The Exposure "Triangle" Simplified
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Hi neighbours
This is the second in a series for newer photographers to help clear the air and reduce the miasma of confusion around technical terms. This time we are going to dissect the concept of the exposure triangle.
Exposure triangle is a popular phrase, but there is actually no triangle. Exposure is the combination of three unique settings. The combination of these settings produces something called the Exposure Value often referred to as EV. Most courses and classes leave this part out, which is, in my perspective, a mistake. Let’s break these settings down.
Shutter Speed
Depending on your camera, the maker may indicate this with an S or Tv. While frustrating that they cannot get together on one contraction, we live with it and move on. All of our modern cameras have a shutter. For most cameras today, this is better called a focal plane shutter. Think of it like this. You have a window. The window has two curtains. The window starts off completely blocked off and no light comes in. The first curtain opens by moving across the window vertically to allow light in. At some later point, the second curtain moves across the window vertically in the same direction to stop light from coming in. The time between the opening curtain and the closing curtain is called the shutter speed, most commonly measured in fractions of a second.
These are the numbers you see displayed in the viewfinder. They are not typically shown as fractions for space reasons but are. So for example, if you see 125 in the viewfinder, it means that the time between the opening curtain and the closing curtain is 1/125 th of a second. Thus the larger the number, the shorter the time between the opening curtain and the closing curtain.
Why Does This Matter?
The idea of the shutter speed has a single purpose. It is used to control action or movement. A rock on the beach is not moving so the shutter speed is less a concern to you. In fact, your body will move long before the rock which brings us to safe handhold able shutter speed, which I will cover in the next section. Now your subject is a child at the beach, jumping off the rock into the surf. The child is moving and if you want to freeze the movement, you need the time between the opening curtain and the closing curtain to be very small, perhaps 1/500th of a second. Or may be you want to blur the jump entirely, which would make for a longer duration. You’re the artist, you choose.
Handholdable Shutter Speed
The human body moves. For something used to capture motion like a camera, we need to be able to counter our natural movement. Experience has demonstrated that most humans can handhold steadily as shutter speeds no slower than 1/60th of a second. I am aware that maker’s tell you that their image stabilization technology will give you “up to five stops” of stabilization. Note the part that says up to. It’s a possibility, not a guarantee, so best practice is not to count on stabilization. Nice if it works, but not to be depended upon.
This number varies depending on the person, the weight of the camera and lens and also the lens focal length. In ideal conditions, a good guideline is that the maximum safe handhold able shutter speed is one over the maximum focal length of the lens. So if the lens has a focal length of 60mm, then the safe handhold able shutter speed would be one over sixty or 1/60th of a second. If the lens is instead a zoom lens of the telephoto variety to bring far things apparently closer, let’s say 150mm - 600mm that the safe handhold able shutter speed would be 1 over 600 or 1/600th of a second. Some people will say that this is wrong, but it’s not because the size and weight of the lens does not change when you zoom. Where some folks get tripped up is when the focal length of the lens is quite short. Let’s say that you have a 20mm lens. By the focal length argument you should have a safe handhold able shutter speed of 1/20th of a second. But you don’t. Why not? Because your body still moves, and we accept that 1/60th of a second is as long a shutter speed as we can safely handhold. Over the decades, I have met many folks who want to argue these points, and that is their prerogative. The collection of blurry photos is a result. By the way if you are more shaky, as some of us are, you may find that the safe handhold able shutter speed for you is a shorter duration. Such is life. As I have gotten older, there are days where 1/60th of a second is too long and so I remind myself on those days to pick a maximum handheld shutter speed of a shorter duration.
Aperture
The aperture in a lens is a changeable item on modern cameras. Much is made of the ability to change the size of this opening, to allow more or less light to enter the camera. In that context, it acts as the counterpoint to shutter speed. A smaller aperture number, means more light passes, so the shutter needs to be open for a shorter time. This is good to understand. If I choose a very small aperture which looks like a higher number (it’s not really), then I need the shutter to be open longer for the same exposure all other things being equal. This is where we start to see that shutter speed and aperture are indirectly related.
The aperture numbers are mathematical constructs. What the numbers are is less important than an aperture of f/4 on one lens means exactly the same thing as f/4 on another lens. The smaller the number, the bigger the opening. The bigger the opening, the bigger the lens must be to gather light, which increases weight and cost, often substantially. This used to be a bigger concern when we shot film, but it is far less a consideration in the world of digital.
Far more important to you as a photographer than the size of the lens hole, is something called depth of field. Depth of field is how much of the scene is in focus from near to far. When we focus on a single subject, the aperture we choose will directly impact how much in front of the point of focus will be sharp and how far behind the point of focus will be sharp.
We care about this for creative reasons. A powerful image has only one subject, and we use depth of field to point the viewer’s eye to that which is most important. In a portrait, that is typically the subject’s closest eye. In a landscape, it is typically something in the middle distance and we depend upon depth of field to manage how much in front of and how far behind the point of focus is sharp.
How Does This Relate to the Aperture Number
Depth of field is affected by three elements;
What aperture is in use
How far is the subject from the camera
What is the focal length of the lens being used
The aperture in use impacts depth of field as follows. The smaller the aperture number, the less depth of field is achieved at any subject distance or focal length. Thus in an example where the lens and camera to subject distance are fixed, an aperture setting of f/5.6 has much less depth of field than an aperture setting of f/22
The distance of the subject from the camera impacts depth of field as follows. At any given aperture or lens focal length, the closer to the subject the less depth of field there is. In an example where the aperture does not change and the lens does not change, an aperture of f/8 will deliver far less depth of field if the subject is 5 feet away than if the subject is 25 feet away.
The focal length of the lens impacts depth of field as follows. The greater the focal length of the lens the less the depth of field is delivered at any given aperture or subject distance. In an example where the aperture does not change, and the camera to subject distance does not change, a lens of focal length of 100mm delivers more depth of field than a lens of 300mm.
We already understand the interdependency of shutter speed and aperture and can now think constructively about how much motion freeze we need in conjunction with depth of field we need, which leads us naturally to our third variable.
ISO
ISO stands for International Standards Organization which respectfully tells us mostly nothing. It came from the consolidation of two other standards, ASA (American Standards Association) and DIN (Deutsche Industry Norm) as a means of describing consistently the sensitivity of a particular film emulsion to light. So what has that to do with digital?
We have ISO in digital because during the transition from film, the makers all agreed that they needed it to be easy for photographers to transition. While film sensitivity was no longer a factor, the ability of the sensor to record light became a factor. Just has higher film ISO ratings (called film speed) required larger elements of silver halide on the film, which after processing would appear as “grain” in the image, to make a sensor more sensitive, more voltage is provided to the photo receptor or pixel on the sensor, As more voltage is provided the signal to noise ratio gets worse and we experience this as digital noise in the image. Film had grain, digital has noise, and both are effects of increased sensitivity.
While film could only get so good. Sensors are regularly improved. Most people think megapixels are most important, but they are wrong. What is most important is how high you can drive the sensor sensitivity before the noise makes the image unusable. In fairness, this is a subject that has gotten right out of hand amongst those who spend more time pixel peeping than actually making images, but that is their issue, not yours nor mine.
By being able to adjust the sensitivity of the sensor, we can accommodate higher shutter speeds than in the past, or small apertures, or a combination of the two because we can balance their limitations with the ISO setting.
A simple Exposure chart showing shutter speed, aperture and ISO courtesy rodlawton.blogspot.com
We can see from this elegantly simple chart that we can achieve the same overall exposure value with different values of shutter speed, aperture and ISO. Consequently, you as the creative have options when you are making an image. There is not a generalized perfect setting.
Your camera will help you get to an acceptable exposure where you have some semblance of control over the settings. This happens most easily in one of the semi-automatic modes, often called Shutter Priority or Aperture Priority. In either case you select the setting that is most important to you, and the camera finds the best pairing based on the light that is present. If you do not want to set an ISO in advance, your camera likely supports something called Auto-ISO where the camera will follow your priority setting with what its algorithms propose as the best combination of the secondary element and ISO.
It is in this space that the incredible compute power of our modern cameras come into play.
Wrapping Up
Learning to leverage the three components of exposure is important if you want to take control of your photography. I hope that this short article is of help in taking you down that road.
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I'm Ross Chevalier, thanks for reading, watching and listening and until next time, peace.
