Exposure Principals Exposure Exposure is a term used to described the distribution of light levels in a picture. Generally speaking, a good exposure has an even spread of darks and lights that are evenly distributed across your image. There are instances in which you can shoot a scene in either a high or low key using a narrow range of extremely bright or dark light respectively, but these are typically more advance in nature requiring skill, but also sensitivity to the aesthetic of the image being create. For our purposes here, lets focus on what is generally associated with conventional image making and what defines a “good” exposure. Exposure is manipulated in photography with 3 mechanical principals that are combined to achieve exposure. These are lens aperture, shutter speed and ISO sensitivity. We will look at each of these individually. Aperture Aperture is controlled by a series of blades inside the lens that close down to a fixed setting at the moment you take the picture, then release back to their widest setting after. These blades essentially create a hole that light can enter the lens. Wider apertures allow more light to pass and smaller apertures allow less. The principal is if you need more light, open the aperture. If it is too bright, stop it down. Aperture in photography is measured in F-stops. Your lenses widest aperture might be anywhere from 5.6 to 1.4 depending on the type of lens you are using. This setting is often marked on the lens. Shutter Speed As the name implies, shutter speed is the measurement of how long the shutter is open to let light on to the sensor or film plane. Longer shutter speeds allow more light and shorter allow less. Here we can begin to see a tradeoff of shutter speed and aperture for a require exposure. If you open the aperture wider, you’re allowing more light in so you would need a faster shutter speed to compensate. Smaller apertures would require a longer exposure time from the shutter. ISO ISO is an acronym for the International Organization of Standardization. In photography this is a number that represents the sensitivity that you are recording with. Film speeds are rated per roll. Digital cameras can be changed for each picture taken. Like everything in photography there is a tradeoff. Lower sensitivity settings (50-200) require more light, but have less grain with film and less noise with digital. Higher sensitivity settings (800-6400) are helpful when shooting in low light, but can produce more grain and noise in the image. So getting an even exposure requires a balance of these three concepts. Decisions have to be made by the photographer as to what potential tradeoffs can be in getting a good picture. So what actually happens inside your camera if you’re shooting in Automatic mode? If you have a modern digital or even a decent 35mm camera made in the last 20 years, you most likely have at least one if not several automatic settings you can set. If you are shooting in full automatic mode, the meter inside your camera measures the light information in the scene you are photographing and comes up with what it feels is the best balance of aperture, shutter speed and in digital cameras, ISO and takes the picture using this combination. There is often a dial of some kind that can “shift” this balance around since different settings can create the exact same exposure. For example, if you meter a scene and your camera tells you that at an ISO of 400 it will shoot the picture with an aperture setting of 5.6 and a shutter speed setting of 1/15 of a second. Turning this dial will shift one up and the other down to balance it out or vice versa. So 5.6 is a good aperture setting, but 1/15 is a long enough shutter speed to create blur if you don’t have a tripod or you’ve had too much coffee. So you can pick a faster shutter speed – say 1/60 which is two stops faster. This means 2 stops of light have been taken away from our shot so we need to open the F-stop of the aperture to let 2 stops of light in. So you will need an F-stop of 3.5 to match the 1/60 shutter speed. In automatic settings, your camera will do this for you, but if you are shooting manually you need to learn how to manipulate. Stops of light aren’t always intuitive, but they are easy to memorize. Digital cameras can be more confusing in that they often measure shutter speed and aperture openings in 1/3 stop increments. Shooting on older 35mm cameras can help you learn stop differences faster, but either way you should know them. The following charts explain aperture full stop increments and shutter speed full stop increments: Aperture 1.4 | 2.8 | 5.6 | 8 | 11 | 16 | 32 | 64 Shutter Speed 1/2000 | 1/1000 | 1/500 | 1/250 | 1/125 | 1/60 | 1/30 | 1/15 | 1/8 | 1/4 | 1/2 | 1 second | 2 seconds | 4 seconds | 8 seconds ISO 25 | 50 | 100 | 200 | 400 | 800 | 1600 | 3200 | 6400 You might notice right away that these numbers are all going up or down by doubling or cutting in half. This is the easiest way to remember these. For example, if you want to brighten up the image by one stop – you can double the shutter speed, cut the aperture in half or double the ISO sensitivity. This is a very broad overview of these concepts and as mentioned above, your camera will figure out all the math for you. In the coming articles, we will take a look at the visual aesthetics of each of these principals as well as learning how to meter without the in camera meter.
Posted on: Thu, 11 Jul 2013 15:36:54 +0000
Trending Topics
Recently Viewed Topics
© 2015