What is focal length?
The focal length of a lens is measured in millimeters. A common misunderstanding is the belief that this lens measurement is the length of the lens from end to end or its overall dimensions. The focal length measurement actually begins at the optical center. A camera lens is made of many pieces of glass and combinations of elements, so as light enters the lens, it converges into “focus” at a point somewhere inside the lens among these elements. This point is the optical center. From the optical center this focused image is then sent to the camera sensor. Therefore to get the focal length, we take the measurement from the focal center to the camera sensor while the camera is focused at infinity (generally means that you are focused on something in the far away distance). Now when you are shopping for a lens and are comparing two different 50mm lenses, you will understand why one of them might be physically longer than the other.
This number or how it is calculated isn’t something you need to memorize or remember and knowing it isn’t going to make you a better photographer. What is important is to understand what focal length affects – which is your angle of view (how wide of a view or how much of your scene will be captured in an image) and magnification (how large subjects will appear). A shorter focal length, like 18mm, will capture a larger width or wider angle of view and subjects in the frame will appear smaller than they do viewing them with our eyes. Longer focal lengths, like 100mm, or even longer, like 400mm, have much more narrow angles of view. You will capture a much smaller width, but your subject will appear larger.
Angle of view is determined solely by the optics of the lens. It does not change if you place that lens on a different camera. On the other hand, field of view (which is often used interchangeably with angle of view even though they are not the same thing), is determined by the lens and the sensor of the camera. What type of camera you have – crop sensor or full-frame – will affect how much of a scene (field of view) ends up in your final image. A crop sensor camera will capture less of the subject or a smaller field of view.
So how does a crop factor work?
Let’s say I wanted to take a landscape photo with a full-frame Canon camera and a 50mm lens. It would look something like this:
If I took that same 50mm lens and put it on a crop sensor Canon camera, the resulting image would look more like this:
The sensor size is “cropped.” Since the senor is smaller, it is like trimming off a large border all around the image. Another way to think of it would be similar to using the cropping tool in a photo editing program where the image is cut, not shrunk. The crop factor is different for cameras from each manufacturer. A Canon EF-S camera has a crop factor of 1.6, while the Nikon DX models are a factor of 1.5.
Going back to my example, both images were taken with a 50mm lens. To figure out how much “loss” of field of view we would get on the Canon crop sensor, we would multiply the 50mm x 1.6 to get 80mm. Therefore, the amount of image we get on the crop sensor camera with the 50mm is equivalent to 80mm if it had been on the full-frame we started with instead.
Here is the same landscape again. This image was taken with the full-frame camera using a zoom lens set to 80mm. Notice how the field of view is basically the same as the 50mm on the crop sensor camera pictured above (and repeated here so you can see them side by side):
We can also work this equation in reverse. Let’s say I want to get the wider field of view of the full-frame camera, as we did with the 50mm, but by using my crop sensor camera instead. I would take 50mm and divide by 1.6 which would result in approximately 31mm. Therefore, if I put a 30mm on my crop sensor camera, it should “see” almost the same thing as the full-frame with the 50mm. You can witness this effect in the images below:
Should you learn how to do all these calculations?
No, there are apps and online charts that have these calculations already completed for you! Like this one: https://mmcalc.com/ which is also available as both iOS and Android apps.
So what is the take-away then for a beginning photographer?
First, know that if you have a crop sensor camera that you will not be able to get the same field of view as a full-frame unless you go down to smaller focal lengths (in effect, zooming out). Second, understand that the focal length, as it gets larger, has the inverse effect on your field of view which gets smaller and smaller and brings the subject closer (in effect, zooming in). Third, know the basic classifications of lens focal lengths and their main uses so that you will have a basic idea of what lenses to use for different shooting situations (and make adjustments as necessary if you are shooting with a crop sensor camera).
Lenses basically fall into five types of focal lengths; super wide-angle, wide-angle, standard, telephoto (zoom), and super telephoto. In each of the descriptions below I am speaking in terms of using a full-frame camera.
Ultra wide-angle lenses have a 24mm or less focal length which allows them to capture a very wide scene. They can be useful for home interiors (real estate photography). Below 24mm, and especially super-wide fish eye lenses, images can become distorted and present an exaggerated perspective that can be artistic and fun to play around with.
Photo by sippakorn yamkasikorn on Unsplash
Wide angle lenses have a focal length range from 24mm up to 35mm. These lenses are also good for confined spaces, such as home interiors, or capturing the whole table of guests at a family holiday or celebration. In addition, these lenses are good for large group photos, cityscapes, landscapes, and architectural photography. These lenses have large depth of field so both far away and near objects can have tack sharp focus and there will be visible distance between your subject and the background when taking portraits.
Photo by Khiem Tran on Pixabay
Standard lenses have a focal length range of between 35mm to 70mm. These lenses “see” the world in much the same range and way our own eyes see it. There is little distortion of the subject so they make flattering portrait lenses. The shallower depth of field allows the photographer to separate the subject from the background as well. These lens are excellent, not only for portraits, but for nature, “on the street” shots, and low light conditions (or when you do not want to use a flash or only want natural light). The 50mm lens is in the standard lens range, and is such a popular lens choice, that it has earned the nickname – The Nifty Fifty.
Photo by Alexei Chizhov on Pixabay
Telephoto lenses are focal lengths of 70mm up to 300mm. These lenses are very popular for wildlife/nature photographers because it allows shooting from a distance without encroaching on the subject (which might be skittish). These lenses bring the subject closer. A lens that is 70-135 is considered a short telephoto and one that is 135-300 is a medium telephoto. Telephoto lenses have a shallower depth of field so crisp focus on the subject is a must. In additional to wildlife, these lenses are also used for any activity or subject where distance is required or unavoidable, such as shooting from the sidelines during a sporting event.
Super telephoto lenses are much like telephoto lenses, except their focal lengths go beyond 300mm. They provide a telescope type magnification which brings the subject and the background closer. Objects behind your subject will look much closer than in a similarly framed shot using a smaller focal length lens.
These lenses are very heavy and can’t be used for handheld shooting. A sturdy tripod is needed to support these lenses. Some even include additional support brackets built into the lens. Any type of photography where you don’t want to fight through a crowd for a position or you are limited on how close you can get benefits from this type of lens: birding, wildlife, sports, astrophotography, moon photography (or any other small distant objects), air and boat shows, car races, and more.
Photo by smarko on Pixabay
Macro lenses are not a type of focal length. They are specialty lenses that come in various focal lengths. They are used for photography of small objects, flowers, products, and insects in amazing detail. Macro lenses create 1:1 or life size reproductions on the camera sensor. (If you have an interest in Macro photography, check out my Macro Photography class)
Your camera may have come with a “kit” lens. An 18-55mm is a common wide angle zoom lens. This lens give the photographer the whole range of lens focal lengths from 18mm – 55mm. An 18-135mm lens would give a photographer the range of focal lengths from wide angle all the way up through short telephoto.
Zoom lenses like these can be great lenses because you don’t have to change your lenses as often and they cover a wide variety of photographic situations. Prime lenses on the other hand have one fixed focal length, like a 50mm. Here is a series of images from a local garden showing the range you can achieve with a zoom lens. In this particular instance, the lens used was the Tamron 18-400mm f/3.5-6.3 Di II VC HLD for Canon APS-C cameras.
What is the advantage of a zoom vs. a prime lens?
The main advantage, as demonstrated in the photos above, is versatility. With one zoom lens in your bag you reduce the weight of your gear and you can shoot everything from wide scenery shots to close ups, people, or details without changing lenses. Additionally, when shooting sports or other fast action, you can re-frame your subject without having to move closer or farther away. Finally, less lens changes means you save time and protect the camera sensor from dust and moisture exposure!
One disadvantage of a zoom lens is that they often have narrower maximum apertures than prime lenses and/or variable apertures which means less and less light makes its way to your image sensor as you zoom in. A prime lens can open up wider and let in more light, as well as achieve a shallower depth of field. Another disadvantage of a zoom lens is their size and weight. Zooms are usually larger and heavier than a prime; however, if one zoom can replace three or four primes in your bag you will still come out ahead with the zoom. Lastly, zooms are usually not as pristine when it comes to image quality, but that should not stop you from purchasing a zoom if it fits your needs.
So, now that you know a bit about focal length and which lenses are best for different photographic scenarios, you can buy gear that will serve your needs best!
If you’d like some suggestions for some “can’t go wrong with one of these” lenses, check out my recommended lenses chart or the shortened list below. These are affiliate links, so if you make a purchase, we make a small commission at no cost to you (full disclosures). Also be sure to check out my blog post Understanding Lens Abbreviations for help deciphering all those letters and numbers on the lens barrel. It will help you buy lenses with confidence!
Copy and paste these descriptions into an online seller site. I would recommend KEH for used and B&H Video for new - see my Buy Camera Gear page for links to these sellers and IMPORTANT advice before buying any gear. Some sellers are not authorized resellers, so buyer beware!
Prime, Nifty 50:
Canon EF 50mm f/1.4
Nikon AF-S 50mm f/1.8G^
Canon EF 24-70mm f/2.8L II USM
Sigma 24-105mm f/4 DG OS HSM Art
Canon EF 70-200mm f/4L USM
Tamron 70-300mm f/4-5.6 Di LD Macro
Mega Telephoto Zoom:
Nikon AF-S 200-500mm f/5.6E ED VR^ (available from used sellers like KEH - see my Buy Camera Gear page)
^Check compatibility for Nikon lenses here.
You are standing in the electronics department looking at the end of the camera lens you want to buy and it looks like alphabet soup or maybe a secret code…
EF, DX, AF-P, ED, IS, VR, USM
What do all those letters and numbers mean?
Over the years camera manufacturers have changed lens mounts and some newer models will not be compatible with older lenses and vice-versa. Nikon also makes some camera bodies that have built in focusing motors and some that do not. This article will cover general abbreviations and is not a guide to which lenses will actually work with your camera. Be sure to refer to each manufacturer's site for compatibility information before you purchase.
A short rule of thumb, except for the letters that indicate it is a crop sensor lens, generally speaking the more letters you see the better quality the lens. I am going to go through a few of the more common abbreviations and explain what they mean. After reading this, you will have a better understanding of those scrambled letters so you can buy camera glass with confidence!
Lens manufacturers build their DSLR and mirrorless camera lenses for basically two categories: crop sensor or full-frame. Full-frame lenses are considered more professional, have a better build quality, and are therefore more expensive. To make things confusing, each lens manufacturer uses a different set of letter codes to signify the “line” of lenses (pro vs. consumer) and the quality of the lens. In this article, I am only going to discuss the two most popular brands, Canon and Nikon. If you want a more complete list of abbreviations for Canon, Nikon, and third-party manufacturers like Tamron, Sigma, and Tokina, use the chart I created (link and download at the bottom of the article). Also at the bottom is a list of some recommended lenses in more common sizes. These are affiliate links, so I do make a small commission if you purchase a lens.
EF – lenses for EOS full-frame cameras
EF-S – lenses for crop sensor cameras (APS-C sensors); you can use EF lenses on a crop sensor camera, but you cannot use EF-S lenses on a full-frame camera body; EF-S lenses are less expensive and lighter in weight
L – “Luxury” lenses are the cream of the crop and have a red band of color around the barrel; usually a more rugged design and/or weather sealed
CN-E – specialty cinematography lenses; black and red like the “L” but manual focus only
EF-M – lenses for the M series mirrorless camera system; EF and EF-S lenses can also be used on EF-M camera mounts with an adapter; EF-M cannot be used on the larger full-frame mirrorless RF mount
RF – lenses for the full-frame mirrorless camera bodies; EF lenses can be used with an adapter
FX – lenses for full-frame cameras
DX – lenses for crop sensor camera bodies; DX lenses are less expensive and lighter in weight
Z – lenses specifically for the Z6 and Z7 camera systems; FX and DX lenses can be used with an adapter
S-line – lenses for mirrorless cameras; “S” means “Superior”; not to be confused with the older S mount lenses from the 1950s and 60s
Gold ring – if the lens has a gold ring around the barrel, that signifies a higher quality lens
For those who are considering buying lenses for their crop sensor camera, but eventually want to upgrade to a full-frame body, I would recommend saving up for the more expensive FX (Nikon) or EF (Canon) lenses. You can use them on your crop sensor camera body for now (with a crop factor) and later when you upgrade to your full-frame DSLR or mirrorless you can continue to use them at their maximum potential. This can save you from having to re-purchase lenses since the cheaper lenses (DX, EF-S) do not have the same quality as full-frame, and for Canon users cannot be used on the full-frame bodies. This is true of third party lenses made for Canon and Nikon as well.
Moving on to focal length, all lenses use the same methodology. Focal length is measured in millimeters (mm). This number is the distance between the camera’s sensor and the lens’ convergence point. The focal length tells us how much of our subject or scene will be captured (angle of view) and the magnification. Short focal lengths have wider angles of view and do not have much magnification, whereas, longer focal lengths have smaller angle of view and a higher level of magnification. Some lenses will have just one number, such as 50mm. This is a fixed focal length which is called a “prime” lens. Prime lenses provide higher image quality and are more compact and lightweight. Additionally, they often have better apertures for low-light.
When you see a range of numbers, such as 18-55mm, it means the focal length is variable, or has a “zoom.” A zoom lens allows you to switch between subjects and different shooting scenarios without changing lenses as often. For a more in-depth look at focal lengths and the differences between wide angle, standard, macro, and telephoto lenses, read my blog post “FocusEd on Focal Lengths.”
The next set of letters and numbers is typically the aperture or aperture range indicated by the letter “f” and then some numbers, such as f/2.8. This means the maximum aperture setting is 2.8, the iris is more open, and lets in more light. An aperture of f/22 would mean the iris is less open and would let in less light. Aperture numbers are harder to remember, they don't go in nice even increments of whole numbers like ISO 100, 200, etc. The reason these numbers appear to not follow a pattern is that they are ratios related to the focal length and the diameter of a circle. When we close down an iris we are making the circle that allows light in smaller and smaller. Think of it this way, it is similar to the iris of your eye. If you were in a dark room and there was only one small light bulb, your iris would open to its widest to let in more light. But if that same dark room had 22 light bulbs burning, your iris would become more narrow to let in less light. You can also think of f-stops as fractions. One fourth (1/4) is smaller than one half (1/2); therefore, f/4 is a smaller, narrower aperture than f/2. The full f-stops from wide to narrow are 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, and 22 (some lenses have f/stops more narrow than f/22, but also typically suffer in image quality).
KoeppiK / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
If the camera lens you are considering for purchase is a prime lens, there will be just one f/. That is the maximum (widest) aperture the lens is capable of using. Lower numbers are better because they allow shallower depth of field and you can get those soft, blurry backgrounds behind your subject in your portraits. If a camera is a zoom, it usually has a range such as f/4-5.6 which means that at the smaller focal length you have f/4 and at the longest focal length you have f/5.6. The more you zoom, the narrower the aperture becomes. Whenever possible, choose a lens that has a low f/ and keeps that throughout the zoom instead of a range, such as a 24-70mm f/2.8. A few things to keep in mind, sometimes the f/ is not included and you will see 1:4-5.6 or 1:4-1:5.6 instead. Additionally, most lenses will shoot at their sharpest when at a medium aperture. So if your lens has a maximum aperture of f/2, your photographs will most likely have fewer aberrations and maximum sharpness at f/4 or even f/5.6.
Some lenses might also have Roman numerals I, II, III to indicated the generation of the lens. A lens with III is newer and probably has some upgrades compared to the same lens generation II. However, be aware that Tamron, and others, may also use similar markings for other purposes. Tamron lenses marked Di-II indicate a crop sensor lens and Di-III lenses are specifically for mirrorless.
Lenses may have markings for magnification such as 1:1. True macro is considered 1:1 where the lens allows the photographer to focus up close and get a life-sized reproduction on the image sensor. A lens with a 1:2 magnification will reproduce the subject at ½ life-size on the image sensor. Macro lenses will often say “macro” on the lens, but be cautious. The label “macro” is sometimes used on lenses that do not really produce a 1:1 ratio, so be sure to check the specifications of the lens if that is something you want.
Even with all of those letters and numbers explained, there may still be a mind boggling amount of remaining letters to decipher. Here are a few of the more common ones for Canon and Nikon. Again if you want a more complete list, use the chart I have created (at the bottom of the article).
IS – Image Stabilization; good for slower shutter speeds because internal mechanisms (gyros) help stabilize camera movement
USM – UltraSonic Motor; faster focus, but quiet
Micro USM - cheaper version of USM for kit and budget lenses and it is not as quiet
STM – Stepper Motor; fast focus, and the most quiet
SC or SCC – Spectra (or Super Spectra) Coating; coating on the lens to decrease reflections and flaring
SWC – SubWavelength Coating; coating to minimize ghosting and flaring; newer version of SC/SCC
DO – Diffractive Optics; lens will have a green ring on the barrel; fewer glass elements so the lens is lighter and smaller with improved optics
AF – Auto Focus; allows focusing from the camera; you may also see AF- followed by D, I, P, or S, which stand for Distance, Integrated Motor, Stepper Motor, and Silent Wave Motor respectively and are all improvements over AF alone (check compatibility carefully some consumer level camera bodies require AF-S lenses for full functionality)
SWM – Silent Wave Motor; very quiet, high speed auto focus
NIC or C (or SIC) – Nikon (or Super) Integrated Coating; to minimize ghosting and flaring
ED – Extra-low Dispersion Glass; used in high-end lenses to reduce aberrations
HRI – High Refractive Index; only used on the very best lenses to reduce aberrations
N – Nano Crystal Coating; labelled in gold; reduces flaring and ghosting
FL – Fluorite – superior lens glass and lighter weight
G – no conventional mechanical aperture ring; aperture set by the camera body
IF – Internal Focusing; focusing is accomplished with internal mechanisms
E – Electronic Aperture; aperture is controlled by electronic signals; not to be confused with the old 1970s E-mount (check compatibility carefully most older bodies and film cameras cannot use E lenses)
Micro – Macro
VR – Vibration Reduction; image stabilization to reduce camera shake
Okay, so let’s put your knowledge to the test by following along. You are considering the following lens:
Nikon AF-S DX 35mm f/1.8G
This lens will have Auto Focus with a Silent Wave Motor (AF-S). It is for a crop sensor camera (DX). It is a prime lens with a focal length of 35mm. The maximum opening of the aperture, or iris, is 1.8 so you can get some shallow depth of field with blurry backgrounds (f/1.8). You have no manual aperture ring (G) which makes this lens compatible with cameras that allow setting the aperture from the camera body.
Let’s try another one. You have two lenses to choose from:
Nikon AF-P DX 70-300mm f/4.5-6.3G ED VR
Nikon AF-P 70-300mm f/4.5-5.6E ED VR
The main differences are the first one is for a crop sensor camera (DX), has an aperture that lets in less light when zoomed (f/6.3), and no mechanical aperture ring (G). The second lens will be the better quality lens for a full-frame camera with a better f/ range and electronic aperture. The first lens costs approximately $400, while the second one costs approximately $550. (Note that some camera bodies are not compatible with AF-P and E type lenses).
Let’s try one final example and this one should be easy. You have two lenses to choose from:
Canon EF 85mm f/1.8 USM
Canon EF 85mm f/1.4L IS USM
The one with the “L” for Luxury is the better lens. It has the f/1.4 aperture and the added image stabilization (IS). However, the prices might give you sticker shock - $350 vs. $1500. Whenever possible, it would be my recommendation to save up for better lenses, but sometimes the price is so much higher that the difference in quality might not be the most important determining factor. In this case the small difference in aperture would probably only be a factor for the most discerning of professionals and if you use a tripod and shutter release the added image stabilization may not add any benefits to your photography.
Buying a lens can be confusing, but by doing your research, checking the specs, and knowing a few key abbreviations, you’ll be able to buy camera glass with confidence!
Lens Abbreviations and Lens Recommendation Charts:
Feel free to print or download the lens abbreviation guide (PDF files below).
Below I have also included a link to some recommended lenses. You can always find more recommendations on our Buy Camera Gear page.
Disclosure: Some of the links below are affiliate links. Please read the full version of my disclosures for more information. If you make a purchase through one of these links, we make a small commission (at no cost to you).