This post originated as a set of notes for a talk I offered to do at Bromsgrove Photographic Society.

“I want a new lens for my camera. What should I get?”

Or probably a better question… “I want to be able to take X type of photograph – what lens do I need.”

Or maybe before asking that, you should ask “I have these lenses – will any of them do the job, and with what limitations?”

So what determines which is the right lens for a job – or what jobs a given lens will be good at?

How many lenses do I need?

There’s a simple formula you can use to calculate this:

LensesNeeded = LensesCurrentlyOwned+1

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Field of view and focal length

The first and most obvious lens property that will determine if it can take the picture you want is the field of view – which is usually described using the focal length. While it’s really the field of view that you care about, no-one ever talks about that, they always talk about the focal length, where the focal length and the sensor size together determine what the field of view will be.

Equivalent focal length

The fact that the FOV for a give focal length depends on the sensor size actually makes it a bit inconvenient using the focal length as a proxy for the field of view, so often you will hear people talk about the “35mm equivalent focal length”, meaning “what focal length lens would have the same field of view using a 35mm film camera”. For a camera with a sensor smaller than 35mm film – a “crop sensor” camera, you multiply by the “crop factor” to get your 35mm equivalent focal length.

So what impact does the field of view have on the image you are taking? A wide field of view – or a “wide angle lens” – will get a lot of the scene in front of the camera into the frame, but individual objects may seem small in the frame. A narrow field of view – usually called a telephoto lens – will select just a small part of the scene in front of you, making distant objects appear large in the frame. Somewhere between the two you have a “normal” lens where the field of view is more-or-less the same as normal human vision.

For 35mm film, wide angle lenses might have focal lengths between 24mm and 35mm. A normal lens would be around 50mm, and 135mm would be a short telephoto lens. 200mm-300mm is a medium telephoto, and anything longer is super telephoto. Telephoto lenses tend to be large, heavy, and expensive compared to normal and wide-angle lenses. You can also go wider than 24mm – into extreme wide angle or fisheye territory – which tend to more specialist lenses.

How wide is wide?

Small differences in focal length make increasingly significant differences to the field of view as you get wider. The difference between a 16mm lens and a 14mm lens is very noticeable.

For a micro 4/3 camera like my Olympus OM-1, the “crop factor” is 2, meaning that a lens with focal length 12mm gives the same angle of view as a 24mm lens would on 35-mm – so it’s a wide-angle lens. A “normal” lens is around 25mm, and something like my 300mm telephoto gives me the same field-of-view as a 600mm lens would on 35mm. Other common crop factors are 1.6x for Canon crop sensors and 1.5x for Nikon crop sensors and Fuji X cameras.

Some lenses are designed to work at a single fixed focal length – these are called “prime” lenses, to distinguish them from “zoom” lenses where the focal length can be moved through a range of values.

Your favourite focal length

I have seen advice online that you should look at what focal length you commonly use (Lightroom, for example, makes this very easy), to help inform any decision on a new lens. While I can see some logic in this, it suffers from the very obvious flaw that you are only going to be using the lenses you already own. If all you have is a 50mm kit lens, all your photos will be taken at 50mm, and by this algorithm all you would ever buy is 50mm lenses. That would just be silly.

I did have a quick look at my Lightroom catalog to see if it gave me any insights on focal lengths, and the main thing that stands out is that for any given zoom lens, I use the two extremes much more than any intermediate length. Make of that what you will.

Teleconverters and other alternatives

Some lenses can be used with a special adaptor fitted between the lens and the camera called a teleconverter, which makes the lens act like one with a longer focal length (but slower – both in terms of aperture and in terms of focusing speed and accuracy). There’s usually a small hit to the image quality too. But compared to the cost – and weight – of carrying additional lenses they can be very useful. Alternatively, if your sensor has a high enough resolution, simply cropping out the edges has a similar effect – reducing the angle of view.

If you need to go wider, wide-angle adaptors are available that fasten to the front of a lens, but I’ve never found them useful except for shooting video. Easier for the occasional wide shot is to shoot several frames and stitch them together.

Also, don’t forget that cropping reduces the field-of-view too. So if you have a high resolution sensor and are prepared to use just 1/4 of the pixels in your final image, you can treat your 50mm lens as a 50-100mm zoom.

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Aperture

The second most important defining characteristic of a lens is the aperture – the size of the hole though which light can pass into your camera. A large hole means a lot of light can get through, and a smaller hole means less light can get through. The actual aperture you use for each photo can be changed from shot to shot, but the maximum size is determine by the design of the lens – in particular, by the diameter of the front element. So a lens that has a large front element can let a lot of light in, and thus has a large “maximum aperture”. Apertures are always expressed as a fraction of the focal length, so f/4 means the diameter of the lens opening is a quarter of the focal length of the lens.

Why are apertures expressed as fractions?

Why is the aperture of a lens described in terms of “F stops” rather than using the width? Because it makes exposure calculations easy. The density of light on the sensor/film goes down as the focal length goes up, at the same rate as it goes up as the aperture (in mm) goes up. So by expressing the aperture as a ratio, a given aperture always corresponds to the same light density (for a given scene brightness) on any lens regardless of the focal length.

Because a wide aperture needs larger glass elements in the lens, a lens that is designed to have a wide maximum aperture is likely to be larger, heavier, and more expensive than one that has a narrower maximum aperture. Such lenses are called “fast” lenses.

Even if your lens has a maximum aperture of (say) f/2, you probably won’t us it to shoot at f/2 all the time – or indeed much if any of the time. So why would you pay the extra for that wide aperture if you don’t use it? There are several reasons it might be worthwhile:

• More accurate focussing
• Sharper lens when stopped down
• Fast aperture is there when you DO need it – low light, long lenses, or fast action
• Bragging rights

A fast lens can be focused more accurately than a slower one. There are various technical reasons for this that I won’t go into, but it boils down to the fact that because your depth of focus is small when wide open, it’s easier to distinguish in-focus areas from out-of-focus ones. This is true when focusing manually but the same principal applies when autofocusing.

Some zoom lenses – typically cheaper ones – have a maximum aperture that varies with the focal length. These lenses will have the aperture range in the name along with the focal length range, such as “70-300mm f4-5.6”. Such lenses invariably have the faster aperture at the wide end (where you tend to need it less) than at the long end. More expensive zoom lenses have the same maximum aperture throughout the zoom range, ad are called “constant aperture” lenses.

Fixed aperture lenses

Not to be confused with constant aperture zoom lenses, some (very few) prime lenses are available that do not have an aperture control in the lens, and thus can only shoot at a single aperture. Examples include the Canon RF 600 and 800 f/11 telephoto lenses. I think the reasoning is that there would never be any reason to use a narrower aperture on such a lens, so why bother adding the capability.

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Lens mount

It could be argued that this is the most important attribute of a lens, even more so than focal length and aperture. Will it work on my camera? If not, it’s just a paperweight (or a doorstop, depending on size). Each major camera manufacturer uses a different, incompatible, lens mount (with very few exceptions, such as Panasonic/Olympus sharing the Micro 4/3 mount), and many manufacturers actually use more than one mount across their product range. Complicating the picture still further, some lenses can be adapted to fit on more than one mount, or may fit your camera but with limitations.

If in doubt, a lens designed specifically for your particular camera family is always going to be the best bet – anything else will always come with some sort of compromise. Your camera manual will tell you what lenses are compatible with your camera.

I tried to summarise what mounts are currently in use by what manufacturer, and it quickly got VERY complicated, so I made what I had into a separate post at Current Lens Mounts – but it barely skims the surface!

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Interpreting lens names

What do all those letters and numbers in a lens name mean? Unfortunately every lens manufacturer has a different naming scheme, but most of them are at least similar enough that once you have cracked the code for one, you can have a pretty good guess at others.

I’m going to use a 24-105 zoom lens from Canon as an example.

RF 24-105MM F2.8L IS USM Z (£3000)
RF 24-105MM F4L IS USM (£1100)
EF 24-105mm f4L IS USM II (£1200)
RF 24-105MM F4-7.1 IS STM (£400)

The lens name starts with the mount – in this case RF (indicating that it is designed for Canon R full-frame mirrorless cameras) or EF (indicating it is for the earlier SLR/DSLR EOS cameras).

Next comes the focal length – in this case 24-105mm. This makes it a versatile zoom lens ranging from a reasonably-wide 24mm to a short telephoto 105mm. A good everyday range.

Next comes the maximum aperture. This is where things start to vary in the examples above. F2.8 is fast for a zoom in this range, making the first example a large and expensive lens. The second example has a constant F4 maximum aperture throughout its zoom range – a moderate speed for general outdoor use but perhaps limiting for studio work or dark environments. The third has a variable maximum aperture – F4 at the wide end but F/7.1 (nearly two stops slower) at the long end. This is a budget lens, but will also be considerable smaller and lighter than the other two.

Thus far, most manufactures will stick to a similar naming scheme. But from this point onwards come a series of letters and codes that will vary from manufacturer to manufacturer, which indicate other features or attributes of the lens. In the above example we have:

L – indicates that this is from the Canon L (luxury?) range of lenses, designed for professional use and indicating high-quality (and high price) lenses with solid construction and good image quality. The first two lenses are L lenses, the third is not. Canon L lenses traditionally have a red ring around the end of the lens. Sony use G and G Master in the name, Olympus use Pro…

IS – indicates that the lens includes image stabilisation. All three of these lenses are image stabilised, though the first two are likely to have better image stabilisers than the third one. Nikon uses the letters VR (vibration reduction) to indicate the same feature, Panasonic use OIS, Sony use OSS…

USM and STM refer to the type of autofocus motor – ultrasonic motors or stepping motors. USM are faster and quieter, and are found in Canon’s more expensive lenses. If neither is specified, it’s a direct drive motor – noisier, cheaper and less accurate than the STM variants. Other manufacturers will have their own codes for differentiating lenses by AF mechanism.

The Z at the end of the first lens indicates that this lens has a number of features specifically designed to appeal to film makers. Film makers tend to care about how the lens behaves while changing from one aperture (or focal length) to another in ways that stills photographers do not, so for example the ability to change gradually from one aperture to another without steps might be important.

The II in the EF lens indicates that this is the second generation of the lens, which may mean changes to the optical formula, improved IS or image quality, or simply cosmetic changes. This can be significant when buying used lenses (or old stock) as the value of earlier versions of lenses is usually lower.

So how do all of these weird letters help me choose a lens? To be honest, I’m not sure that it does. Once you get past the mount, focal length and aperture designation, the other bits are really there to makes sure that each variant has a unique name. To determine exactly what the features of each variant are, you need to go to the manufacturer’s website and look it up.

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Other lens attributes

Besides the properties already mentioned, there are many other lens properties that will affect how suitable a lens is for a task, including

• Size and weight
• Focusing
  • Speed, accuracy, throw, type
  • Focus breathing
  • Minimum focus distance
  • Maximum focus distance (usually, but not always, infinity)
• Image quality
  • Colour rendition
  • Distortion
  • Vignetting
  • Bokeh
  • Flare-resistance
  • MTF
• Build quality
  • Reliability, repairability, repeatability
  • Water resistance
• Ergonomics
  • Internal zoom
  • Internal focus
  • Rotating front element
  • Focus limiters etc
• Special capabilities
  • Tilt/shift
  • Macro
  • Fisheye
  • Soft focus
  • Built-in teleconverters
  • Filter compatibility
  • Infra-red compatibility
• Availability and price

More expensive lenses tend to have better ergonomics, better build quality, better image quality, and better focusing, but are also usually bigger and heavier. Don’t underestimate just how much difference that might make – there’s no point splashing out on a super-quality expensive lens and discovering you never take it anywhere because it’s too big and heavy.

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Buying lenses

Unlike cameras, lenses tend to hold their value much better (at least until a new model comes along or a new camera mount renders them obsolete).

Used lenses can be a good buy.

Lenses can also be rented, or potentially borrowed. I don’t mind lending lenses to people I know, where I would be more reluctant to lend a camera (which feels more personal somehow). Perhaps it’s because a (modern) camera has 3000 settings that I would not want messed up, whereas a lens just has a switch or two that can trivially be set back in place.

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Caring for lenses

Don’t scratch the glass. Don’t drop them (especially not into the sea). Even weather-sealed lenses are only sealed while mounted on the camera, so be particularly careful when changing lenses.

Scratches, fungus, dust and cosmetics affect the resale value but don’t really affect image quality (unless really bad). Fungus really scares off buyers though, as they fear it will spread to other lenses.

Some people advocate putting a filter on every lens, to protect against scratching or otherwise damaging the front element. Others reckon that adding more glass in front of the lens degrades the quality. In the film days, a UV or skylight filter was needed (for colour film at least) to avoid sunlight washing out the colours in the exposure, but digital cameras have a UV filter built-in to the sensor. Personally, I’m in the “no filters” camp (unless I need a polariser), having experienced at first had the effect that a filter can have on a lens’s ability to autofocus. More expensive filters may work better in that regard.

A lens hood, on the other hand, also works to protect the front element from bumps and scratches, but improves the image quality and focus ability by avoiding the glare that can result if sunlight falls directly on the lens surface. I use a lens hood whereever I can. Just make sure the hood is designed (and fitted) properly for your lens so that it does not cause vignetting.

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Lenses for genres

So far the information here has been largely technical. But what if you are trying to decide what lens you need for a particular style of photography? A very quick guide/summary below.

Landscape

• Typically wide angle
• Max aperture not that significant
• Sharpness, and corner sharpness in particular, may be important to you
• Stitch to go wider

Portrait

• Normal to short telephoto usually favoured
• Wide aperture if you like the shallow DOF effect
• Wide aperture for accurate focus
• Wide angle lenses can lead to distorted features (* see below)

Nature

• As long a telephoto as you can afford/carry
• As wide an aperture as you can afford/carry
• Fast autofocus

Street

• Inconspicuous
• Slightly wide angle usually favoured
• Undemanding on lens qualities
• Prefocus may be preferred

Macro

• Specialist lenses with close focus and low distortion
• Focal length translates to working distance
• Manual focus may be fine

Astro

• Need all the light you can get
• Wide angle for starfields, telescope+adapter for deep sky.
• Manual focus fine

Still life

• Bit like portrait but unlikely to want shallow DOF and can take your time focusing

Sports

• Fast autofocus
• Zoom preferred
• Mid to long to ultra telephoto
• As wide an aperture as you can afford/carry

Weddings

• Low light
• 24-70 F/2.8 seems to be preferred…

Travel

• Easy to carry
• Phone?

General

• Kit zoom covers quite a lot of bases!

Of course, these are just starting points, not rules, and anyway, rules are meant to be broken. Nothing to stop you using a wide-angle lens for nature photography or a fisheye for your still life. You might just get something out of the ordinary. Note that I am not advocating walking up to grizzly bears with your wide-angle lens to get that “different” shot. Please respect the wildlife and your own safety!

Wide angle lenses distort faces?

No they don’t. Using a wide angle lens in a portrait can make the face look distorted IF you use the wide field of view as an opportunity to take the picture from very close up. But it’s the fact that you are very close that has caused the distorted look, not the lens.