Lenses: An Introduction

Lesson #2 in Photography for Ignorant Geniuses

Find earlier photography lessons here.


Photo by Keith Williamson / Licensed under Creative Commons 2.0

At the very basic level, a lens is what controls and directs light onto the camera’s sensor. Lenses are usually named and defined by their focal length in millimeters. Focal length affects two major aspects of photography: magnification and composition (through field of view). Do not worry if you’re lost – the aim of this lesson is to explain how these will affect your pictures.


The Major Categories of Lenses

Wide Angle

A wide-angle lens is anything which captures more than our natural field of vision. Any lens with a focal length of under 35mm is usually considered to be a wide-angle. There are various degrees of wide-angle lenses, including ultra-wide and fish-eye lenses (which distort pictures a lot).


A normal lens is one which captures pictures approximately at a par with our (human) field of vision. When you take a photo with a normal lens it usually replicates what you see with your eyes. It is not magnified and it does not include more width than you would see naturally. Hence the name normal.


A telephoto lens magnifies the subject, but narrows field of view in the process. Anything longer than 70mm is usually considered to be telephoto.

Prime vs Zoom

A zoom lens is any lens which shoots at a range of focal lengths. A great example is the ubiquitous 18mm-55mm lens we find as standard on many digital SLRs (DSLRs). Zoom lenses make up the majority of the market because they are very practical. They give you a choice of focal lengths in one lens, allowing you to switch smoothly.

A prime lens is one which only shoots at one fixed focal length (for example 50mm). Even though this might seem like a terrible restriction, they offer a major advantage in being able to offer better image quality and a wider aperture at reasonable prices.


When marketing point-and-shoot cameras, most manufacturers describe their focal length as a magnification level (“12X Optical Zoom” for instance). This is the easiest factor to explain and is usually the most important factor for amateur photographers. The photos below show the difference between different focal lengths:

Zooming in

Did you know?

If you wondered how camera zoom levels are determined, it is very simple. Divide your maximum zoom level by your smallest. In the case of the photos above, for example, we have approximately 17.5X magnification from the first to the last photo.

There are times when a camera’s zoom is crucial for these purposes for practical reasons. Sports and nature photography are two of the major legitimate reasons for wanting to zoom in – if you’re on safari, for example, you wouldn’t want to get too close to a lion. Portraits are subjects which lend themselves very well to telephoto lenses too because the results are usually more flattering. Wanting to avoid taking a few extra steps, however, is never a valid reason.

N.B. Focal length and field of view will vary depending on sensor size. See note at end of lesson for more info.

Field of View

There is more to different focal lengths than making an object in the distance look closer. In actual fact, when we shift through the different focal lengths (for example moving from 27mm to 476mm progressively) we are not only magnifying things but also closing the field of view (from 67 degrees to 4.3 degrees respectively). It might seem like an obvious fact, but when you do have a choice you should always shoot your subject at the right focal length. If you don’t think it makes a difference, see the four pictures I took below:

Field of view

As you can see, this phenomenon directly affects the composition of a photograph and it is crucial to understand that when taking a photo (in an ideal situation) we need to choose the focal length we want to shoot at based on composition, not on avoiding having to take twenty steps.

Other Effects

There are many other differences that arise from changing focal lengths:

  • Distortion: Wide angle lenses tend to distort images, which sometimes can give them an unreal look. If you look at the sky in the first photo in my first comparison you can see the clouds appear unreal for that very reason.
  • Compressing/ exaggerating distances: This can be seen greatly in the second comparison. If you only saw the first photo you’d think the bottle and vase are farther away than they actually are. The last one makes them appear closer.
  • Depth of field: It is much harder to achieve a shallow depth of field with a wide angle lens. A shallow depth of field is what you need to throw foregrounds or backgrounds out of focus while keeping the subject of your photo in focus.

Practical aspects

This lesson serves more as a base for future subjects than anything else. Learning about lenses alone is not going to give you a lot of immediate practical use on its own, but I would like to point out a couple of practical tips you can take immediately from this lesson if they haven’t appeared to be obvious enough by now.

A wide angle lens distorts dimensions in the most unflattering way possible, so never shoot portraits with wide-angle lenses unless you intend shooting a caricature.

A telephoto lens narrowing the field of view has many advantages. As we saw in my second example, if you’re shooting a subject in a location you can’t control, the narrow field of vision helps you avoid any clutter in the background.


There isn’t much specific coursework to be given out this week, however it would help you greatly if you could try shooting the same subject at different focal lengths. First try zooming in without physically moving and then try replicating my second comparison. Once you see it yourself you will see what a big difference focal length makes to the composition of your pictures.

That’s all for this week. I’ll be back with more next weekend. If anything is unclear please do not hesitate to get in touch with me to ask for further explanations – I’ll be glad to help out in any way possible – at the end of the day any problem you might have will probably be shared by other people following these lessons.


Photography for Ignorant Geniuses is a set of tutorials I give out on a weekly basis about the science behind photography. For more information (and to subscribe) click here.

(#29 of my 366 X 2012 project)


Note: Focal length on different sensor sizes

Even though focal length of a lens does not change from one camera to the next, the effect that it has on a sensor (or film) will vary depending on the sensor’s size. Unlike the days of film, when the majority of cameras used a standard film size, camera manufacturers, have been creating sensors in every shape and size, so in order to keep things simple they just kept referring to their lens focal lengths in a “35mm equivalent” (referring to a 35mm film/ sensor, NOT a 35mm lens), since this was a standard that was easy for everyone to understand.

Going into the mathematics behind this will take me a whole lesson in itself, and most photographers simply avoid the matter completely because:

  1. it does not affect their photography in any way and
  2. there is a simple work-around to convert the numbers to a standard.

Since 35mm was the standard film size for a very long time, most people use the way focal length worked on that system as the reference point. If you look at Canon’s advert for one of their point and shoot cameras:

Whereas if you look to the specs page you can see that the actual focal length is much smaller, however it is simply marketed at its 35mm equivalent:

This is not as straightforward with DSLRs or EVIL systems, where the actual focal length of a lens is quoted. In this case you need to keep the “multiplier” in mind. This is simple enough to understand.

In my Canon 50D, for example, the sensor is 1.6 times smaller than a 35mm film, so I have to multiply my focal length by 1.6 to arrive at the 35mm equivalent. This means that if I’m shooting at 17mm (actual focal length), it is really the equivalent of shooting at around 27mm. How did I find the multiplier? Well, Google’s your best friend ;)

3 thoughts on “Lenses: An Introduction”

  1. Hi
    Thanks for the info on lenses I am following you on G+
    I have just got Canon EOS 7. I worked out that I need to x lens f by 1.6
    and it is my first step into digital being into film for a long time.
    canon AE1 with various prime lens, so I should know the question to this.
    looking to get a reasonable priced portrait lens and also a long 200 mm but not sure which to go for Prime or telephoto any suggestion or push in the right direction would be great.

    1. Hi Nick,

      I personally have a Canon 85mm f1.8 prime lens for portraits. It is a great length (works out to the equivalent of a 135mm lens) and I don’t thing I’d change it for anything.

      I then have a basic zoom telephoto (70-300mm, Sigma) because my telephoto needs are not as great. I find the zoom lens practical when I absolutely need it, but at the same time it is not as bulky (or expensive) as any decent zoom with good glass would be.

      I hope this helps, but if there is anything more specific that would help you just fire away :)


  2. Hi Richard
    Thanks for the update on lenses think I was looking at something similar and its good to know get advise / suggestions. I will most probable go for the 85mm f1.8 prime. although I do have a 28-135 – f3.5 – f5.6, but the f stop is not as good as f1.8
    I will have a look at the price difference / specification between the Sigma and canon and let you know which one I get
    Thanks again.

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