The Aperture

Lenses have controls that allow us to change the focus distance, and the width (magnification level). There is one more important attribute of lens design that we have control over - The Aperture.

The Aperture is the opening of the lens. It’s a circle that we make larger or smaller. It’s not a perfect circle - it’s a series of blades that open and close to form a rough circle. Before somebody invented aperture blades, the aperture was adjusted by sliding in and out different pieces of metal that had different size holes cut in them.

[[Image of an Aperture at different openings]]

The Aperture is pretty interesting. It get’s smaller and blocks some rays of light from hitting the image sensor. Yes, as this happenes, the image that is formed on the sensor has a larger and larger depth of field.

  • The Aperture gets literally smaller, the f-stop number gets bigger, Less light in, more things in focus.

  • The Aperture gets literally larger, the f-stop number gets smaller, more light gets in, less things in focus.[[ED: Placement of these? Flow of this section...]]

An image sensor is flat, but the amount of in-focus light that hits it comes from a 3 dimensional shape. The width of the lens determines how much light side-to-side, and up-and-down get through, while the aperture determines how much light front-and-back get through. [[ED: remove this?]]

[[Diagram of a camera, with a 3 dimensional shape of ‘what’s in focus’ cut out, to demonstrate.]]

When I refer to light, I am referring to rays of light that will reach the image sensor in focus.

Notice one picture, where only part of a bee is in focus, while in another focus, an entire street into the distance is in focus. This is the depth of field, and the less that is in focus, the smaller the aperture.

I’d be remiss if I didn’t at least mention that aperture isn’t the ONLY thing that affects depth of field - different lenses, and the width also play a factor, but let’s not worry about this right now.

As photographers, we can change the Depth Of Field by adjusting the Aperture. Neat! It gives us control over the exposure (how much light) and the depth of field.

Lenses can only have an aperture that is so large, and lenses that are capable of really ‘open’ apertures - ones that let in a lot of light and have extremely shallow depth of fields - are more expensive than regular lenses.

Units Of Measurement

You are going to see a lot of abbreviations when looking at lenses. Know that we have at least a glimmering idea of the most important lens attributes - the focal distance, the width, and the aperture

Focal Distance - Feet or Meters

The easiest unit is the focal distance. This is measured in units of distance - usually feet or meters. Many lenses have markings along them that indicate what the current focus distance is.

[[photo of a lens with this feature highlighted]]

Width - Millimeters

The Width of a lens is measured in mm. A prime lens will be listed as, say, “50mm prime” or just “50mm” while zoom lenses will use two numbers between their widest and narrowest possible settings, with a dash between them. “18-55mm” or “24-120mm” for example.

Aperture - F-Stop.

For the sake of clarity, I am going to ignore the apertures affect on Depth Of Field - we will get to it later, once you have a camera in your hands, and can see the results of photos.

An F-stop is a measurement of quantity of light. But unlike most other units, it is relative. An f-stop’s value is often written with an f then a slash, than a number. Like ƒ/8, ƒ/11, ƒ/20, or ƒ/2.4, or ƒ/1.8.

Actually, when written about the aperture, it’s an f with a little fancy descending hook: ƒ. Nobody will fault you for just typing f. When talking about f-stops, it’s just a regular f. Think of this way: It’s the regular f except for the abbreviated aperture nomenclature.

We use f-stop in terms of “twice as much” or “half as much”. Each major stop is twice as much. ƒ/1.0, ƒ/1.4, ƒ/2.0, ƒ/2.8, ƒ/4.0, ƒ/5.6, ƒ/8.0, ƒ/11.0, ƒ/16, ƒ/22.

WAIT… Those numbers aren’t twice or half as big as each other! What gives? First, square all of those numbers. That means multiply them by themselves.

Yep, now it works out. 1.4 times 1.4 is about 2, and about 2 is about twice as much as 14. The f-stop is proportional to the square root of light that gets through. Uhg. Confusing. Good thing this isn’t really important to you, as a photographer. You won’t need to square or ‘sqrt’ any numbers in your head. You won’t need a calculator, it’s just useful to know how the scale works.

Modern cameras use a scale where you can increase or decrease f-stops by a third. The scale that we move around on is now the following: 1.0, 1.1, 1.2, 1.4, 1.6, 1.8, 2.0, 2.2, 2.5, 2.8, 3.2, 3.5, 4.0, 4.5, 5.0, 5.6, 6.3, 7.1, 8.0, 9.0, 10.0, 11.0, 13.0, 14.0, 16.0, 18.0, 20.0, 22.0. Don’t memorize that.

Many lenses go higher than ƒ/22. ƒ/22 is a pretty standard upper limit on many consumer level lenses. The mechanics that control the aperture blades can only be adjusted so precisely, and more precise control is an engineering challenge naturally more expensive. Lens designers are usually competing to see who can make the most open aperture, not the most closed. I’ve yet to ever use a lens for a digital camera with an upper limit higher than ƒ/32. [[See the f/64 section in the ‘Styles/Philosophies’ section]] .

Fast And Slow

You may hear talk of ‘fast’ lenses. A fast lens lets in more light than a slow lens. Fast lenses are better at low-light photography, and at the very least they give photographers more options. Thus, they are better.

Yeah, it’s a stupid analogy - the lens isn’t going anywhere, and light travels at a constant speed - but if you learn one thing during this book, it’s that photographers aren’t great at analogies.

Lenses with minimum f-stops of f/2.4 or lower are usually considered ‘fast’.

Close It Down

Because we keep halving light, we can never reach a value of no light getting through. Not even at f/abillionkajillion. Even f/abillionkajillion would theoretically let in some uselessly small quantity of light.2

That’s one of the reasons why we use this scale, instead of inverting it. We can always have bigger numbers5

As f-stops get larger and larger, less and less light is getting to the image sensor. We are making the aperture smaller and smaller.

The smaller the f-stop, the more light is getting through. The larger the f-stop, the less light is getting through.

Closing down an aperture has it’s own image quality drawbacks due to light diffraction. This gives us a practical upper bound. In fact, the maximum f-stop is rarely used for this reason. Many photographers hate going above f/11. Read why in [[link to light diffraction section]].

Open It Up

An aperture hole can only get so open - eventually it would be larger than the lens! You might expect a value of f/1 to be a limit, but there are lenses with values of f/0.7, f/0.9 and other absurd limits. These lenses are very expensive and use lots of clever optical engineering to achieve this. The size of the back of the lens - the image sensor - does not affect how open an aperture can get.

As you make the hole of an aperture smaller, the f-stop unit number we use gets larger. And as you make the aperture more open (the hole is bigger), the f-stop unit get’s smaller.

If we are looking for a lens with the largest aperture, that can let in the largest amount of light, we are looking for the smallest f-stop.

Sometimes, the width matters.

Due to the way lenses are designed, sometimes the minimum f-stop (open aperture) is different at different lens widths (zoomed in or zoomed out).

[[photo of old lens with different markers]]

2 Hypothetically we could get to a point where the hole is smaller than individual molecules, atoms, or even photons, but let’s not bother thinking about that.3
3 I mean, you’re already confused as it is.
4 1 squared (1 times 1) is 1. In case you had forgotten.
5 In college I took a mathematics course where I once had to prove that there are an infinite number of numbers on a test. This is bringing back some bad memories. It was one of the easier proofs I had to do in that course, but still I shudder at the thought. I never studied enough and I barely passed the course.6
6 I, of course, mean that math is awesome and everybody should study it and do their homework and always pay attention during class.7
7 But in all seriousness, mathematics is really cool once you get past all of the lame arithmetic and memorization that many schools force students through. If you happen to be trudging through arithmetic and algebra and memorizing formulas and math feels like the worst thing on the planet, fear not. It gets better! That math course I mentioned a few footnotes ago? I don’t think we used a number higher than 10. What was I talking about? Oh, right. Photography!

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