Remember the Basics of Photography – Part 2: Exposure
Plain and simple, exposure is the amount of light recorded by the camera’s sensor (or film). That light forms an image. UNDERexposure is the loss of shadow details while OVERexposure is the loss of highlight details. Ideally, you don’t want either in your images… you want a “normal” exposure with no details lost in either shadows or highlights. Your camera has a histogram that you should use to judge your exposures. Generally, you should strive for something like this (a bell curve):
If it looks like this then part of your image is underexposed:
If it looks like this then part of your image is overexposed:
Sure some of that under/overexposure can be fixed in post processing (especially if your camera’s sensor has a wide dynamic range), but you should strive to achieve normal exposure in the camera (unless you are doing bracketing for later high dynamic range processing, but that’s a topic for another day).
If you have a good understanding of exposure, there is no need for you to read any further. If you don’t understand the basic exposure triangle, this is your starting point as all other advancements in your photography hobby are dependent on your understanding of it. I’ve been stopped at least five times in the parks in the past two years by folks with DSLR’s that have no idea how to use them. They ask me to “show them a few things” or “show them in a nutshell how to use it.” Don’t get me wrong, I don’t mind helping people, especially with photography related questions, but in the parks is not the place to learn how to use your camera outside of full-auto mode. In addition, you can’t possibly understand how to operate your camera effectively if you don’t understand the basic exposure triangle. Don’t panic though because it’s really not a difficult concept to understand once you see how simple in design cameras are.
A camera is basically a light-proof box with moving parts that allow light in (shutter) through a small hole (called the aperture). The image recording medium is the sensor (or film) and the light allowed in through the shutter and aperture falls on the sensor which records the image. A sensor is sensitive to light and this sensitivity can be adjusted (ISO or film speed).
Let’s relate the camera to our human eye. Simply stated (and oversimplified just for this analogy), light enters our eye though the pupil/iris and to stop light from entering all together, we close our eyelids. The light we see is collected by the retina which sits at the back of the eye wall and then sent up to the brain for processing. Now, let’s translate the eye parts to the parts of a camera:
- Iris/pupil = aperture – “the hole” that can widen or narrow to control the amount of light allowed in
- Eyelid = shutter – covers “the hole” (aperture) and opens and closes to either let light in or keep light out
- Retina = the camera sensor who’s recording speed can be controlled through ISO. Often described as ISO “sensitivity”
The aperture is adjusted to a certain width to control the amount of light to be let in, the shutter opens to allow the light in through the aperture, the light is then picked up/recorded by the sensor at a certain speed. Note that the higher the ISO speed, the faster the sensor will record the image because a higher ISO makes the sensor more sensitive to light. The lower the ISO speed, the less sensitive the sensor is to light and the more time it will take to record the image. The sensor then sends the image to a memory card.
Regarding aperture, the smaller the number, the wider the aperture. So an aperture of f/2.8 is wider than an aperture of f/8 and will therefore let in more light once the shutter is opened.
Regarding shutter speed, it’s measured in fractions of a second. So 1/30 means the shutter will stay open for 1/30 of a second. The faster the shutter speed, the less time the shutter is open to allow light into the camera. The longer the shutter is open, the more time light has to enter the camera.
Starting to see the relationship? To increase the shutter speed, at least one or two other things must happen to achieve a normal exposure: 1) open up the aperture wider to allow in more light to compensate for a shorter period of time the shutter is open, or 2) ISO can be set higher to make the sensor more sensitive to light (so it can record the image faster). Reducing the aperture size (i.e. going from f/2.8 to f/8 for example) necessitates either setting a slower shutter speed to allow more time for light to enter, or increasing the ISO sensitivity so the sensor can record the image faster. That’s the basic gist of the exposure triangle without getting into explanations of f stops, which put actual numbers to the above examples. If you want a better explanation of exposure, I highly recommend the book Understanding Exposure by Bryan Peterson.