Part III: Exposure.
Exposure is literally the most important thing in photography. The term refers to the amount of light reaching the sensor as controlled by shutter speed, aperture and sensitivity. While this is a solid definition it never helped anyone learn anything. A neutral exposure can be defined as an exposure which produces an image where there is no detail lost in the highlights or the lowlights. However it's often said that the correct exposure produces the exact image that the photographer wants. Knowing how to expose an image using all the settings on your camera in manual mode is crucial to knowing how to take good pictures.
The key to exposure is control and balance. Like a zen master you need to find the right balance of the three most important settings on your camera.
A balance of these three settings can produce any result the photographer wants.
The first in the triad is ISO, sometimes known as sensitivity or speed, it is a measure of how sensitive the sensor is to light. This setting provides the broadest level of in-camera control over the lighting in the shot. Lower ISO numbers mean that the sensor is less sensitive to light and higher ISO numbers mean that the sensor is more sensitive to light. The best practice is to use the lowest practical ISO in any given scenario in order to avoid noise and loss of detail. At ISO 100 (generally the lowest most cameras will go) your images will be sharp and free of noise but the camera will need a well lit scene. At higher ISOs the image will be brighter and it will be easier to shoot in the dark, or at narrower apertures, but noise will start to appear and detail will be lost.
Aperture has been covered pretty well in my previous post but it's still an important part of the triad of settings that a photographer has to use to get the effect they want. The measurement is given by an 'f-number' which goes from as low as 0.95 on some lenses to as high as 36. Aperture, while just as broad a setting as ISO in terms of lighting, has a huge effect on not only the lightness of the image but the sharpness and the focus as well. Lower (or wider) apertures lower the depth of field while allowing lots of light into the image. This means that typically if you focus on a subject while using a wide aperture then that subject will be the only thing in focus and everything else will be blurred. A wide aperture will also let more light onto the sensor which allows you to use a lower ISO and avoid noise. At higher (or narrower) apertures the image becomes sharper, the depth of field increases and less light is let in. When shooting at narrower apertures you need to use a higher ISO to avoid underexposing the shot.
Shutter speed is the last in the triad of settings. Usually expressed as a fraction of a second, it's literally a measurement of how long the shutter is open and the sensor is exposed to light. Higher shutter speeds mean that the sensor is exposed for less time and lower speeds mean that the sensor is exposed for longer. This is the finest adjustment of the three and it has its own side effects. Ultra fast exposures will freeze motion in the frame and eliminate any camera shake or motion blur but because they allow so little light in they need to be used at high ISO. Shutter speeds of one second and slower are known as long exposure. Long exposure will allow for shooting in extremely dark conditions at low ISO and narrow apertures but to shoot long exposure the camera has to be mounted on a tripod or the shot will be extremely blurred.
Your camera has tools that help you determine how your shot is exposed. The main in-camera assistant is the EV Meter. The EV meter measures the exposure value of the scene and allows you to control your exposure by showing you if your shot is overexposed or underexposed and by how much. An EV meter reading of 0 will mean a neutral exposure, positive readings mean brighter images and negative readings mean darker images.
Controlling your exposure is the most important part of taking quality photos. Every DSLR has these settings but they're accessed in different ways. Reading your camera's manual is the best way to learn how to use your individual camera but for reference purposes I will use a Canon 500D because the settings are well laid out on screen and the Canon's control layout is pretty standard and most DSLRs can be used in a very similar way.
First off we need to set the camera to manual mode. This is done with the mode dial usually found on the top of the camera. The mode dial has many icons for all the different modes on the camera but for manual we will set it to M.
Then we will locate the control dial on the camera. More professional DSLRs have more than one dial for quick adjustment of various settings but almost every DSLR will have one dial which is usually set to shutter speed in M mode. To adjust the shutter speed, just rotate the dial until you see the shutter speed you want on the screen.
Changing the ISO is done by pressing the dedicated ISO button and then rotating the dial to select the desired setting. Usually you can dismiss the setting menu by pressing the ISO button again or with a half press of the shutter button.
The aperture setting is slightly more complicated. Some cameras will have a button that lets you change the aperture with the dial but many don't. In order to change the aperture on this Canon we have to press SET in the middle of the directional pad and then use the directional keys to select the aperture setting. The dial will then work as normal. You can also modify other settings this way. A half press of the shutter will lock in your settings. When configured for a neutral exposure the EV meter will show a reading of 0.
It's important to learn to use your own camera by reading the manual. If you bought yours without a manual then just google the manual for your camera and read it online. This will be the last How to do Photography until after I come back from Japan but in the next Photography How To we will cover focusing.