Understanding Aperture and Shutter Speed

When learning photography, you will hear much about the exposure triangle. The three sides of that triangle are Aperture, Shutter Speed, and ISO. But while ISO has been around for decades (in the film days, it was called ASA), the two factors that affect the actual exposure have always been aperture and shutter speed.

ISO will determine how the light that hits the digital sensor is handled, but aperture and shutter speed together control how much light. But if they both control light, why do you need both? Why not just learn one or the other? This article will help you understand aperture and shutter speed and how they relate to one another.

Photography literally means writing with light.

In digital photography, you need to be concerned with how much light reaches the sensor. Controlling that is the job of the aperture and shutter speed. The aperture is the size of the hole that light passes through, and the shutter speed is how long that hole stays open. This makes it sound simple, and it is. But, there are other factors at play.

The amount of light that gets to the sensor is measured in stops. At this point, you don’t need to know what a stop is, but you do need to understand how to use it. Each stop of light doubles or halves the amount of light hitting the sensor. You can adjust this light and the number of stops by modifying the aperture, the shutter speed, or both.

Why you need both aperture and shutter speed

So, once again, if they both control the light, why do you need both? Why not just memorize one thing and get on with it. Well, it is because both things also affect other aspects of the image. So, you need to understand your goal and what you are trying to accomplish and then understand how each of these factors will affect the outcome.

But wait, it gets more confusing because the aperture is also measured in f-stops. And to make matters worse, the f-stop number gets larger as the size of the aperture or the hole that light passes through gets smaller. So f16 is a smaller aperture than f4. How many full stops your lens has depends on several factors, not the least of which is cost. It may get no larger than f5.6 or go all the way to f2 or larger.

Lenses with maximum apertures above f4 are usually a lot more expensive. A notable exception is the ‘nifty fifty,’ a 50mm prime lens that is f1.8 and reasonably priced. Almost all lenses will get smaller, or stop down, to f22.

Aperture controls more than just the light passing through

Aperture does more than just control the amount of light that passes through the lens. It also affects the depth of field (DOF). The depth of field is how much of the total scene is in sharp focus. For a landscape image, the depth of field may be miles. For a portrait, it is probably feet or even inches.

For a flower or a macro shot, it could be a fraction of an inch. Just know that the larger the number, which means the smaller the aperture, the greater the depth of field. For landscapes, for instance, you will usually want to be at f11 or f16. Note that maximum depth of field at f22 or smaller may have a cost in total sharpness of the lens itself. Everything in photography is a trade-off.

Understanding the relationship between aperture and shutter speed

Shutter speed is how long the shutter stays open, allowing light to pass through the aperture. This is reasonably simple to understand; the longer it stays open, the more light gets through. Know that in many digital cameras, especially mirrorless models, there may not be a physical shutter.

In that case, the shutter speed will be how long there is the power to the sensor that is capturing the light. But for purposes of this discussion, it means the same thing. Just like with aperture, shutter speed affects other aspects of the image, primarily concerning motion.

Using slow shutter speeds

At a very slow shutter speed, it can affect two things. One is camera shake. If you are hand-holding your camera, a slow shutter speed will cause blurry images. How slow you can shoot handheld also has many factors, including how steady you are and if the lens or camera has vibration control.

A slow shutter speed can also blur objects moving in your image. This might be good or bad, depending on your goal. If you are shooting moving water, a slow shutter speed can give a nice cottony look to the water. But if you are trying to photograph children running across a baseball field, having them blurry is probably not ideal.

Use fast shutter speeds for crisp action shots

On the fast end of possible shutter speeds, which is measured in thousandths of a second, you will freeze motion. How fast you need to shoot to freeze motion will depend on how fast the subject is moving. Generally, you will want to start with at least 1/1000th and go from there.

But there is a huge difference between trying to freeze a puppy walking toward you and freeze an airplane flying at 100 MPH. Speaking of which, that is another example of how subjective the use of shutter speed can be. If you photograph an aircraft with propellers, you want to shoot fast enough to freeze the plane but still show blur in the rotors. Otherwise, it looks like the plane is just suspended in the air.

Understanding aperture and photography is a balancing act

So, as I said before, everything in photography is a balancing act. Your shutter speed controls motion, and your aperture controls the depth of field. The combination of the two determines how much light reaches the sensor. Finding a happy medium between all of these factors takes knowledge, practice, and skill.

Understanding light is the primary lesson in understanding photography. And understanding aperture and shutter speed is the primary lesson in understanding light. Master these two factors, and you will master your camera. Everything else will come easily once you master exposure.

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