Photography Notebook: Exposure
There is no word as basic to photography as exposure--the total amount of light reaching your camera's sensor when taking a photo. Two things go into exposure: illumination and time. Exposure is the amount of light applied to your camera's sensor over the time the shutter is open.
As a new user, I was confused about the word exposure. Many photography books advised that if a photo was too bright or dark, I should "adjust exposure." I then wondered, "Where is the exposure button on my camera?" (It didn't help that some books would ofen say, "open it up a few stops" or "stop it up a bit"--directives which makes no sense to me even now.) I paged through the instruction book for the Rebel and online information, and I could find no mention of this "exposure button" or "exposure dial" that I needed to adjust to make my photos perfect. The truth is that there is no single exposure button on the camera. Moreover, as far as I can tell, every button, dial, and switch on the camera has some contributing function in determining exposure or the appearance of the photo. The challenge in photography is to manage all these controls--which can be adjusted in many (literally millions of) combinations--so that you can capture a subject with your photographic intent.
What determines exposure?
The definition of exposure in terms of time and light implies the term exposure applies to only those settings which set time and light level for the photograph. Many photography books seem to imply that just adjusting the shutter speed and aperture value is all that is involved in exposure. However, there seems to be that many, many more factors that influence exposure. Every single dial on the camera and every single thing in the scene plays a role in exposure or how the camera interprets that light and produces an image.
From what I can tell, these factors directly determine exposure:
- The time the shutter is open: controlled by Time value (Tv) or Manual (M) mode. The shutter time is set automatically when the camera's Automatic exposure (AE) circuitry kicks in for Program (P) or Aperture Value (Av) mode.
- The f number: controlled by Aperture value (Av) setting or set in the M mode. This controls the size of the opening allowing light to the lens. The Av is set automatically when the camera's Automatic exposure (AE) circuitry kicks in for P or Tv mode.
- The sensitivity of the sensor: the ISO speed setting mimics the sensitivy of film. A low ISO value, such as 100, makes the film less sensitive and longer exposures are necessary to capture an image. Higher ISO values, such as 1600, make the film more sensitive to light and less time is needed to capture an image.
- The interpretation of the scene: Metering mode determines how your camera's sensor interprets different parts of the scene. These modes make different parts of the sensor sensitive or less sensitive to light.
- The Exposure compensation value (EV) setting: this skews the AE circuitry to either underexpose or overexpose the scene.
- The scene's light levels and how the camera is situated within these light sources, reflections, and shadows. Shadows, clouds, wind, smoke, fog, mist, leaves, trees, grass, reflections, and moving people and objects have a big part in the exposure in addition to the camera controls. Of course, in studio photography, these factors are controlled--almost completely with good lighting skills.
What also contributes to how a photo looks?
The appearance of a photo also seems to be influenced by factors which don't seem to fit precisely the definition of exposure, but which impact the final appearance of the photo:
- Lens optics
- White balance
- Picture style
- Color space
- Noise reduction
- Custom functions like highlight tone priority
- The scene itself--light and shadows, air conditions, movement
- Techniques such as High Dynamic Range (HDR) which combine several shots at different exposures into one photo.
SummaryPreparing these notes helped me understand the key ideas behind exposure:
- Exposure is the amount of light falling on the sensor while taking a photo.
- There is no single dial on the camera for exposure, but setting the time value for the shutter (Tv), the aperture value (Av or f number), ISO speed, exposure compensation, or metering mode sets exposure.
- The or camera's position within the scene controls exposure.
- Other settings influence the appearance of the photo, such as filters, lens optics, focus, white balance, picture style, color space, noise reduction, custom functions, or techniques like HDR.
- A big influence on the exposure is also what is going on in the scene--light, shadow, movement of the sun, clouds, reflections, fog, mist, smoke, and many other changing and uncontrollable factors--also play a role.
- Busch, David D. (2007). Digital SLR Cameras & Photography For Dummies. Indianapolis: Wiley.
- Frost, Lee. (2003). Teach Yourself Photography. London: teach yourself / Hodder Headline.
- Hedgecoe, John (2003). The New Manual of Photography. London: Dorling Kindersley.
- Wikipedia contributors. (accessed 2009-07-22). "Exposure (photography)," Wikipedia, The Free Encyclopedia.