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Apertures & F-Stops … these words usually cause confusion in most photographers – new and old.

Its a rather complicated subject, so here is the quick and easy explanation:

1. An aperture is simply the mechanical opening in the lens that allows light to pass through the lens onto your sensor/film plane [hereafter referred to as “the sensor”].

2. The smaller the number, the bigger the aperture… the bigger the number – the smaller the aperture.

3. Each increase in aperture size allows twice the amount of light to pass through. [ f4 allows thru twice as much light as f5.6 ]

And that’s it!!!

Hooray. Easy, huh?

Well not exactly… but you could live forever without knowing the technical stuff and still take great pictures.

But for the curious amongst you… here is the technical stuff:



A device called a diaphragm controls the size of the aperture. The diaphragm functions much like the iris of the eye – it controls the effective diameter of the lens opening.

The aperture of a lens can be adjusted to control the amount of light reaching the sensor.

In combination with the shutter speed, the aperture will regulate the amount [or volume] of light that falls onto the sensor.

A fast shutter speed will require a larger aperture to ensure sufficient light exposure. [short time + big aperture]

A slow shutter speed will require a smaller aperture to avoid excessive exposure. [long time + small aperture]


A typical F-Number Scale:

2.8 / 4 / 5.6 / 8 / 11 / 16 / 22

One of the biggest causes of confusion for most new photographers is getting your head around the fact that f1.4 is bigger than f8

Bigger = Smaller.

When someone says “you need a bigger aperture” do they mean a bigger area or a larger f-number? Its always good to clarify this because it does get confusing. Generally you are best using terminology like “increase or decrease a couple of stops”.

The aperture of a lens is specified as a “f-number” [also called a “f-stop”], which is the ratio of the focal length to the aperture diameter.

However, the f-number does not relate to the actual area of the aperture itself. The f-number is simply the only constant in a rather complex formula which is used to determine the area of the aperture.

Remember earlier when I said that each increase in aperture size allows twice the amount of light through as the previous f-stop?

So you are probably thinking that would mean that f4 has twice the diameter of f5.6 … well no, you would be wrong.

The area of a circle does not double when you double its diameter. [the increase in area would be a lot more than double in that case]

It is not the diameter of the aperture that is important, but rather the area of the aperture, and for each lens it is different.

An aperture of f5.6 on a 35mm lens will have a different area to f5.6 on a 200mm lens, yet f5.6 on both lenses will allow the same amount of light to reach the sensor [assuming the same shutter speed in both cases].

So when calculating the area of the aperture you need to use the formula above, and the only thing that remains constant regardless of what focal length lens you are using is the f-number.

“But Byron, when do we ever need to calculate the area of the aperture?” I hear you ask…

Wellll… we don’t. Not anymore, but there was a time not so long ago when you did. Back in the days of manual everything cameras.

No Auto-Focus, No Auto-Exposure, NO AUTO ANYTHING. You had to set everything manually.

Can you imagine taking a photo like this:

■ You look at your scene,
■ Take your light meter from your pocket and take a reading [taking into account your film speed]
■ Get your camera and dial in the shutter speed and set your aperture [as dictated by your light meter]
■ Now you have to guess how far away your subject is and set the focus by reading the focal plane markings on the side of your lens…
■ Now look through the viewfinder [range-finder] – there’s no through-the-lens composing here folks!
Now you can press the shutter release.

If you didn’t have good maths, a good memory, and a fair deal of patience – then you took crap photos… which is what lead to the invention of the “Sunny 16 Rule”.

The Sunny 16 Rule basically said that for certain lighting conditions and film speed and shutter speed, you could set your aperture to a certain size and you should get an ok image.

If it was Sunny [like midday, no clouds] then you could set your aperture to f16 [hence “Sunny 16”]


A “Stop” is each step up or down the f-number scale.

A “Stop” is also each step up or down the shutter-speed scale.


To make everything work with the combination of aperture and shutter speed, each increase in shutter speed needs to be twice as fast as the previous shutter speed. [that’s just techie stuff and don’t ask me to explain it!]

1/500sec is twice as fast as 1/250sec…

Because of the relationship between apertures and shutter speeds each increase or decrease was referred to as a “Stop” [god knows why they picked that word. They just did, ok!]

So a “stop” relating to apertures is called an “F-Stop” [" F " being the annotation given to the “f-number”]

So when you hear someone say “you could increase shutter speed by 2-Stops” or “you could reduce the aperture by 3-Stops” you can now understand what they mean.


Confused now? Feel sorry for asking?… and this is the really really R-E-A-L-L-Y simple version of Apertures and F-Stops.

Personally, I have been taking pictures for more than 15 years, and I pretty much know my stuff, and even I get a headache thinking about this.

Look, with today’s cameras, you really don’t need to know the techie stuff, all you need to remember is this:

The smaller the number, the bigger the aperture. And each increase in aperture size allows twice the amount of light to pass through.

Did I mention that aperture size effects Depth of Field [DOF]? maybe that is something for another tutorial…






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