What I Know Of: Exposure

Exposure is the total amount of light that falls on the image sensor (or film, if you're old school) while taking a photograph.  In layman terms, exposure affects how light or dark the image is.  Getting the right level of exposure is critical to getting a good picture and can be tricky as the dynamic range (the range of light and dark) of a real-life scene is often more than the camera can actually capture.  That said, there's no single "correct exposure" for a photo -- photographers over- and under-expose photos all the time for aesthetic reasons.  I'm just going to talk about the basics of how exposure is affected by the camera controls that influence it, and why you would pick one camera control versus another.  In future posts, I'll go into detail on those controls.

Underexposure, "Proper" Exposure, Overexposure

There are several ways to control how much light falls on the sensor:

• At the source.  The time of day, weather conditions, use of flash, how much and what kind of artificial lighting -- all these things affect the amount of light in a scene and therefore exposure.  Source lighting is a whole topic, or series of topics, unto itself.  For now, we'll consider source lighting to be a constant and talk about the stuff you control on the camera.

• Aperture - The aperture adjusts a diaphragm inside the lens that changes size in order regulate the amount of light the amount of light that is passed through.  It is analogous to the iris of an eye, expanding and contracting to control light.

• Shutter speed - Cameras have a shutter which is conceptually like a curtain that is normally closed and which opens for a prescribed period of time to let light hit the sensor.  The amount of time the shutter is open is the shutter speed.  Using our eye analogy, the shutter is analogous to the eyelid, except that with a an eye the lid is normally open, and with a camera the lid (shutter) is normally shut.
• ISO sensitivity - The ISO sensitivity setting on a camera controls the sensitivity of the sensor to the light that hits it.  At a high ISO sensitivity setting, the sensor can get a suitable exposure with less light than at a low setting.  Stretching the eye analogy to its breaking point, imagine that you're outside on a bright sunny day for awhile, then you come inside and it's dark until your eyes adjust to the indoors.  This is your eyes desensitizing to light when outside and raising sensitivity when you go inside.  Adjusting this sensitivity is what the ISO setting does for your camera.  For the curious, ISO sensitivity is named after the standards body that specified it -- the International Organization for Standardization.

The Exposure Triangle and Reciprocity

The Exposure Triangle

The three camera parameters for affecting exposure -- aperture, shutter speed, and ISO -- are called "The Exposure Triangle".  Each affects exposure using different aspects of the camera's physical design.  The available values for shutter speed, aperture, and ISO are cleverly chosen so that a change to one parameter is equivalent, from the standpoint of exposure, to a change in either of the other two parameters.  In other words, selecting the next faster shutter speed results in the same reduction in light hitting the sensor as using the next smaller aperture or selecting the next lower ISO sensitivity setting.

More importantly, it means that I can change one parameter, and make the inverse change in another parameter, and the resulting exposure will remain the same.  So, if I open up the aperture by one increment (letting in more light), and increase the shutter speed by one increment (decreasing the amount of time the light is let in), the resulting exposure is the same before and after the changes.  The principle holds true for ISO sensitivity as well.  This inverse relationship between shutter speed, aperture, and ISO is called reciprocity and it's fundamental to understanding photography.  The key take-away from reciprocity is that we can get the same exposure with different combinations of the shutter speed, aperture, and ISO settings.

Side Effects of Each Parameter

So what's the point then?  Why would I use a different combination of settings if it results in the same exposure?  Because the parameters also affect image properties other than exposure.  A photo taken with one set of parameters will look very different from a picture taken with a different, but exposure-equivalent, set of parameters.

Here's what the Exposure Triangle parameters affect other than exposure:

• Aperture affects depth of field (DoF) and sharpness.  DoF is the distance between the nearest and farthest objects in a photo that appear acceptably in-focus in an image.  If you look at a photo in which the subject is in-focus and everything in the background is out of focus, that's a narrow DoF.  Narrow DoF is a terrific technique for making the subject pop and reducing visual clutter emanating from the background.  You'll see this a lot in portraits or close-up photography.  Conversely, a photo where everything in the foreground and background is in-focus is said to have wide DoF.  You'll see wide DoF in landscape photography.   Large apertures (big diaphragm openings) result in narrow DoF, while small apertures result in wide DoF.  In the example photo below, the DoF is only about 1 inch and only covers the middle section of the trilobite. 
  
  

  

  Narrow Depth of Field

  Aperture also affects the overall sharpness of a photo (not just the objects within the DoF).  Generally speaking, when you open up the aperture on a lens all the way, the sharpness is slightly reduced from what the lens is capable.  Reducing the aperture (or "stopping down" in photographer speak) a couple settings from wide open, will increase the overall sharpness of a photo noticeably.  Likewise, on the other side of the scale, stopping down a lens too much will introduce egregious amounts of an optical phenomenon known as diffraction, which also causes reduced sharpness.  For most lenses, one or two units smaller than wide open to about f/8 or f/11 is the "sweet spot" range where a lens delivers its best sharpness.

• Shutter speed affects the camera's ability to freeze or blur motion.  When objects in the photo are moving, a fast shutter speed can "freeze" them, providing a sharp image of the object.  With a slow shutter speed, the sensor captures light reflected off the object for a longer duration of the motion, causing the object to be blurred.  Written down that sounds like a bad thing, but in practice motion blur can give a photo a sense of, well, motion that is useful for pictures of dynamic subjects, such as sports, wild life, automobiles, dancing, etc.  It also does beautiful things with moving water such as waterfalls and rivers.  That said, sometimes freezing the action and capturing an instant in time gives you a perspective that you just won't get from watching the event in real-time.  A great example of this is the ubiquitous photo of a water drop falling into a glass of water.  So shutter speed, like everything else, is a creative choice.  Shutter speed also affects the camera's ability to take a sharp photo when the camera is handheld.  Depending on the focal length of the lens, it can be impossible to get a tack sharp photo with a handheld camera if the shutter speed is too low.  In a future post, I'll describe in detail how to deal with that.

Motion Blur

• ISO sensitivity affects the noise level of the resulting photo.  As an analogy, let's say you're listening to music and you crank up the stereo.  In moments when the music isn't playing and the stereo should be silent, you'll usually hear some hiss and noise.  Cranking the stereo is essentially raising its sensitivity to the recorded material and increasing not only the music level but also the noise level from the recording and the stereo circuitry.  This is the same concept as ISO sensitivity and image noise.  The noise is seen as a grainy look in the photo, and while it is sometimes used deliberately to lend a lo-fi look to a picture, it is generally not considered a desirable  thing.  However, sometimes in order to get the shutter speed or aperture you require for a shot (for example, you're trying to freeze the action of wildly moving children at an indoor party), you have to raise the ISO sensitivity and deal with the noise.  High noise photos are not only grainy but they're also less sharp because the grain has a deleterious effect on image resolution.  There is software available that does a phenomenal job at removing noise from a photo, however there's also a bit of a sharpness penalty with the software too.  But dealing with noise is usually preferably to dealing with a blurry photo because the shutter speed was too slow (software can't do much for that!).  In general you want use the lowest ISO sensitivity required to use your desired aperture and shutter speed settings and still achieve proper exposure.  Cameras vary pretty wildly on how well they deal with high ISO sensitivity settings, with newer and more sophisticated cameras having on board processing to reduce the noise, allowing you to get acceptable images from higher ISO sensitivity settings.

Excessive ISO Noise

The Balancing Act

So choosing between the various combination of aperture, shutter speed, and ISO is really a balancing act of deciding how you want to affect the image properties of exposure, depth of field, sharpness, motion blur/freeze, and noise.  Sometimes what you're trying to achieve puts the various camera parameters at odds with one another and then the balancing act becomes a matter settling on a compromise.  The sheer number of combinations makes photography a highly technical craft, but the myriad resulting aesthetic choices make it an art.