Back to the Basics: Aperture / F-stop

Exposure is the capturing of an image on film or digital sensor through the use of light. Three factors go into making an exposure: Aperture, Shutter Speed and ISO. Changing one of those factors will effect your exposure AND one or both of the other two factors.

Let’s take this one at a time. I’m going to spread this out over a few posts. First, we’ll talk about Aperture.

Aperture

The Aperture controls the amount of light that hits the digital sensor / film. Let’s think of this in terms of the human eye. Imagine the aperture is your pupil. And the digital sensor / film is your retina.

Aperture = Pupil
Sensor / Film = Retina

In the simplest terms, the pupil is a hole that allows light to enter your eye. When it’s dark, your pupil dilates (the hole gets larger) to allow more light to hit your retina. When it’s bright, your pupil constricts (the hole gets smaller) to allow less light to hit your retina. In photography, the hole is the aperture, and it’s measured in f-stops.

In photography . . .
When shooting in lower light, you need a larger aperture setting on your camera (larger hole). When shooting in brighter light, you need a smaller aperture setting (smaller hole). Changing the size of the hole controls the amount of light that hits the sensor / film (retina).

Make sense so far? Good. It’s about to get more complicated.

Aperture is measured by f-stops. You may have seen them written this way: f/2.8, f/4, f/5.6, f/8, f/11 f/16, etc

The LCD screen of a digital SLR shows the current f-stop (f/22).

The f-stop markings on a manual lens from f/22 through f/2.8 (along the bottom).

Here’s where its gets crazy: the smaller the f-stop, the larger the hole opening and the more light reaching your sensor / film.

That’s not a typo. I’m not messing with you. I know, it’s wiggy and it sounds backwards, but this is the way it is.

Take a moment to study the diagram because it’s about to get even weirder.

From one stop to the next, the light is doubled or halved. f/11 allows twice the amount of light than f/16. f/22 allows half the amount of light as f/16. This is called the Factor of Two. More on that in a later post.

Let’s re-cap what we know so far:

- The measurement of the amount of light entering your camera lens is the f-stop.
- The smaller the f-stop, the larger the opening.
- A 1-stop difference is the equivalent of half or double the amount of light.

With me so far? Then lets talk some more lingo . . .

Stopping Down
Stopping down refers to choosing a smaller aperture (larger number / smaller opening / less light). For example, changing from f/11 to f/16 is stopping down by 1-stop. And, as we know, that 1-stop is the equivalent of half the amount of light.

Stopping Up
The opposite of stopping down, Stopping Up is moving to a larger aperture (smaller number / larger opening / more light). For example, stop up from f/8 to f/5.6.

True Aperture
True aperture refers to full stops in photography. Many lenses and cameras have 1/2 stops and 1/3 stops.

True Aperture: f/1, f/1.4, f/2, f/2.8, f/4. f/5.6, f/8, f/11, f/16, f/22, f/32, f/45, f/64, f/90
1/3 stops: f/2.8, f/3.2, f/3.5, f/4.0, f/4.5, f/5.0, f/5.6, f/6.3, f/7.1, f/8, f/9, f/10, f/11, f/13, f/14, f/16, f/18, f/20, f/22, etc

Your camera or lens may not have 1/3 stops.

Fast Lens
Your lens also may not have f/1. It may only open as wide as f/5.6. The wider the aperture opening, the faster the lens. A lens open to f/2.8 lets in 4x more light than a lens open to f/5.6. To make an equivalent exposure with the f/5.6 lens, you need to expose the digital sensor / film 4x longer. Therefore, the f/2.8, which will capture the photo 4x faster, is the faster lens. This is where Shutter Speed comes into play and that deserves a post all to its own.

That wraps up the basics of f-stops and their effect on exposure. F-stops also control Depth of Field, which I discussed in my post Control Depth of Field with F-stops, lens selection and distance to subject.

Additional Reading:

Back to the Basics: Aperture / F-stop part 2


Control Depth of Field with F-stops, lens selection and distance to subject


Back to the Basics: Shutter Speed


Back to the Basics: ISO


Back to the Basics: Exposure Triangle