Understanding Aperture

Wait bigger is smaller? and is it wide open or not?

When I was first learning photography aperture completely boggled my mind. People were always talking about getting a “fast lens” and how Canon’s 50mm f-1.4 was a must have for any photographer. Without a clue I went out and bought the lens and started shooting. Ironically this is still my favorite lens in my kit today, nearly 8 years later.

First let’s get a couple of things straight, when people are talking about a “fast lens” they aren’t typically talking about autofocus or how quickly the lens attaches to the camera, they are typically talking about aperture. People also use the term “wide lens.” I’ve found that “wide” is less commonly used probably because it could also reference the focal length. People use the word “fast” because a lens with a wider (wide) maximum aperture is capable of achieving the same exposure as a lens at a similar focal length with a higher (faster) shutter speed.

When it comes to aperture “bigger is smaller.” Meaning that the larger the number, the smaller the aperture. When you have a smaller aperture the lens lets in less light.

The definition of “aperture” is: a hole or gap. When it comes to a lens aperture is the hole that allows light to flow through the lens and onto the sensor/film of the camera. In the early film days aperture was manually controlled on the lens. Now it is all controlled digitally through buttons and dials directly on the camera, the lens has a small chip inside that receives the message from the camera and relays that to the aperture blades which open or close to create the desired aperture within the lens.

For the camera’s autofocus system to work properly the lens moves the aperture blades out of the way leaving the lens “wide open” so that the maximum amount of light can reach the sensor to allow the autofocus system the best odds of getting it right. When you pull the shutter release button the aperture blades move into place to reach the desired aperture that was set in the camera for the duration of the exposure. After the exposure the lens goes back to wide open. DSLR’s have an exposure simulation button, typically on the front of the camera near the lens, pressing this button will put the aperture blades into place as you look through the viewfinder giving you an idea of the amount of light the camera will see during the exposure.

Aperture is the element of the process that controls depth of field. When you look at an image some of them are tack sharp from the foreground all the way to the background and some of them have a dreamy blurred out foreground or background. This effect is controlled by aperture. As you let more light in by changing the aperture (smaller number, larger hole) less of the image becomes in focus.

The reason that the images becomes sharper as you allow less light in is because when you use a lens wide open, or near wide open, a ton of light is pouring through. Lenses are not perfect so a lot of the light around the edges of the lens is just a wash and not perfectly controlled, which gives that nice blurred area around your focus point. As you make the aperture smaller (increase the number) it blocks off the edges of the lens and gives you a more focussed light which in turn creates a sharper image. Each lens does have a sweet spot however. There comes a point when you start getting softness from making the aperture too small. This varies from lens to lens and takes experimenting to figure out. Typically to get an entire frame in sharp focus I use an aperture between 11 and 16 which varies based on the focal length. The images below show various apertures and their effect on the focus of the image. All of the images were shot with a 50mm lens. As you can see in the beginning just the edge of the telephone pole is in focus. As we progress from an aperture of 1.4 to an aperture of 22 more and more of the image comes into focus and by the end of it even the electrical boxes behind the eaves pout are starting to become sharp.

So what do the numbers mean?

The F-stop is the ratio of the lens focal length to the diameter of the aperture, or pupil. One F-Stop is defined as a factor of 2 in light intensity. Therefore if you decrease your aperture by 1 stop you let in twice as much light, if you increase by 1 stop you allow half as much light. In practice an aperture of 1.4 allows in twice as much light as an aperture of 2. The most common whole stops in photography are: 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22 as demonstrated in the gallery above. It is important to note that cameras often allow the aperture to be adjusted in fractional increments of a whole stop, typically 1/3 or 1/2.

Since the aperture is proportional to focal length it is not a constant diameter, a 50mm lens will need a larger opening to achieve an aperture of 2.8 than a 24mm lens. This is the reason that some zoom lenses have a variable aperture as the lens zooms.

Thanks for reading, have a great day!


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