The Exposure Triangle, Part II: What Is “Exposure” and What Is A “Stop”?

Each of the three parts of this photograph, from left to right, denotes a one stop increase (a doubling of the amount of light) in the exposure.

“Exposure” and “Stop” Explained:

In Part I of “The Exposure Triangle” (see that article here) we established the essential definitions of Aperture (f-number), Shutter Speed, and ISO.

But, two vitally important questions must be answered if we want to gain complete control over our camera’s settings:  1) What is the exposure of  a photograph, and 2) How do we measure what happens to exposure when you raise or lower Aperture, Shutter Speed, and ISO?

Exposureis the amount of light captured by the camera’s sensor.  The exposure is manipulated by The Exposure Triangle we learned about in the previous article: Changing Aperture, Shutter Speed, or ISO alters the exposure of the photograph.

When we adjust Aperture, Shutter Speed, or ISO, we use a stop to measure the increase or decrease in exposure.  You’ve probably seen or heard the word stop used before in photography, but what does it mean?

In photography, a “stop” is a doubling or halving of the amount of light which reaches the camera’s sensor.

If you increase the exposure by 1 stop, you literally double the amount of light reaching the sensor.  If you decrease exposure by 1 stop, you literally halve the amount of light reaching the sensor.

The following will discuss how Aperture, Shutter Speed, and ISO can change the stop of a photograph:

Stops and ISO:

The standard ISO settings.

As we discussed in the prior article on The Exposure Triangle, ISO is a number which denotes the sensitivity of your camera’s sensor to light (the higher the ISO, the more sensitive your sensor is to light).

Doubling the ISO number increases your exposure by one stop (doubles your sensor’s sensitivity to light).  In other words, moving ISO from 200 to 400 increases exposure by one stop (doubles the sensor’s sensitivity to light).  Conversely, moving ISO from 400 to ISO 200 drops your exposure by one stop (halves the sensitivity of your sensor to light).

Stops and Shutter Speed:

Some of the basic shutter speed settings.

As we discussed in the prior article on The Exposure Triangle, shutter speed measures how long your sensor’s shutter is open…in other words, the shutter speed is how long your sensor is able to collect light.  The slower the shutter speed, the longer the shutter is open and the longer the sensor is able to collect light.

As is the case with ISO, doubling your shutter speed (slowing your shutter speed by double) will increase exposure by one stop, and halving your shutter speed will decrease exposure by one stop.

For example, slowing your shutter speed from 1/30 to 1/15 will double the amount of light collected by the sensor, thereby increasing exposure by one stop.  Conversely, quickening your shutter speed from 1/30 to 1/60 halves the amount of light collected by the sensor, thereby decreasing exposure by one stop.

Stops and Aperture (f-number):

The basic aperture (f-number) stops.

As we discussed in the prior article on The Exposure Triangle, aperture (f-number) denotes the size of the opening of your lens.  The larger your aperture (or the smaller your f-number), the more light reaches the sensor.

The measure of a stop of the aperture is a little bit weird.  Since f-numbers are measured differently than ISO and shutter speed (the f-number is the ratio of your lens’ focal length to the diameter of the aperture), a stop doesn’t relate to doubling or halving the f-number.  Instead, to decrease one stop you multiply the f-number by 1.41 (which is the square root of 2 rounded to the nearest hundredth), and to increase one stop you divide the f-number by 1.41.

For example, going from f/11 to f/16 is a decrease of one stop because 11 * 1.41 = 16.  Conversely, going from f/11 to f/8 is an increase of one stop because 11 / 1.41 = 8.

In other words, changing your f-number from f/11 to f/16 halves the amount of light passing through your lens.  Changing your f-number from f/11 to f/8 doubles the amount of light passing through your lens.

Don’t worry about learning the math behind f-stops.  All you need to do is know the basic f-stop settings (f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22), and know that between each of those basic f-stops is one stop of exposure.

Stops and The Exposure Triangle:

So how does this all relate back to the Exposure Triangle?

A “stop” is interchangeable among Aperture, Shutter Speed, and ISO.

For example, let’s arbitrarily say your optimum exposure is f/8, 1/30s, ISO 200, and you want to decrease your f-number by one stop to f/5.6 (thereby letting in twice as much light). If you lower the f-number by one stop, you’re letting in twice as much light, and you’re photograph will appear too bright if you keep ISO and shutter speed unchanged.  You need to find a way to decrease the amount of light by half in order to get back to optimum exposure.

To counteract that doubling of light when you lowered the f-number by one stop, you need to change shutter speed or ISO to halve the amount of light.  You can choose to either quicken shutter speed by one stop to 1/60s, or decrease ISO by one stop to 100, and your exposure (the amount of light collected by the sensor) will go back to the original (optimum) amount.

See the image below for basic description of what must happen to maintain the same exposure if you increase or decrease any setting by one stop:

How to balance exposure after moving a setting up or down a stop.

What Happens When We Adjust Aperture, ISO or Shutter Speed?

This is the key to basically everything.  Why would you ever want to alter your settings and have to deal with correcting exposure like I described above?  Well, there are considerations to understand when adjusting your exposure:

Aperture: When you adjust aperture (change the f-number), you alter your photograph’s depth of field.  The depth of field is the amount of area in front of and behind your focal plane which appears to be in good focus.  The lower the f-number, the narrower your depth of field.

So, let’s say you want to isolate your subject from the background. One way to accomplish this is to use a very low f-number…doing so will create a narrow depth of field.  A narrow depth of field will make your subject appear to be in good focus, but the area in front of or behind the subject will appear soft, or blurry (i.e. the blurry area is what is commonly referred to as bokeh).  This is how you isolate a subject in photography.

Shutter Speed:  If your shutter speed is too slow, your photograph will appear blurry due to motion blur.  There are two types of motion blur:  1) Your camera moving in your hands while the shutter is open, or 2) Objects in your composition moving too quickly while the shutter is open.

If you wish to freeze all the motion in your photograph and mitigate the risk of motion blur, you need to make sure your shutter speed is quick enough.  In other words, if you take a picture, and the photo is blurry due to camera movement in your hands, you need to quicken the shutter by enough stops to eliminate this type of motion blur.

Some photographers purposefully slow down shutter speed to create a long exposure photograph.  Long exposures typically have very distinguishing characteristics in that they all introduce the element of movement into the photograph:  Long exposures make running/flowing water look soft and silky, they can make moving people disappear or blurry, and they can even turn moving lights (i.e. lights on a moving car) into laser beams.  For this reason, controlling your shutter speed can be a vital tool in the toolbox of a fine art photographer.

ISOWe discussed this in the previous article on The Exposure Triangle:  As your ISO number increases, and the sensor becomes more sensitive to light, the sensor will pick up more noise, or grain, in the image.  So, while bumping up ISO is one viable alternative to speeding up the exposure, you will always want to work at the lowest ISO number possible due to the introduction of noise into the photograph. That is, unless you actually want grain in your photograph…then working at a higher ISO is necessary.

The amount of noise your sensor picks up at a given ISO depends greatly on the quality and size of your camera’s sensor.  Generally speaking, the larger the sensor in your camera, the more you can raise ISO without introducing significant noise (i.e. full-frame sensors usually pick up less noise at comparable ISOs than do APS-C sensors).  Furthermore, some sensors are specifically designed to function better (i.e. pick up less noise) at higher ISOs, like the sensor in the Sony A7s and A7sii.

Controlling and adjusting aperture, shutter speed, and ISO is a constant balancing act.  There is no “right” or “wrong” way to properly expose a photograph.  What is vitally important is that you choose the settings which will create the characteristics of the photograph you want, while at the same time minimizing the risks of any downsides to your settings.  With some practice you will be able to manipulate these three settings from rote memory, knowing immediately how each change will alter both of the other two settings, and ultimately how it will alter the characteristics of the photograph.

Happy Shooting!


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