Essentially, the camera itself is just a light-tight box with a shutter! Photography is all about capturing light on a photographic emulsion or electronic sensor. And as such the lens and film or sensor, are the most important components affecting image quality.

Lens is to change the angle of view in the image, So they can have a major impact on the resulting photograph. In this beginners guide I intend to describe the technical terms and different properties of lenses.

Primes VS Zooms

Prime (fixed) focals have the advantage of being fast (bright) and very high quality by virtue of the fact that they can be designed just for that focal length. A zoom lens allows a choice from a continuous range of focal lengths. They are useful where you require a range of focal lengths but want the convenience of a single lens, whether for weight (only one lens), always being ready to take the picture or shooting in a dusty environment and you want to keep debris entering the camera to a minimum (you also need to think about how to change films).

This all sounds great but there are drawbacks. Zooms are slower than primes (smaller minimum aperture) and can thus make hand holding and focusing (whether manual or auto) problematic. Also due to their complexity zoom lenses suffer from more abberations than primes. Lenses from the major camera makers tend to be very good. Third party zooms vary considerably. Like everything else, you tend to get what you pay for.

Zooming is more than just getting closer. It alters the focal length and affects the perspective and depth of field of the picture. Consider whether you should zoom in and use a longer focal length, or get closer and use a shorter focal length?


Focal Length

Without getting into technical definition of focal length, It is directly related to the angle of view of a lens. The shorter the focal length, the greater the angle of view. For example, and focal length of 50mm has an angle of view of about 46 degrees, whereas a 28mm lens has almost double that, around 75 degrees. On the other hand, a 100mm lens has an angle of view of around 23 degrees. If you have not already noticed then focal lengths are mentioned in mm.


Aperture

As you will know from other Beginners Guides, the choice of aperature affects the depth of field, how much is a scene is in focus. A wide aperture (a small f number) results in very little depth of field, whereas a narrow aperature (a large f number) results in lots of depth of field.

Lenses have a maximum aperture, say 1.8 or 3.5. This is the widest aperture that the lens has. The lens can use narrower apertures (to provide more depth of field), but not wider ones. This limits the minimum depth of field possible.

The longer the lens, the less depth of field is present for a given focal length and distance. So, a 50mm at f8 has more depth of field than a 100mm lens also at f8 (This is good for portraiture as little depth of field blurs any distracting background)

if you were to take a 50mm lens with a 6.25mm diameter aperture you’d have a lens set to f/8 (since 50/6.25 = 8). Generally each increase or decrease in f-stop value either doubles or halves the aperture size. Since f-stop values are relative to the focal length, each camera lens should let basically the same amount of light through at the same f-stop value regardless of focal length. (barring complex technical factors such as light loss from large numbers of elements and so on, but we won’t get into that here)

The usual f-stop range on 35mm and digital SLR camera bodies is:

1.0 1.4 2 2.8 4 5.6 8 11 16 22 32

though most camera lenses are only optically capable of a subset of that overall range.

The relationship between these values involves halving and doubling the amount of light. Going from f/2.8 to f/4, for example, involves a halving of the aperture size. Each number is approximately 1.4x more than its previous stop since 1.4 is the square root of 2 (to one decimal place), though since the specific numbers derive from tradition they are not always spot on. Lenses for larger camera systems such as large format cameras usually have even smaller apertures - going to f/64, for example.

This series of numbers may look difficult to work with, but in fact there’s a fairly simple way to recall it. Just remember that the first two values are 1.0 and 1.4 respectively. Each following value then doubles by every other value. So 1.0 becomes 2, then 4, then 8 and then 16. 1.4 becomes 2.8, then 5.6, 11 and 22. (the only minor glitch, of course, to this handy mnemonic scheme is between 5.6 and 11)

Confusingly enough, when the number is small (eg: f/2.8) then the lens diaphragm is open wider (“opened up”) and thus more light enters the lens. If the number is large (eg: f/22) then the lens diaphragm is closed smaller (“stopped down” or “closed down”) and thus less light enters the lens. In addition to altering exposure times, the aperture setting also affects depth of field.

The letter f is frequently italicized for good looks, and a slash is often placed between the letter f and the numerical f stop value to indicate that the f-stop value is a fraction of the focal length. eg: f/4 means that the aperture is a quarter of the focal length. The letter f stands for “focal,” “factor” or “focal length” depending on who you talk to, and the number is also often stated as a ratio. (eg: 1:2.8)


Shutter Speed

Due to the relationship between aperture and shutter speed, a wide aperture requires a higher shutter speed. This means that the wider a lenses maximum aperture the easier it is to work in low light. For example, say at f1.4 the correct shutter speed is 1/60. A lens with a maximum aperture of f4.5 requires a shutter speed of 1/15, low enough to induce camera shake.

For this reason, an f1.4 lens is considered "faster" than an f4.5 lens.

As a general rule of thumb, using a shutter speed slower than 1 / [focal length of the lens] without a tripod can cause camera shake and result in blurred shots. So using a 50mm lens at 1/60 is OK, but you need a tripod for 1/15.



What determines price?

If you are looking at lenses and their prices for the first time, you may be quite bewildered at the pricing scheme. It may be counter intuitive at first to see that the lenses with the widest ranges (the highest "optical zoom") tend to be the cheapest while the ones with the most restrictive ranges tend to be the most expensive.

But once you understand how several key factors play into the final cost of a lens, you’ll understand why certain lenses can cost thousands while seemingly similar ones can cost only hundreds.

There are two primary factors that determine the price of a lens. These are focal length and aperture.

Focal Length

The cheapest lenses are the ones that are easiest to make. The easiest lenses to make are those that are close to the 50mm mark*. The farther the focal length deviates from 50mm (percentage-wise), the more costly it will be. It is much trickier to design a 10mm or a 500mm lens than 50mm lens.

Moreover, we must take into account the fact that most digital SLR's use sensors whose area is smaller than that of 35mm film. This is where the crop factor originates and is why people commonly say to multiply a lenses focal length by 1.5x or 1.6x to get its true length.

* Precisely speaking, the "normal" focal length is defined as the length of the diagonal dimension of the medium. For film this is approximately 43.2mm (24mm x 36mm). Calibrating this to 50mm is a simplification, so that we can make our calculations easier.

(Comparison between a 50mm to a 1200mm lens)

Aperture

The bigger factor into cost is how large the aperture is. The larger the aperture is (indicated by a small f-number), the more costly a lens is. This is why two lenses with the same focal lengths can differ by $1000.

There are other features that factor into the final cost such as build quality and the inclusion of ED (extra low dispersion) elements for reducing Chromatic Abberation (abbreviated CA and known as "purple fringing").


How big an aperture do I need? Do I need IS?

Bigger apertures are better, but they cost you a lot of extra money. Why do you need a bigger aperture anyways? There are several reasons for wanting a big aperture.

Bigger apertures are most commonly used in situations where there’s poor lighting and if you cannot use flash. To take a picture, the camera will either need a bigger aperture or it will need to make the shutter speed slower to expose the shot properly. Using a larger ISO value (1600, 3200) will help too, but going too high may sometimes introduce unwanted noise into the picture. If you are handholding the shot in this situation, the need for a bigger aperture is that much more important because a slower shutter speed will lead to camera shake and blurriness.

You will also need a bigger aperture when you want to blur the background in a shot. When you use a larger aperture, the depth of field becomes shallow, placing only closer things (to the focus point) in focus, leaving the rest out of focus, or blurred. Conversely, when you use a smaller aperture, the depth of field becomes deeper, making a larger part of the image in focus.

Diaphragm Blades and "Bokeh"

The other major factor when it comes to blurring the background is how many diaphragm blades there are. This affects the the "bokeh" (a Japanese word which literally means "blur"), describing how well a lens handles out of focus areas of an image. Having more blades (7-9) is better than having fewer (5). Bad bokeh can be characterized by the presence of distinct geometric shapes (like pentagons or hexagons) instead of smooth circles when certain elements are blurred out.

Do I need IS?

Image Stabilization (IS) is a technology that can reduce the need for a larger aperture if you are handholding shots in low light. In layman's terms, it is a "virtual tripod." In situations where you cannot bring along a tripod, IS can make a difference between a photo that's a keeper and one that heads to the trash bin.

In practice, IS will gain you about 2 full stops (3-4 for newer generation lenses), but your mileage will vary depending on how advanced the technology in the lens is and technique. To put things into context, I've heard of people who were able to handhold shots at shutter speeds of 1/4 sec!


(Right - IS off, Left - IS on)

Beware that IS does NOT help when the subject moves. While IS may still help you handhold the shot, you will still need a fast lens to obtain a faster shutter speed that will freeze the action.

Like everything else, IS is a useful tool but is not a substitute for skill. You still need to hold your camera still and use proper technique.


What are all these numbers printed on my lens?

Lenses have bits of text printed on the barrel or around the front element which convey a lot of important information about their properties. Be really careful when shopping for lenses. Two lenses may have very similar-looking names, but may actually be completely different one from the other.

Here are two examples. Note that all the technical terms used here are described later on in this FAQ.

CANON LENS EF 28-80mm 1:3.5-5.6. Ø58mm.

EF means that the lens is of the Canon EF type. EF lenses fit Canon EOS cameras and virtually no others.

28-80mm refers to the focal length of the lens. In this case there are two values since the lens is a zoom lens which can go from 28mm at its widest to 80mm at its longest. These numeric values in millimetres essentially indicate the coverage area of the lens.

1:3.5-5.6 refers to the widest aperture of which the lens is capable. The 1: is there since f/stops are, technically speaking, ratios. Since the lens is a zoom there are two aperture values - f/3.5 and f/5.6. This particular lens is an inexpensive amateur lens which can be opened up to f/3.5 at the widest end (28mm) but only f/5.6 at the longest end (80mm). This means it’s a fairly slow lens - it can’t let in much light, even when its aperture diaphragm is fully open.

Note that the autofocus motor type is not indicated, which means that the lens uses either an AFD (arc-form drive) or micromotor drive. Both types are slow and noisy compared to ultrasonic (USM) motors.

Ø58mm refers to the filter ring diameter. In other words, screw-on filters 58mm in diameter will fit this lens.

CANON LENS EF 200mm 1:2.8L II USM. Ø72mm.

EF means that the lens is a Canon EF lens for EOS cameras.

200mm refers to the focal length. Since only one value is indicated, this lens is a prime lens (ie: a lens with a fixed focal length).

1:2.8 refers to the widest aperture of which the lens is capable. f/2.8 is reasonably wide, especially for a 200mm telephoto lens, and so this is considered a fast lens.

L indicates that the lens is an L or “luxury” series Canon lens. Such lenses are generally the best that Canon manufacture. They are marked with a characteristic red stripe around the end of the lens barrel.

II indicates that this is the second version of this lens with these particular numerical specifications that Canon have made.

USM refers to the autofocus motor type used by the lens. In this case it uses an ultrasonic motor - see below. Non-L USM lenses are marked with a characteristic gold stripe around the end of the lens barrel. L lenses usually have USM motors, but have a red stripe instead (ie: the red stripe takes priority and you don’t see lenses with two stripes).

Ø72mm refers to the filter ring diameter. In other words, screw-on filters 72mm in diameter will fit this lens, making it a fairly large lens.



I want to take photos of wild birds. What lens do I need?

You probably won’t want to hear this answer, but nature photography of small and fast-moving wild animals is a difficult field and basically requires really expensive lenses. 500mm and 600mm lenses are commonly used by bird photographers - your typical 100-300mm zoom lens is just not long enough for great photos. And lenses longer than 300mm are both incredibly expensive and really heavy.

So the harsh reality is while you may be able to get nice snapshots of fairly tame birds with a 100-300mm lens you won’t be able to get those amazing wildlife book or calendar shots - small birds filling the entire frame - with one. You can always use your lens at 300mm and then crop off the edges of the picture, but then picture quality will suffer.

I want to do sports photography. What lens do I need?

Unfortunately, this answer is going to be somewhat like the previous one. The challenges of sports and other action photography are twofold. First, by its very nature, sports photography tends to involve rapid motion - fast-moving players or cars or whatever. Second, usually there’s some distance between the action and the camera.

Solving the first problem requires lenses which can let in plenty of light, the use of flash or faster film or high ISO settings on a digital camera. Each of these solutions has drawbacks, however. Fast lenses are large, heavy and expensive. Fast film or high ISO settings result in higher grain or noise and thus lower picture quality. And flash may be inadequate to illuminate the subject effectively, particularly if the subject is some distance away.

Solving the second problem basically requires the use of long telephoto lenses. However, most affordable autofocus telephoto lenses are very slow - they don’t let in much light. So this amplifies the first problem.

Now obviously there are some cases where these two issues aren’t a massive problem. For example, perhaps you’re shooting a basketball game and you’re in the front row. Basketball courts are of a modest size and so you could probably do okay with flash (assuming you’re allowed to use flash - some places won’t let you as it can temporarily blind or distract the players) and you won’t need an incredibly long lens accordingly. Such a situation is a little less challenging than shooting hockey on a big, poorly lit, rink.

Nonetheless, pro photographers rely on fast lenses, and this is the primary stumbling block for amateur photographers on a budget. Fast telephoto lenses, especially fast telephoto zooms, are really expensive. And there’s not much you can do to work around that fact without a lot of compromises. To cover these points further:

Fast lens. Get the fastest (largest aperture) lens you can afford. A 70-200 2.8 lens is great for basketball, for example. A 75-300 4-5.6 is probably not, since even shooting wide open means you’ll have slow shutter speeds, which will result in unwanted motion blur.

Telephoto lens. You’ll need a long telephoto zoom unless you’re planning on shooting very close to your subjects. For example, you won’t need a long lens to shoot skateboarders in an urban setting, but you will if you’re covering a football game.

Cropping. You can always make up for a long lens to a certain extent by cropping the picture - trimming off the edges. The problem with this is that enlarging the picture also enlarges the grain in the case of film and lowers the apparent resolution in the case of digital.

Image stabilization. Useful for reducing blurring caused by camera motion, but of no value whatsoever in freezing subject motion.

Flash. Useful both for illuminating the subject and freezing subject motion. Not every venue permits flash usage, however.

Film/ISO speed. Fast film or high ISO settings are needed to keep shutter speeds to a minimum. Once again this involves tradeoffs with picture quality.

Camera with fast focus. A fast pro camera (such as the EOS 1 series) can lock focus surely and accurately and has minimum lag time when the shutter release is pressed. A consumer camera is not going to be as surefooted and decisive, and will make it harder to nail the perfect shot.

Fast lens motor. A Canon ring USM lens can autofocus rapidly, whereas a Canon AFD (arc form drive) lens cannot. A lens with a rapid motor frequently makes the difference between achieving a shot and getting nothing.

To summarize - if you plan on putting a 75-300 4-5.6 consumer lens onto your camera, don’t expect photos like those which grace sports magazines. This isn’t to say that you can’t get satisfactory photographs with such equipment, just that it’s challenging to do so. It takes a lot of skill, experience and luck to come up with consistently good results. And you should be operating on the expectation that you will face problems with blurring of the subject and general low sharpness and low contrast if you use an affordable consumer telephoto zoom lens.



What is a macro lens? My lens has MACRO written on it.

Macro photography is the somewhat confusing name for closeup photography. Just as the human eye can only focus up to a certain distance (a distance which moves alarmingly further away with age), not all lenses can focus as closely as others. Most lenses are designed to focus up to a metre or two with long telephoto lenses having much longer minimum focussing distances than that.

Now this obviously isn’t going to help you if you want to take a super closeup of a small flower - you need a much shorter minimum focussing distance. Basically you want to be able to fill the frame with your small subject. And another concept comes in - the magnification factor. Traditionally, true macro photography refers to 1:1 photography and smaller. In other words, a lens with 1:1 magnification is able to image an area as small as the exact size of the image format in question. In the case of 35mm film this means an area of 24x36mm in size. Sometimes magnification is written as a decimal factor, such as 0.25x or 1.0x.

Unfortunately, lens manufacturers tend to throw around the word “macro” with cheerful abandon as a marketing gimmick. The fact a lens has MACRO printed on it basically means nothing, and you have to look closely at the lens specs. If a lens can do 1:1 or 1:2 photography then it’s a real macro lens, optimized for closeup photography. It may also be designed with a flat field so it can be used to take photographs of flat objects like stamps without focus problems. Lenses that can only reach 1:4 or whatever can’t take really close-up pictures.

True macro lenses are generally of much higher optical quality than ordinary lenses and usually cost more. They are also usually optimized to take photographs of small flat objects with even focus across the surface - flat field. They’re still a good buy if you don’t do a lot of macro photography, however. You can always use them for regular photography as well - they just have the bonus that they can focus much closer than ordinary lenses can.



What is a tripod mount on a lens for?

Normally you put a camera onto a tripod by attaching the camera body right to the tripod head itself. But if you have a really heavy lens this is a bad idea. Large lenses can often weigh far more than the heaviest SLR cameras. So the right way to do it is to attach the lens to the tripod head via a tripod mount. The camera then sort of hangs off the back of the lens unsupported, but this isn’t a problem - lens mounts are designed to handle that kind of weight easily.

Lens tripod mounts or collars are mounting rings with built-in clamps. These rings make it easy to rotate the camera from portrait (vertical) to landscape (horizontal) position. If the maker of your lens sell a tripod mount for the lens then it’s probably wise to get one.



What is the issue with a rotating end of a lens?

Some lenses have an outer end which rotates when you adjust either the focus or the zoom setting or both. Others do not.

This matters a lot if you’re using a polarizing filter or a graduated neutral density (ND) filter, since the properties of the polarizer vary depending on its angle of rotation and the graduation line runs across the filter in a straight line. It can be very annoying to set a filter to achieve the effect you want, touch up the focus, and find that the polarizing or graduated ND effect has changed because the end of the lens has rotated.



What is a focal length multiplier (or cropping factor) for digital and APS cameras?

35mm film has an image area of 24mm by 36mm. These are the exact dimensions of the area on the film to which an image is recorded.

Medium to low-end digital cameras sold today have sensor chips smaller than 24x36mm in size, since producing a 24x36mm image chip is still quite expensive to do. Similarly, APS film records to an area of film 16.7x30.2mm in size.

The upshot of this is that if you use such a digital or APS camera you’ll be taking photos which do not record the same image size as 35mm film. So it’s like taking a photo using 35mm film and then cropping out (snipping off) the edges. Imagine drawing a smaller rectangle within a given 35mm photo and then cutting it out - you’ve got a digital or APS photo.

This cropping factor is often confusingly referred to as a focal length multiplier. This is because the cropping makes, say, a 50mm lens on an APS camera behave rather like a 70mm lens on a 35mm camera. Not because the focal length has actually changed - it hasn’t - but because of this cropping of the image. The cropping factor is sometimes specified as a numeric value - 1.3x or 1.6x, say.

If you want to use your lens to take photos of things far away then this might actually be to your advantage. But if you want to use a wide-angle lens then this cropping factor can be a problem, since wide-angle lenses yield less dramatic results when you crop out the edges.

Some people object to the term cropping factor as well, arguing quite rightly that the issue is a matter of a change in format of the image recording area and using lenses designed for a different size format. This is true, but people are so used to equating a given 35mm film focal length with a given coverage area (or field of view) that I think the concept of a cropping factor is convenient and easily understood.

To give an example, let’s say you have a 100mm lens. When used on a 35mm film camera you get a certain coverage of the scene. But if you were to put the same lens on a digital camera with a 1.6x crop factor (ie: a smaller than full frame sensor) then you would not get the same view of the scene - you’d get less. The view you would see on your 1.6x digital camera would be the same as if you had a 160mm lens, were there such a thing, on your 35mm film camera.

What is a slow lens or a fast lens?

These are colloquial expressions describing the maximum aperture value or values of which the lens is capable. Slow lenses have a very small maximum aperture, which means less light enters the lens and so longer time periods are required to expose the film or image sensor. Fast lenses have a very wide maximum aperture and so shorter time periods are required to expose the film or image sensor.

The larger the maximum aperture of a lens the more light it lets in. And so faster lenses are generally more desirable than slower lenses. First, fast lenses let you take photos in lower light levels using available light rather than blasting the scene with ugly light from a flash unit. Second, you can see through the viewfinder better since fast lenses let in more light and so the view through the finder will be brighter with a fast lens.

As explained above, lens f stops are ratios, and so smaller numbers indicate larger apertures. A lens with a maximum f stop value of 1.4 is, therefore, fast. And a lens with a maximum f stop value of 5.6 is slow by comparison. However, f stop numbers are the ratio between the focal length of the lens and the aperture, which means that it’s very easy to design a fast 50mm lens (1.8 is a typical maximum aperture value) but very hard to design a long 200mm telephoto lens with a maximum aperture so large.

In fact, designing fast lenses in general is more complex and expensive than designing a lens with a small maximum aperture, so fast lenses tend to cost more than slow ones. It’s also harder to design and build a fast zoom lens than it is a fast prime (fixed focal length) lens. Faster lenses are also usually physically larger than slower lenses of equivalent focal lengths. The reasons for all this are tied into the complex mathematics of optics.

Note that autofocus lenses for EOS cameras have the focus motor built into the lens, not the camera. And some lenses focus more rapidly than others. So sometimes you hear people talking about a lens having a fast or slow focus motor speed, which is a separate matter altogether from the optical properties of the lens.



What is bokeh?

A term borrowed from the Japanese, pronounced with short vowels. (ie: more like French pronunciation - bo-ké - versus long English diphthongs - bow-kay.) Essentially bokeh, which is derived from the Japanese for blurring, refers to the highly subjective quality of the out of focus areas of a picture. Good bokeh is generally held to be smooth and soft, whereas bad bokeh is generally held to be distracting in some way - perhaps clumpy or doubled-up bright spots and so on.

Bokeh can be important for portraiture - you want out of focus areas behind the subject to be smooth and as non-distracting as possible. Highly patterned or sharp-edged areas don’t look as good. Mirror lenses are notorious for bad bokeh - they have annular (ring or doughnut shaped) out of focus highlights.

Bokeh is also written without the H, but the H is usually added to remind English speakers that the word has two syllables and not one.

What is an extender?

An extender is a special kind of lens which mounts itself between the main lens and your camera. As its name suggests, an extender increases the focal length of your lens by some set amount. At this moment, there are two kinds of extenders, 1.4x and 2.0x. As a quick example, if you have a 100mm lens, the 1.4x and 2.0x extenders will transform the focal length into 140mm and 200mm respectively.

For this reason alone, extenders can be quite useful in extending the reach of your lenses at a relatively low cost while taking less space.

As always though, there's a tradeoff when you gain convenience. You sacrifice image quality, and the lens becomes "slower." Here's an explanation for both points.

1) You might recall that f-stop is a simple ratio between the diameter of the aperture of the lens (how big the hole is) and the focal length of the lens. For example, a 50mm lens with a 25mm diameter has an f-stop value of 2, hence the f/2 branding. Naturally, this means then that doubling the focal length will double the f-stop value, causing you to lose TWO whole stops. Using a 1.4x extender makes you lose ONE stop. If the f-stop value gets too high, sometimes auto focus will no longer function. Check with your manufacturer for exact details.

2) Extenders also degrade image quality, especially if you use a 2x extender. Think of an extender as a magnifying glass for a moment. If an extender is a magnifying glass, all of the "faults" in a lens will effectively become that much more noticeable. For examples, images may be softer, be more distorted or may lack contrast. But this is the price you pay for convenience. I'm sure that most of us would be happy lugging around a 200mm lens with an extender rather than a 400mm lens and a tripod.

To sum it up, extenders are useful tools for getting more utility out of your lens collection. I personally recommend buying a 1.4x converter as the degradation in speed and image quality is less noticeable in practice. If you start with a very good lens, you may not notice a difference at all!