by David Tong
The whole technical aspect of photography is about exposure. To put it simply, how light is harnessed through a lens and projected as an image onto a film’s surface or a camera sensor to produce an image.
While the proper application of exposure control is not an end-all basis for a good photograph, it does benefit the photographer to know how an exposure is made and how you can control the exposure to produce the images that you want.
With today’s advancement of computer chips in cameras, it has become easier to achieve a “proper” exposure in most scenes that users will enounter. The amount of presets in most consumer pocket cameras are so abundant, it has become quite tricky NOT to get a proper exposure in most cases with automated cameras. However, most camera users think this is some sort of computer “magic” that the camera knows what to do when you change your setting from “Beach” to “Fireworks” to “Winter”, for example.
The reality is, the camera manufacturer merely made some preset adjustments to the fundamental parameters of obtaining an exposure – namely, shutter speed, aperture opening, and film/sensor sensitivity! The uninformed user simply doesn’t know this.
Not knowing the relation of these three parameters will limit you to what the cam decides is right, you’ll end up being a camera operator without understanding why and how the equipment works.
As mentioned, exposure is based on three fundamental parameters:
Shutter speed – How long the shutter stays open to let in light.
Aperture size – How wide the aperture blades of the lens can open. The larger the opening, the more light can enter.
ISO/ASA – How sensitive the sensor is to light or how sensitive the film is to light. The more sensitive, the more light can be captured at a given time.
Take a look at Figures 1 & 2. Figure 1 illustrates the relationship between shutter speed, aperture, and ISO/ASA. Assuming ASA/ISO values didn’t change (red line) the smaller your aperture, the slower the shutter speed required to obtain the same exposure. As you increase shutter speed (vertical line) you’ll require a larger aperture opening to obtain the same aperture.
If you alter the film speed or ISO sensitivity of your sensor, the red line will shift and so will the shutter speed and aperture, but the proportion is still the same, as illustrated in Figure 2.
Here’s a simple analogy – A faucet, water, and a pail. The end goal is similar – to fill the pail with water.
If you open the tap more by turning the faucet, that’s the same as using a SHORTER shutter speed. Let’s say it takes 100 drips to fill the pail, by opening the faucet to 5 drips-per-second, you’ll fill the pail in 20 seconds (5×20=100). If you increase the flow to 100 drips/sec, then you’ll fill the pail in one second. If you turn down the faucet to 2 drips/sec, you’ll need 50 seconds to fill the same 100-drip amount of the pail.
You’re changing the TIME VALUE it takes to fill the bucket. That’s why Shutter Priority mode in most cameras are called “Tv” or Time Value.
Another way to fill up the pail quicker is to increase the flow of the water is to enlarge the faucet’s opening itself. Light is relatively infinite and the more you open the aperture, the more light enters.
The larger the spout of the faucet, the less time you need to keep the taps open. Assuming that you’re still flowing the same amount of water, however, the amount of water that’ll end up in the pail will still be the same. That’s the reason why large aperture lenses allow you to shoot at faster shutter speed than a slow lens, you’re allowing more light through in lesser amount of time.
Increasing Film/Sensor Sensitivity
The last parameter we can change is the pail itself. By making the pail size smaller, we can fill it up a lot faster. The same thing happens when you increase your sensor’s sensitivity or using a fast film. You allow a lot less time to expose the sensor or film, but at an expense of grain or noise. In terms of the pail, we’re sacrificing the volume of water we can store.
Of course, you can combine all three parameter changes and see how it affects your exposure as well.
Naturally, Figure 6 will fill up the cup in a jiffy, while Figure 7 will take a lot longer to fill up. If you don’t stop Figure 6 somehow (by stopping the exposure), you’ll overfill the bucket (i.e. OVEREXPOSURE), conversely, if you stop the water flow too soon, you won’t fill the pail at all (i.e. UNDEREXPOSURE).
It’s that simple.
Here are some real-world examples of a scene taken within a few seconds of each other in two ISO settings (100 and 200).
With the ISO 100 setting example all four images have the same exposure, but the shutter and aperture combination are different. For every stop of shutter speed increase (e.g. 100 to 200, 200 to 400), we need to open up the aperture by the same full stop (eg. f/11 to f/8, f/8 to f/5.6). The two setting maintains the same ratio.
In Figure 10, we increased our ISO from 100 to 200, a full stop as well. As you can see, the proportion from 1/200 to 1/1250 shutter speed increase is maintained by the opening of the aperture value from f/10 to f/4. The four exposures are still identical.
I hope these examples helped you understand how changing each of the three parameters affect your exposure. There are no rules in exposure as certain scenes or artistic preference may call for intentional under or over exposure, understanding the three parameters discussed will allow you to apply that decision to your photographs.
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