a future post.
Notes on updated post: When I originally wrote this post I was very new to fully manual SLR photography and based it entirely what I had read on the Digital Photography School (DPS) website. DPS is an amazing resource for new and mid level photographers and many of the articles there are must-reads for those new to SLR cameras. Since the original posting of this article I have leaned a lot from that source as well as many others and I have tried to update this post accordingly. I have also added photos to the post to illustrate some of the concepts discussed.
Originally Published: 08/02/2009
Updated and republished on the date of this post.
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