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A lens is made up of both clear optical-elements (glass or plastic), and light-blocking fan blades.

The optical-elements allow for light transmission and determines our viewing perspective (covered in Focal Length).

At the very rear of the lens, the fan-blades are referred to as the lens Aperture and the position of these blades (referred to as the Aperture's "f/stop" value), controls the amount of light being allowed to pass through the lens - much like a throttle controls the amount of gas getting to an engine.

A low f/stop value (say, f/2.8 or less) means the most light is passing through the lens on the way to the shutter-plane and the film/sensor.

At the other end of the spectrum, f/22, the light is being severely limited.

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Always remember this: In photography, we are attempting to capture light.

Sometimes we need all the light we can get, while other times there is just too much of it and we have to find a way to cut back, or our photograph will be too bright (over-exposed).

So Aperture is the throttle controlling how much fuel is getting to an engine.

Aperture can also have a powerful visual impact on a photograph.

A side-effect to Aperture is that it allows us, under the right circumstances, to control the area of focus in our photograph, something referred to as the "depth-of-field."

Aperture f/5.6

Aperture f/5.6

Here, I'm photographing a granite slab, at a distance of 10-15 feet from me.

At the lower-end of the f/stop scale (with the blades wide-open), my subject is in focus but the background is blurry. 

This is referred to as having a very limited depth-of-field, and is very common for portraits.

Hence the "portrait" mode setting on your camera.

Increasing the f/stop closes down the blades, resulting in less light passing through the lens, but at the same time, it will also open up our focusing range considerably. 

Having pretty much everything in focus is very common for landscapes, hence a camera's "landscape" mode feature which will set a high Aperture value for us.

In this example, we see how the Aperture value manipulates our depth-of-field, from being very limited so our eyes are drawn solely to the rock, to where the background becomes clearly visible and a central player in our final photograph.

That's it, that's the big mystery to Aperture.

As the person in charge, we use it to set not only the throttle, but also if we want everything in focus or not.

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"I learned from looking at his [Ansel Adams] work the places he loved the most, and where he spent the most time, was where he did his best work. I learned from him that you have to love what you photograph, and you have to give it time."

Ion Zupce

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