In today's installment, I'm going to review the three primary controls used to set proper exposure of your camera. When you use the camera's “automatic” mode, these settings will be set by your camera to provide the best overall exposure for the scene. While convenient, this is one of the main reasons that images look like snap-shots, rather than photographs! By understanding these controls, you will be able to obtain properly exposed images, but you now have much more creative control. So let's abandon the automatic controls and get one with producing photographs.
This is the size of the opening in the camera that allows light to enter; typically controlled by an adjustable diaphragm in the lens. Wide apertures let in more light than small apertures, providing the most rudimentary exposure controls. However, the aperture size has another effect; it changes the depth of field or focus (DoF) of the captured image. Any kid who's made a pinhole camera knows this, because a pin-hole camera only works when the entrance aperture is… well… A pinhole.
A wide aperture will have a shallow DoF, where the primary object will be in focus but the foreground and background will be blurred. A narrow aperture will provide a well focused image of the foreground, object, and background. Most tablet and smart phones, and miniature cameras such as the GoPro, have narrow apertures to enable them to shoot very crisp images without the need for a focus mechanism. Unfortunately, this can cause a picture to be very cluttered, because everything is in focus, not just the main subject. On the other hand, there are times when you want everything in focus, but an automatic camera setting might open the aperture to better exposes the scene, thus blurring out the fore and backgrounds.
The solution is to use the “aperture priority” setting on your camera. This allows you to set the aperture for the correct DoF, and then the camera will set the shutter speed and/or ISO to yield the proper exposure. I find myself using this setting at least 60% of the time for still photography.
The aperture setting is often referred to as the f/stop. Since the amount of light admitted through the aperture is proportional to the area of the opening, the f/stop is measure of the Apertue. Each f/stop represents an aperture that is the square-root of two larger or smaller in radius, and thus admits a fair of two in light. The actual f/stop number is the reciprocol of the aperture size, so changing the f/stop from f/5.6 to f/8 reduces the amount of light entering the entrance aperture by a factor of two. This last fact is really all you need to know, since it will relate directly to the settings for shutter speed discussed next.
Shutter Speed –
The shutter speed determines how long the film or sensor are exposed to the incoming light; slow shutter speeds increase the time that the shutter is “open” thus increasing the amount of light accumulated, and vice versa. Typically, slow shutters are used in low light situations, while fast shutters are used in bright light.
But there's a catch: moving objects will tend to be “motion blurred” with slower shutter speeds. This might be undesirable if you're shooting fast moving subjects such as athletes, cars, planes, or a child at play. Or, you might want a slow speed to intentionally blur the scene to make a waterfall more dramatic, for example.
In these situations, you can use the shutter priority on your camera to set the desired speed, and the camera will set the aperture and/or ISO to achieve the correct exposure.
I noted above that a change in one f/stop changes the amount of light by a factor of two. Shutter speeds setting are designed to also change the amount of light by a factor of two. Thus changing the shutter speed from 1/60 to 1/125 will reduce the light by a factor of two. If you were to also change the f/stop from f/8 to f/5.6, that would increase the light entering by two, and the overall exposure of the image would remain unchanged. This interplay between f/stop and shutter speed is the primary key to creating photographs as opposed to snapshots!
Here are two Wikipedia reference pages for shutter speed.
ISO Setting –
Finally we need to discuss the ISO setting, as I used to know it in the US, ASA. The International Organization for Standardization (ISO), is an international organization that maintains standards for a variety of measurements and other specifications. In the current context, ISO refers to the sensitivity of the sensor to light. In the film days, this mean that the emulsion on the negative had larger crystals that absorbed light and changed their opacity to make the image. Higher ISO meant larger grains and more light sensitivity. Today, for digital imaging sustems, the ISO setting refers to the “signal gain” on the detector. Larger ISO numbers mean that the camera is more sensitive, and it will therefore make an image in lower light.
However, higher ISO settings also bring added noise. In film, the larger crystals make the image more grainy. In the digital age, the higher ISO, and thus gain, increases the random noise on the image. In addition, higher ISO will also make bad pixels and any structure in dark current be more promenant. Lower ISO will always provide a better quality photgraph, but some situations will require higher ISO settings. It's always best to start with the lowest setting and only increase the value if you are unable to achieve your desired explore by adjusting the aperture and the shutter speed.
Higher end cameras now have noise reduction that can significantly improve image quality even at higher ISO settings. And some cameras will actually take a “dark” image to remove bad pixels and dark current structure; this age-old standard for astronomy imaging has made its way into commercial photography!
Here is a rather dense technical description of ISO that includes a discussion of ISO as applied to digital cameras.
Now that you have some understanding of the non-automatic controls on your camera, you should challenge yourself to not to use the automatic setting….ever! Ok, there is one time when the automatic setting is very useful, and that's when your kid is doing something impromptu, and you don't have time to mes with the controls…
1) Shoot flowers in your yard. Set aperture priority and then photgraph the same flower, but with different f/stop settings. What happens?
2) Shoot running water, such as a stream, fountain or even a running faucet. Use shutter spread priority, and then take a series of shots at different speeds. What happens?
3) Shoot a flat, well lit scene, such as a wall-hung picture. Set the camera to manual mode. Select a f/5.6 and shutter speed 1/125. Take a shot. Now change the f/stop to the next higher number (e.g., f/8) and shutter to the next longer speed (e.g., 1/100). Take a shot. Now try the opposite experiment. What happens? This should give you a feel for the interplay between f/stop and shutter speed.
4) Shoot a simple scene in low light. Set the camera to manual. Adjust the shutter speed and aperture to get a good exposure. Now take a series of shots with different ISO settings, starting with ISO 100 and increasing by at least four or five settings. What happens?
Next time, I'll post my results from these excercises.
Until then, happy shooting!