Photo Basics: The Exposure Triangle

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A photo like this requires a delicate balance of the exposure triangle.  But I pushed the envelope ever-so-slightly to make this scene a bit more dramatic:  Despite the low light, I intentionally kept the ISO low and the shutter speed slow to emphasize the contrast.

Like any art form, Photography is a matter of balance.  Finding balance is what ultimately makes a great photograph:  Balance in your composition, balance in the subject matter and balance in the exposure.  Today, we’re going to focus on Exposure.  Exposure is ultimately the way in which the photograph is recorded.  It’s a term we use to describe how the light was captured in the frame.  A photo, after all, is simply a record of the light at a given moment.  So we are ultimately crafting with the use of light.  So exposure is a very important aspect of photography and we need to make sure we use the light perfectly.  When it comes to exposure, we’re specifically talking about the balance of three technical details:  ISO, the the Aperture size and the Shutter Speed.  Collectively, these elements make up the Exposure Triangle.   Finding balance in your exposure will help you communicate your story through the finished photograph.

So let’s break it down…

Components of Exposure:

ISO

Put simply, ISO is the measurement of the sensitivity of the shooting medium.  On film, it’s simply a way to measure the sensitivity of that film.  So choosing the film is important because you’re basically locked into that ISO until you finish the roll.  With digital cameras, the medium is actually a digital sensor.  The digital sensor can be changed on the fly; you can even have a different ISO for each photo.

A higher ISO number means that the film or sensor will be more sensitive to available light.  So you’ll be able to shoot with faster shutter speed at higher ISO, assuming no other conditions are changed.  The catch is that as the ISO increases, you will lose more and more detail due to grain and noise.  With film, the higher ISO films tend to have a larger grain, larger cells if you will.  You may get the shot, but you will see a notable grain across the finished photo.  With digital sensors, a higher ISO also results in a grain, but usually in the form of noise.  Depending on the camera, high ISO could result in colored dots (hot pixels), though some cameras try to correct for it.

Generally, you want select the lowest ISO feasible (usually 100 ISO) because it will yield the best detail and the cleanest image.  But it will change the way you need to use your Shutter and Aperture, but more on that in a bit.

Aperture

The Apeture is a physical gate that sits inside or just behind the camera lens.  It ultimately controls the depth of field.  The depth of field is a measurement of how much depth inside your photograph will be in focus.  There’s a mathematical way to figure out exactly how much will be in focus, but we’re not getting into the math today. Just think of it this way:  The wider the aperture, the shallower the depth of field.  A small aperture will result in a deep depth of field.

A warning:  Aperture is measured in terms of fractions or ratios where it’s always 1 over some other number.  So an aperture setting of f/50  (sometimes noted as f:50 or f50) is actually a smaller aperture than f/500.  That ‘f’ throws people off sometimes, so don’t be surprised if you find a few photographers with the mantra of “big number equals small aperture”.  It’s a bit of a misnomer because we are dealing with fractions and ratios: Mathematically, if f=1, 1/500 is significantly smaller than 1/5.

The ‘f’ in aperture measurements actually stands for focal length of the lens.  The resulting size of the aperture is not a fixed size as many are led to believe.  The actual size varies depending on the focal length.  Because of this relationship, you will see different behaviors of the same aperture setting at different focal lengths.  But the ratio is really just a measurement of the amount of light that will get through.  Aperture settings are also referred to as f-stops.

Shutter Speed

I saved the simplest for last:  Shutter speed is literally just a measurement of how fast the shutter will open and then close.  It’s measured in a fraction of a second.  So a shutter speed of 1/100 is fast, but not nearly as fast as 1/1000.  If you look at that setting in your camera, you may find that number can climb quite high.

If you’re trying to freeze a moving object in your photograph, you’ll want the shutter to act as fast as possible.  But as you increase the speed of the shutter, you will significantly cut the amount of time that light can reach the film or sensor.  Your image will appear darker every time you increase the shutter speed, assuming no other aspects have been altered.

The Exposure Triangle:

If you were to chart it out so that the three components of exposure were plotted on separate axes, the chart would look like this:

Photo Exposure Triangle
So imagine that you are setting up to create an exposure and you must find the balance between all three axes.  Each axis has a relationship with the other two and if you favor one axis, it pulls resources from the other two as if everything were connected by a fixed length of rope with pulleys.  By this very nature, you cannot accommodate all three.  That is to say that if you want a faster shutter, you will probably need to have a larger aperture and therefore have a smaller depth of field.  Or you may be able to counter-balance that by increasing your ISO to make your aperture a little smaller, but you will sacrifice some of the image quality.  There’s always a relationship and you can’t escape it.  So here’s a quick summary of how each affects the end result that will become your photograph:

  • Aperture: Affects Depth of Field
    • A small Aperture yields a deep depth of field (more things in focus), but will require slower shutter speeds or higher ISO
    • A large Aperture yields a shallow depth of field (blurry backgrounds and foregrounds, but subject is in focus), but will permit faster shutter speeds or lower ISO.
  • Shutter Speed:  Affects Motion Blur
    • A fast shutter speed can freeze motion, but it will require a lot of light (either increase ISO or large aperture)
    • At slower shutter speeds, motion will blur, but light requirements are more forgiving (smaller apertures or better ISO are possible)
  • ISO Sensitivity:  Affects Image Quality
    • A higher ISO will have more of a grain, but it will permit faster shutter speeds or smaller apertures
    • A lower ISO will yield a higher quality image, but more light will be required (slower shutter speeds or larger aperture)

Choosing which axis to focus on depends on your artistic vision or the situation.  For example, simply getting the shot is half the game with sports, so you will want a fast shutter speed.  This could mean that you work with a wide open aperture (less forgiving on focus) or a higher ISO (more grain, but it may be acceptable).  But suppose you are doing something artistic with a dark and looming environmental portrait where your subject is fairly well lit but the background is not.  You may push a faster shutter speed to increase the contrast and drama in the photograph.  Or, if you want to keep a shallower depth of field, you may lower the ISO as much as possible.

My point is that this all

 

Final Thoughts:

As you can imagine, I could very quickly go down a series of tangents as to how you can push the envelope of your exposure triangle to create some interesting images.  But we’re just scratching the surface at this point.  The exposure triangle will eventually become second nature to you.  It’s not always an exact science, but it’s important to know how it all works.  Having a basic understanding of how this works is essentially to thriving in the world of photography.  This is the science behind the art.