DP Tips # 4: The Holy Trinity of Exposure

In the next installment of my DP Tips, I thought it would be useful to give all the “newbies” out there a brief run down on the different factors that directly impact a digital photograph’s registered exposure.

Exposure: What is it exactly?

Put quite simply, exposure is an intrinsic characteristic of a digital photograph which quantifies and qualifies the quality of light that ultimately reached the digital camera’s digital sensor. In the olden days, photographers had to be concerned with a multitude of factors before arriving at the proper exposure for a given photograph. Fortunately, for us in the digital age – digital cameras are capable of “automatically” making very well informed “guesstimates” as to what it believes the exposure for a given photograph should be. In certain situations, the camera is very adept at performing this task and in other situations its’ calculations are less than ideal. Improving your photography in the digital-age requires us to understand exposure in one of 2 contexts: (a) how the camera formulates its exposure “guesstimates” and (b) how we can manually override the settings to create the perfect exposure.

Camera on Automatic: What’s happening under the hood?

When you shoot your digital camera on its pre-defined “automatic” settings, the camera is making all the decisions with the goal of establishing a properly balanced exposure. Depending upon the the type of camera you have, the camera will have more or less “smarts” built inside of it to make an educated guess for the proper exposure. On my Canon 40D, the camera actually has a mini-database of different types of photo scenarios with pre-established exposure settings. Of course the success of this strategy is limited to the camera’s ability to compare what it thinks it sees in your frame to what is actually in its exposure preset database. By the time you have pressed your shutter button half-way down, the camera has formulated what it thinks is the proper exposure for the foreground, subject and background that you have chosen.

Aperture … and Shutter Speed … and ISO – Oh my!

After your camera has calculated an exposure for your photograph, its characteristics can be quantified by three attributes. These attributes are crucial in understanding exposure and ultimately in making manual adjustments to improve upon what the camera has setup for you. The following three terms are therefore introduced for all the newbies in the audience:

• Aperture: The size of the opening on your camera’s lens through which light will be allowed to pass. A smaller assigned aperture value translates (albeit counter intuitively) into a larger opening for light to pass. Aperture therefore has a direct impact on the quantity of light that reaches the camera’s sensor in a given period of time.  Specified aperture values are expressed in numbers like: 2.0f, 2.8f, 4.0f, 5.6f, etc. This number is called the f-stop number.
• Shutter Speed: As described above, the camera will allow the sensor to be exposed to light for a given period of time. The longer the shutter speed, the more light hits the sensor. On the other hand, a shorter shutter speed restricts the amount of light that reaches the sensor – thereby affecting the quality of the exposure. Shutter speeds are typically expressed in fractions of a second (or longer).
• ISO Sensitivity: This means something slightly different than it did in the days of film photography, but fundamentally the ISO value assigned to an exposure indicates how the sensor chose to react to the light that hit its surface during the specified timer period. Typically rated with numbers in the 100 to 1600 range, ISO sensitivity for digital cameras is based upon a reference ISO value. When a digital camera employs a high ISO setting, it is making a decision to amplify the light that has reached the sensor. However if the camera utilizes a low ISO value (like 100 or 200), it is making a decision to process the light in a fairly standard (non-amplified) manner.

Aperture, Shutter Speed & ISO are ALWAYS inter-related

The above statement is pretty straight-forward, but deserves some additional discussion. When the camera calculates an exposure for a given photograph, it has determined values to use for the aforementioned three attributes. If any of these attributes are changed (by you or the camera), the exposure will be impacted in very specific and qualitative way. There are some practical scenarios where you might want to use a longer shutter speed than what the camera suggests or maybe you want to allow more light to enter then lens in a given period of time. In either of these two situations, you or the camera will have to adjust one or both of the other 2 attributes in order to maintain an equivalent exposure. Future DP Tipsarticles will get into more practical scenarios and utilize basic math to explain how exposure can be manipulated to suit a particular photographic need.

The Histogram: How Good is Your Exposure?

So – your camera was set to automatic and you trusted that the camera would properly evaluate the exposure for your beautiful outdoor garden shot. How do we know if the camera nailed the exposure or if a manual change to some of the settings might result in a better photo? All new digital cameras offer the ability to display a histogram representing the distribution of colors across a given photo. A well-exposed photograph, should have a clean distributions of colors between pure black and pure white. The histogram representation for a well defined photo typically looks like a mountain range (see image 1 below), whereas a poorly exposed photo has a histogram where all the data points are concentrated at one or both sides of the graph (see image 2 & 3 below).