Lens

“The camera is an instrument that teaches people how to see without a camera.” - Dorothea Lange.

Before thinking about complicated lenses with 11 different pieces of precisely curved glass that rotate and move and focus and zoom in and out… think of a magnifying glass.

[[Image of Magnifying Glass]]

Magnifying glasses (a simple lens) have a focal distance and a magnification level. The focal distance is how far away something needs to be from one (either) side of the glass in order to appear in focus (A magnifying glass is symmetrical, so let’s assume your eye is one of the “something’s”).

The Magnification Level is called the Focal Length. This will be confusing, so I am going to try to avoid this term.

[[Side Diagram pf magnifying glass, with 3 ray lines drawn to focal point]]

Lenses contain multiple pieces of glass that the light has to travel through. As a whole, all the pieces of glass operate as one big magnifying glass. The difference is that they are not symmetrical. One side (the image sensor or eye) can be much closer than the other side (the subject).

Lenses have limits on the focal distance. Some lenses, called “macro” lenses, are designed to be able to focus on things very close up - a few inches or less! Other lenses, most that we will use, are designed to focus on things as close as a few feet, to as far as infinity.

[[Cutaway of a Lens]]

Infinity? This is where the lines that all focus on the image sensor are parallel before they hit the lens. Parallel lines never touch; they can be thought of as ‘touching’ at infinity.

[[Diagram of infinite focus ray lines]].

By varying the distance that these pieces of glass are to each other, and that they are to the image sensor, one can change various properties of the lens - like it’s focal distance, or it’s magnification level.

The magnification level is the zoom level. Some lenses (usually bigger ones) can zoom in, and capture detailed images of objects that are very far away. When you zoom a lens in, all you are changing is the width of the field of view.

Magnification level is separate from the focus distance - but if you are zoomed in really far, let’s hope your focus distance is that far as well. Wide lenses can see things close to them, while zoom lenses ‘zoom in’ to things far away. This is measured in millimeters. The lower the ‘width’, the more the lens “see’s” - a wider Field Of View. Technically, it’s the distance from the lenses optical middle point to the image sensor. This can sort of make sense, if you think about how lenses extend when you zoom in - they get farther from the image sensor. But you don’t need to know about how any of the optics works. Most photographers don’t, and even the most hard core gear-head photographers would be hard presses to design their own lens from scratch. Let’s leave these tasks to optical physicists and engineers.

All you really need to understand is that low numbers, anything below 40mm is a wide angle lens, from 40-60mm are considered ‘normal’, and anything above 60 or 70 is considered a zoom width.

The mm unit refers to the Focal Length AKA the magnification level AKA how zoomed in you can get.

The Kit Lens

If you are purchasing a new DLSR the lens that gets shipped with the camera is called the Kit Lens. It is usually a get-the-job-done lens that is not great at any one task, but versitle enough to be used in many sitations. A common kit lens is a f/3.5 18-55mm lens. Keep reading on to learn what those numbers mean!

Ultrawide Lenses

Some lenses are really really wide. These are called ‘ultra-wide’ and are anything below 20mm or so. One of the widest lenses ever made was a nikon fisheye 6mm lens. It had a 220 degree field of view! It actually looked behind itself! This lens weighed more than 11 pounds!

Technically a 0mm lens might be possible, this is where the optical center of the lens would be touching the image sensor. For all photographic purposes, this is impossible. Although, to be fair, I don’t think anybody has bothered trying. It would also be useless, with a field of view so wide it would be photographing the image sensor!

Fisheye Lenses

A fisheye lens is an ultra wide lens that is not rectilinear. In layman terms, it distorts the image - straight lines don’t appear straight.

[[Photo from a fisheye lens side by side with a photo of a fisheye lens.]]

Fisheye lenses are pretty awesome. You’ve probably seen their work if you have ever seen skateboard videos from the 90’s.

To Zoom Or Not To Zoom

Some lenses can’t zoom in or out. These lenses are fixed at a single width. These are called “prime” lenses. Lenses that can zoom in or out are called “zoom” lenses (Clever, huh?). More on prime lenses and their advantages later. [link to that section]

Most lenses are capable of, zooming in and out to different widths, and any lens (that isn’t broken) is capable of focusing on different points. The exception to this pinhole cameras, but a small hole in some cardboard is hardly the pinnacle of lens design.

[[Top down diagram of 3 different lenses, with the field of view drawn on them]].

Lenses are designed as barrels, since the pieces of glass are round.

Why are Lenses Round, and Images Rectangular?

Lenses are round - and they generally1 don’t care which way is up. A circle of light is going through the lens to the image sensor, which only pays attention to a rectangular part in the center. The image sensor might care which way is up.

Film negatives are rectangular because images are rectangular. Images are rectangular because canvas is rectangular. Prints are made out of paper, and it’s easy to cut paper along right angles. It is possible to have round prints. Think about old portraits that have heavy vignetting - those dark corners come from the circular design of the lens.

[[photo of such a portrait]]

Image sensors are square because they are designed to emulate film. But also, because pixels and screens are square! Every round image you see on an LCD screen is made up of a bunch of small squares. Further, image formats store images in 2 dimensional arrays (like a grid, or a spreadsheet). These, of course, are rectangular.

Basically, design restraints. In a hypothetical world, it would be possible to make a lens rectangular - like an eyeglass maker shapes lenses to be more-or-less rectangular. It would be a whole lot of trouble for no benefit. There is no practical reason to make square camera lenses!

Let’s not forget that our controls on the lens, the focus and zoom rings, will only work on barrel shaped lenses. They can rotate different pieces of glass and - like a screw - move it forward and backward precisely. This clever bit of engineering wouldn’t work with rectangular lenses! Why fix what ain’t broken?

If somebody ever asks you why lenses are circular and not rectangular, The correct answer is “Just Because” or “Don’t worry about it”.

1 The exceptions being: Tilt-shift lenses, lenses with abnormal aperture shapes, and old lenses that have been turned into coffee mugs.

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. Yet it doesn’t make cookie cutter shapes of light on our image sensor. It blocks some of the light that is hitting the sensor in focus, and as it gets smaller, the image stays the same size but the depth of field gets smaller.

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.

[[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]]

Deciphering The Jargon

When looking at lenses in stores, online, or while peering over the shoulder at a rival photographer; it’s important to know how to read how we describe lenses.

Lenses are listed with their most open f-stop (most open possible aperture, most amount of light getting through). So a “f/1.8 50mm lens” is a 50mm prime lens with it’s fastest f-stop at f/1.8.

Sometimes, particularly when talking about a lenses most open f-stop, the notation 1:X will be used instead of f/X.

An f/2.8 18-55mm lens is a lens with a minimum f-stop of 2.8, which is widest at 18mm, but can be zoomed in at 55mm.

Because lens width’s sometimes affect the minimum f-stop, this needs to be indicated. It is usually noted with a dash between the two minimum f-stops.

An f/3.5-5.4 24-120mm lens is a lens that can be zoomed between 24mm and 120mm. It has a minimum aperture of f/3.5 at 24mm, and a minimum aperture of f/5.4 at 120mm.

Many times, the “f/“ and the “mm” get ignored. I have a Sony lens that simply says “2.8/16”. It is assumed that the unit with the decimal is the minimum f-stop, and thus the other one is the lens width.

If you see a lens listed with “TF”, that means it is a Tile-Shift lens. [[See this section on tilt-shift lenses.]]

If you see a lens listed with “AF”, that means it has autofocus features - the lens has a motor built in, and electrical contacts with the camera body. The determines when to change the focus, and adjusts the lens automatically.

Other letters, like 'PZ', 'EF', 'EFS', 'F', 'MC' and 'L', and countless more are brand-specific markings to identify the lens - things like the lens mount type, the series of lenses, or specialty features of lenses. You will be okay if you can read the minimum aperture, and the focal length range. To cover all possible lens markings would be a waste of all of our time!

[[diagram of scale of f-stops]]

[[So many diagrams during this part. Make it all work.]]

Down To Business

Don’t worry if all of these concepts are a little foggy. F-stops in particular will take some practice - you know, while holding camera - to get used to.

In other words, I know you might be confused! Relax! I just threw a whole lot at you, so just relax. Breath. Take a few photos at different apertures [[link to section on aperture priority mode]]. Let’s review the important things to remember.

Lenses take light that is bouncing around (as light does), and direct some of it at as an image on an image sensor. The image sensor reads this light and ’takes’ a photograph. They image that is projected onto the sensor can be zoomed in or out by changing the width of the lens, called focal Length. This is usually done by twisting a control around the barrel of the lens. Some lenses have electronic controls, and a motor to zoom in and out for you. This focal length is measured in millimeters (mm) for legitimate reasons that are legitimate, don’t worry about why.

Zooming out means a lower focal length (small mm), and zooming in is a higher mm distance (big mm)

The part of the image that is in focus can be adjusted by adjusting the focus ring on the lens. Most lenses have the focus ring farther forward than the zoom ring, control wise. Twisting this will change the distance away from the lens that objects are in focus. The distance is measured in units of distance (duh). Geet or meters, usually. This distance is the middle point of what is in focus, and the range of distances of what things are in focus is called the “depth of field”.

How much light that gets to the image sensor can be adjusted by changing the aperture of the lens. This is a ring that shrinks and grows. Measured in f-stops, we adjust the ‘f-stop’ in order to change the size of the aperture. This also affects the depth of field.

That Wasn’t Too hard!

Feel free to ignore and forget anything about optics or lens design or square roots. I mention these complicated (and unnecessary) things because when I was learning photography, I was frustrated about how my resources didn’t mention anything about them!

Understanding the design constraints that lenses have won’t make you a better photographer, but it will make you a smarter one. When hunting for lenses on eBay, it’s nice to know why a lens is or isn’t better. Sure, knowing the lowest f-stop and range of width is nice, but what about the coating of the glass? The number of aperture blades? How about how fast the motor can focus an image, or even what type of motor is used? Which lenses are worth their weight, and which are plastic pieces of crap? Or what about a plastic piece of crap that takes really really beautiful images? (I’m looking at you, Holga).

I have covered lenses to a more advanced degree than most introductory guides - but there still is a lot I left out. I encourage you to continue researching.

Lenses are incredibly advanced, sophisticated pieces of technology, and I believe it is a passive benefit to at least learn some of how they work.

The lens is arguable the most influential piece of gear - it will affect how your photos appear photos the most. (Although light sources make a close contender).

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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