Is this a good explanation for "What is aperture?"

I was asked to explain "aperture and f-stops" and so I decided to write and article about the subject. I am looking for feedback to determine if the article does justice to the subject. Here it is:

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Take the front lens cap and the rear lens cap off your lens and look at the base or rear of the lens in a well-lit area. Very likely you will see an opening at the base. That opening is called the APERTURE of the lens.

The APERTURE refers to the opening at the base or rear of the lens that allows light entering the front to pass through to the sensor or film.

The light passing through the aperture can be increased or decreased by changing the size of the aperture. In photographic terms this is referred to as varying the f-stop.

F-stops are numbers – actually fractions – that represent different sizes of the aperture. Common f-stops seen when using a digital SLR are:
1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22

These values are sometimes written as f1, f1.4, f2, f2.8 etc or f/2, f/4, f/5.6 etc.

It is very important to note that the “smaller” values like f/2 and f/4 represent larger openings or apertures and the “larger” values like f/16 and f/22 represent smaller apertures.

CONFUSING?

This apparent complexity or confusion is easily solved by thinking of f-stops as fractions, which they really are.

Which fraction is larger: ½ or ¼? A half (½) is larger than a quarter(¼). Likewise f/2 is larger than f/4, which is larger than f/8 and so on.

You may wonder why not make life simple by having just one aperture of a fixed size. Well, if there was just one aperture, some of the creative effects you see in photographs would not be possible. The ability to vary the size of the aperture by varying the f-stop gives photographers a creative tool to produce stunning images.

A large aperture (f/4.5) was chosen to create the blurred background in the image below. This prevents objects in the background from becoming a distraction as your eye focuses on the yellow-crowned night heron. Generally, large apertures are used to create blurred background.

Looks pretty good.
You might add that 'aperture' is merely 'an opening'. And 'f/stop' is the 'numeric fractional measure of the size of that opening'

That might help people to understand why a 'large aperture' is the same as a 'small f-number' designation for f/stop.

I dunno if this helps but I usually at a comment about human or cats eyes.

A cats iris in the dark is very wide so it can see more/allow more light in or lack there of. Vs in the daylight where it is extremely small.

You might wanna say something like "The size of the aperture, coupled with the appropriate shutter speed, make a correctly exposed photograph."

I'm not sure who your audience is supposed to be, but here are a couple of things that would make your writeup more technically accurate without getting too detailed:

tonybear007 wrote in post #11825385
Take the front lens cap and the rear lens cap off your lens and look at the base or rear of the lens in a well-lit area. Very likely you will see an opening at the base. That opening is called the APERTURE of the lens.

The APERTURE refers to the opening at the base or rear of the lens that allows light entering the front to pass through to the sensor or film.

The device which controls aperture (typically an iris diaphragm) is not at the base of the lens but somewhere in the middle of most modern camera lenses.

tonybear007 wrote in post #11825385
The light passing through the aperture can be increased or decreased by changing the size of the aperture. In photographic terms this is referred to as varying the f-stop.

F-stops are numbers – actually fractions – that represent different sizes of the aperture. Common f-stops seen when using a digital SLR are:
1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22

These values are sometimes written as f1, f1.4, f2, f2.8 etc or f/2, f/4, f/5.6 etc.

It is very important to note that the “smaller” values like f/2 and f/4 represent larger openings or apertures and the “larger” values like f/16 and f/22 represent smaller apertures.

The f-stop is a ratio of the effective diameter of the aperture to the focal length of the lens. For example, a 100mm lens set to f/4 has an approximate aperture opening of 25mm - 1/4 of 100mm.

What photographers call aperture is really focal ratio, as optics and physics defines it. I did astronomy for years, and with telescopes, and in optics in general, aperture refers to the diameter of the main optic and focal ratio is the ratio of focal length/aperture. Somehow, photographers got into the (bad) habit of referring to focal ratio as "aperture". They got away with it because the focal length of primes is a constant.

That's why the aperture scale works inversely. When photographers say they have a f/2.8 aperture, they're really saying that they have a f/2.8 focal ratio, which implies that compared to another lens of the same focal length, there is a larger objective diameter or opening to admit the light. The actual aperture (in terms of physics and optics) is the diameter of the opening at that focal ratio, usually given in millimeters.

In SkipD's correct example, the focal ratio = f/4. A photographer calls f/4 the "aperture", but it's really the focal ratio.

And also, the relationship of the f-stop to focal length might be good to add. As f/4 at 20mm does not give near the same depth of field capability as say, 400mm. Or even f/11 in a macro setting, for example. Always a function of focal length and distance to focal point at a given f-stop.

I know I could count on you brilliant people.

MANY THNAKS! Great answers, valid points and thanks for the technical pointers.

Skip's comment ("I'm not sure who your audience is supposed to be") is timely. The audience I was addressing certainly shaped the response I gave.

The "audience" for this article is a neighbor who does not want anything technical. She just wants to know enough to help take better photographs. This article actually needs a follow-up to discus shutter speed and the relationship between aperture and shutter speed in getting the right exposure.

I wanted to make the explanation as simple as possible and without the technical aspects. In another response to a similar question I wrote the following:

The different f-stops of a lens is expressed as a ratio of the focal length and the diameter of the aperture.

Let's consider a 50mm prime (fixed) lens. When the diameter of the aperture is 25mm, the f-stop is 1/2, which we get by dividing the diameter of the aperture by the focal length (25mm/50mm = 1/2).

But instead of getting into the maths of it I simply stated the 1/2 is greater than 1/4 so f/2 represents a bigger aperture than f/4 because we are dealing with fractions.

I know it can be confusing and there are probably 10 different ways to explain it depending on your audience.

Again many thanks to all who gave their input.

tonybear007: If the focal length = 50mm, and the real aperture is 25mm, the true focal ratio = 50/25 = f/2. So the photographer says he has an "aperture" of f/2. In truth, he has a focal ratio of f/2, and an aperture of 25mm.

One simplistic way somebody explained it to me was imagine a dark room with one door. As soon as you open the door it lets light in. The bigger the door, the larger the aperture and however long you hold the door open for is the shutter speed, for example you could have a small door (F11) and open it for a longer time, or a large door (F2.8) and open it for a shorter time. Best way I thought to explain it somebody who has never heard of the concept.

nebula_42 wrote in post #11830249
tonybear007: If the focal length = 50mm, and the real aperture is 25mm, the true focal ratio = 50/25 = f/2. So the photographer says he has an "aperture" of f/2. In truth, he has a focal ratio of f/2, and an aperture of 25mm.

Heh! Accurate you are!

But, of course, the "shorthand" is pretty solidly entrenched, and perpetuated by the camera and lens companies. And, good luck in changing that! Can you imagind having a conversation in which you could not refer to an "aperture of f/4" but were always constrained to referring to it by its "correct" term -- "What aperture are you shooting at?" "My aperture to focal length ratio designator is f/4, so, let's see, I'm using my 24-70 at 35mm so 35/4 is, well, hmm...let's round the 35 to 36 and 36/4 is 9mm, so hey I'm shooting at an aperture of 9mm!"...