Many people misunderstand the concept of focal length in photography. Contrary to what some photographers think, the focal length of a lens is not the lens’s physical dimension. So, what is focal length then? In this article, I will answer that question and discuss how to decide which focal length is right for your style of photography.
Definition of Focal Length
Without getting into an in-depth physics discussion, the focal length of a lens is an optical property of the lens. It measures the distance, in millimetres, between the optical centre of the lens and the camera’s sensor (or film plane). It is determined with the camera focused to infinity. Lenses are named by their focal length, and you can find this information on the barrel of the lens. For example, a 50 mm lens has a focal length of 50 mm.
In the definition of focal length, I mentioned the “optical centre” of a lens. You may be wondering what this is. Well, a camera’s lens is not made from a single piece of glass. Instead, it is a combination of lens elements and groups of elements. These combinations help to focus the light and cut down on distortions. The location where all the light rays converge to form a sharp image is known as the optical centre of the lens.
Focal length is a property of the lens itself, not the camera. What I mean by this is that a 50 mm lens is a 50 mm lens, regardless of whether on a full frame, cropped sensor, or medium format camera. However, the size of the sensor does play a role in the lens/camera combination’s field of view, but more about this in a minute.
The Important Stuff
While the definition of focal length may be relevant to some people, as photographers, it is not something you need to remember. What is more important to understand is what focal length tells us. Focal length describes the angle of view of a lens. That is, how much of a scene before us the lens captures. And in addition, how large subjects within the frame appear. The longer the focal length of a lens, the narrower its angle of view. Subjects appear larger using long focal length lenses than they do viewing them with our eyes. On the other hand, lenses with short focal lengths take in a much wider angle of view. Thus, elements seem much smaller in the frame than they do to our eye.
Take a look at this illustrative sketch. According to Nikon, its 500 mm f/5.6 lens has an angle of view of 5o, while its 50 mm f/1.4 lens has an angle of view of 46o. And lastly, its 20 mm f/1.8 lens has an angle of view of 94o. As you can see, the longer 500 mm lens takes in a much thinner slice of the scene. As a result, only a portion of a single boat is captured in the shot. On the other hand, the 50 mm lens has a wider angle of view. Standing in the same location, you can capture a much broader section of the scene, including several boats and more of the distant rocks. However, with the 20 mm lens, you can capture the entire scene in a single frame.
On a recent trip to Lake Tahoe, I came across two fellows rock climbing while I was hiking. Although I did not have my camera on a tripod, I was sitting down on the rocks and captured all four shots from the same spot. Notice how my images narrow in on the scene as I zoom in on the climber. At a 25 mm focal length, you can barely spot the men. Alternatively, at 140 mm you can see the expression on the climber’s face. Incidentally, he managed to climb another 3 m up the face before he lost his grip, slipped off the rocks, and repelled safely back down to the ground!
Field of View and Equivalent Focal Length
The terms “angle of view” and “field of view” often get used interchangeably. However, as I said above, the angle of view is an optical property of the lens. It does not change regardless of what type of camera is being used. Field of view, on the other hand, is a result of the lens/camera combination. Field of view not only depends on the focal length of the lens but also the camera’s sensor size.
A full frame camera has a sensor that is the same size as a 35 mm film negative (36 mm x 24 mm). However, digital cameras today come with a wide variety of sensor sizes depending on the manufacturer and camera model. Sensors that are smaller than full frame are considered cropped sensors. This term comes about from the fact that these smaller sensors see less of a scene, in a similar manner to what cropping an image does.
The phrase “effective focal length” (also known as equivalent 35 mm focal length) is used to equate what a lens captures in 35 mm sensor terms. Because most people are used to working with 35 mm film cameras, at least those of us with a few grey hairs, the full frame format was adopted as the standard. Equivalent focal length describes the focal length of a lens you would need to put on a full frame camera to capture the same field of view as a given lens on a cropped sensor camera. This is where crop factors come into play. The equivalent focal length is found by multiply the focal length of the lens by the non-full frame camera’s crop factor. For Nikon, it’s DX cameras have a crop factor of 1.5. Canon’s EF-S cameras have crop factors of 1.6. Additionally, micro four-thirds cameras have a crop factor of 2.0, and Sony and Panasonic’s 1″ sensors have a crop factor of 2.7.
This image below was taken with a 24-70 mm f/2.8 lens at 44 mm on my full frame Nikon D800. If I had put this exact lens, at the same zoom, on my Nikon D500 (cropped sensor), it would still be a 24-70 mm lens at 44 mm. However, the reduced field of view – due to the cropped sensor camera – would not see the same area. In this situation, I would only capture what is outlined in red. My effective focal length on the D500 would be 44 mm x 1.5 or 66 mm. In other words, if I wanted to capture what is outlined in red on my D800, I would have to put on a lens with a 66 mm focal length. Of course, I could also zoom my 24-70 mm lens from 44 to 66 mm.
Nasim wrote an excellent article on Equivalent Focal Length and Field of View a few years back. For a much more detailed explanation, check his article out.
Classifications of Focal Lengths
Camera lenses are classified into five descriptive categories depending on their equivalent focal length. Ultra wide-angle lenses have a focal length of less than 24 mm in full frame terms. They capture incredibly broad views. However, because of this, they often present a distorted view of the world. They are fun lenses to use and have a very close minimum focusing distance and a large depth of field. If you photograph interiors, these lenses are well worth having in your bag.
Wide angle lenses have an equivalent focal length in the range of 24 mm to 35 mm. These lenses still take in a wide view and are often used by landscape and architectural photographers. When you use a wide lens, it is a good idea to try and include some foreground interest. This will give your photos a sense of scale and help to lead viewers into your image. Because these lenses have very large depths of field, it is easy to get both near and far objects in sharp focus.
Standard lenses have focal lengths between 35 mm and 70 mm. They capture the world in a way very similar to how our eyes see. They cause minimal distortion, so are a favourite of portrait photographers. Another feature of lenses in this focal range is their ability to isolate a subject from its background using much shallower depths of field than wide angle lenses.
The focal lengths between 70 mm and 300 mm are considered telephoto lenses. They are regularly used by wildlife photographers to get closer to their subjects without being seen. These lenses have shallow depths of field, even at small apertures, so acquiring sharp focus is critical.
Super telephoto lenses have focal lengths exceeding 300 mm. They are often used for photographing birds and other small distant subjects. These lenses can be very large and heavy and may require the use of a tripod to support them. They are also very expensive! Nikon’s AF-S NIKKOR 800mm f/5.6E FL ED VR lens is a whopping $16,300 at the time of writing this article! Luckily there are some much cheaper options available now that allow photographers with normal pocketbooks to get into bird photography!
Again, all the numbers I just mentioned are in full-frame terms. If you have a crop-sensor camera, you will need to find your equivalent focal lengths by dividing these numbers by 1.5, 2, or whatever your crop factor is.
Zooms vs Prime?
Prime lenses have a single, fixed focal length. A zoom lens, on the other hand, has a variable focal length. Some popular zoom lens ranges include 16-35 mm, 24-70 mm, and 70-200 mm. A great lens for travelling is one that covers both the wide and telephoto ranges, such as an 18-200 mm lens. The advantage here is that you will not have to carry a multitude of lenses with you, or change lenses to shoot wide vistas and close-ups of architectural details.
There is a downside to zooms, though: they are often not as optically sharp as primes. Although this gap is closing with newer and better technology, it still exists, especially when dealing with superzooms like 18-200 mm lenses. Another drawback is that they tend to have narrower maximum apertures than prime lenses. While a top of the line zoom might have a fixed aperture of f/2.8, primes in a similar focal length may open up much wider and often let in several more stops of light. This can make prime lenses more desirable in low light conditions. For more information on primes versus zooms, see our larger article on the subject.
Conclusion
Don’t get hung up on the definition of focal length, or even the difference between the angle of view, the field of view, and the equivalent focal length of a lens. What is important to remember is that lenses with long focal lengths bring objects closer, like a telescope. And, on the flip side, wide angle lenses are great for capturing large vistas. If you need to get closer to your subject than you can physically, opt for a telephoto lens. If landscape and architecture are genres you enjoy shooting, then make sure you carry a wide angle lens. For portraiture, and anything else in between, you can’t go wrong with a nifty-fifty in your bag.
If you have questions, please drop me a line in the comments below, and thanks for reading!
thanks for the article. i have read numerous versions of this topic but liked your definition of angle of view . now the
burning question. is the image size of a 500mm F4 lenses always bigger than a 300mm even when using crop bodies. The reason i ask is because my 500mm F4/7dmkii combo ( 800mm field of view ) produced larger images in frame than my current olympus 425mm f4 ( 2x crop -840mm field of view ). thanks for clarifying this
You’re welcome, Sam. I’m not sure I completely understand your question. If you mean that when you look through the viewfinder you see more on your Canon than on your Olympus, it may be the coverage in the two viewfinders. The view finder on your Canon has 100% coverage of the scene. I’m not sure what model Olympus you have, but it may not have 100% coverage. Not sure if this answers your question.
Thanks for reading!
Hi Sam, I believe that it gets very complex, when you try to compare different camera lens combinations. If I am informed correctly, Olympas utilizes a 4:3 aspect ratio and canon hast a 3:2 aspect ratio. So I am no expert on this either but, in my thinking that changes up the variables when you try to calculate the equivalent focal lengths.
Focal length is NOT measured from the “center” of a lens. It is measured from the second principle plane.
This is easy to explain in a diagram without confusion or ambiguity. Please ask someone who actually knows optics and lens design to write your lens articles.
The article states its measured from the “optical centre of the lens and the camera’s sensor (or film plane).” Is this different than what you claim as being measured from the center of a lens?
The article states its measured from the “optical centre of the lens and the camera’s sensor (or film plane).” Is this different than being “measured from the center of a lens?”
Why do you have to be rude about it? She said “optical center” and elaborated its definition. Clearly (at least to me) she didn’t mean physical center.
The writer’s description clearly said ‘optical center’, not center in the physical sense; you *can* disagree without being rude, you know…
Did you pay for reading photographylife?
You didn’t?
Then instead of being rude be polite and make a contribution.
Anyway this is a site dealing with photography, and neither quantum mechanic about sensors, nor difraction limited lens desing for steppers.
Adorei sua dica.
Thanks so much, Nilce!
Very interesting article.
Thank you very much.
Glad you enjoyed it, Les. Thanks for reading.
Great article. It is worth mentioning that 35mm is the nearest to what the human eye sees and is popular with street photography.
When selecting a focal length you should first consider the subject that you want to capture then the focal length to match.
Chris
www.cjhphotography.co.uk
Thank for your kind words and comments, Christopher.
There is some debate about which focal length is most like our eyes. Many folks say that 50 mm is most like them. However, while a 50 mm lens may make things appear about the same distance away, our eyes see a much wider angle of view than a 50 mm lens. Especially if you take into consideration our periphery sight. It is an interesting discussion.
Thanks for reading and contributing.
Thank you for preparing an article on photography,about focal length of lenses.anyway,
I want to know whether Canon lenses are better or Nikon lenses. For example,the lens16-35mm f.4 nikon.or 16-35 ,mm f2.8 Canon. Seyed
Hi Seyed,
I’m glad you liked the article. I’m afraid though that this wasn’t a lens review, but just a discussion on focal length. The debate over which is better, Canon or Nikon, has been going on for years. As to which is “better,” that can depend not only on the technical specs, but also on brand loyalty, personal preference, budget, and what you like to shoot!
your photography tutorials on focal length is excellent. but being an amateur photographer I could not understand the caption of the photograph of the room . What is X-T3+XF10-24mmF4 R [email protected] Is it the name of the camera used and the lens specification?
I’m glad you enjoyed the article, Partha.
Good question, and apologies for assuming that everyone knows what all that stuff under the photo means. At Photography Life, we label the technical specifications about images consistently. The first part (X-T3) is the camera name, then the lens (XF10-24mmF4 R OIS), and lastly the focal length that the image was taken at (@10mm). After that we include the ISO, shutter speed and aperture that the photography was shot at.
Hope that helps.
Hey, I enjoyed this article and it aptly cleared all my confusions.
Thank you.
Thanks, Tani! Glad it could clear things up for you!
thanks elizabeth for your reply. sorry about the confusion. basically i have a olympus omd-emi mkii. just to clarify will the image captured be larger on my canon rather than olympus combo based on the fact that i was using a 500mm lenses despite both cameras having an equivalent focal length ( 840 v 800mm ). or to put it another way is the angle of view of a lens affected by the crop factor .!
Hi again, Sam.
As Gunner said above, the size difference you are mentioning could be because of the different aspect ratios used by both cameras.
However, if you take two images from exactly the same spot, objects in the photo shot with the Olympus will be larger. This is why many bird photographers prefer to use crop sensor cameras as opposed to full frame cameras. With the cropped sensor, they can get much closer with their lens as the total magnification is a result of the equivalent focal length.
Finally, the angle of view of the lens is not affected by the crop factor, but the field of view is!
Let me know if that clears things up!
Hi Elisabeth, thank you for your article. I think it is very well structured and I liked that you found a simple understandable terminology for a rather complex topic. It makes for a good read and I agree one shouldn’t get hung up too much on the focal length. For taking good images it is much more important that photographers can grasp the general idea what effect a certain focal length has to the composition of the image one has in mind. At least that is my takeaway from your article.
Thank you for your thoughtful words, Gunnar, and for helping Sam with his question. And you are correct, the most important take away is that certain focal lengths are more appropriate for certain tasks. Knowing when to use a wide angele vs. telephoto lens is what is most important.
Hello,
Great article. I am the picky guy and would like to comment on this:
“The longer the focal length of a lens, the narrower its angle of view. Subjects appear larger using long focal length lenses than they do viewing them with our eyes. On the other hand, lenses with short focal lengths take in a much wider angle of view.”
It is actually false. The angle of view a lens can deliver is not only due to its focal length. For instance the 5mm lens of my iPhone can deliver about the same angle of view than a typical 30mm made for FF, without considering the sensor behind. Of course the long wavelength lenses have in general narrow field of view because the size of camera sensors, for which they are made, is limited
What is always true is that a lens convert angles (angle of one object seen from a given distance) to a distance (a length on the focal plan). The larger the focal length the largest will be this distance for a given angle.
To put an example : www.sylvainphoto.com/p4766…#hb8facc3b
On the link above the same scene from a screen is seen by two lenses (image projected on a paper). Both are 50mm lenses however they do not have the same angle of view (not the same number of circles) but the “angle-to-distance” conversion is the same.
*wavelength -> Focal length
Sorry, Sly. Please don’t take this the wrong way, but I have read over your comment several times, and visited the webpage you mentioned, but I can not understand what you are saying. When you say “The angle of view a lens can deliver is not only due to its focal length,” I’m not sure what else you are saying plays a role. The filed of view does depend on both focal length and sensor size.
The reason your iPhone lens has a similar field of view (not angle of view) as your 30mm lens on a full frame camera is because of the small sensor size and large crop factor associated with the iPhone’s sensor.
Again, apologies for not understanding.
I think Sly is actually right that focal length does not determine angle of view, or at least not in a straightforward way. What is clear is that focal length determines magnification at the rear focal plane. This magnification refers to how large the image of an object is at the focal plane, compared to how large it is in real life; and it is not influenced by the sensor size (crop factor). The image of a given object at a given distance, when projected by a longer lens, will be bigger than when projected by a shorter lens. In other words, lenses with longer focal lengths magnify more than lenses with shorter focal lengths.
As for angle of view, it is not determined by focal length unless you add the requirement that all lenses to be compared have the same size image circle. Since lenses can have different size image circles, it is certainly possible that two lenses of the same focal length can have two different angles of view, even if you don’t consider sensor size or crop factor. For example, if you have a 100mm full-frame lens and a 100mm APS-C lens, assuming the APS-C lens has a smaller image circle, the maximum angle of view of the full-frame lens will be bigger than the maximum angle of view of the APS-C lens. However, the magnification of the two lenses will be the same.
Hi Elisabeth,
I think @Chulgor covered pretty well my point. I can make a 100mm lens and call it wild angles as soon it can deliver an angle of view of at least 64degree. Of course you will need a very big sensor to use fully that lens. But the angle of view is still a property of the lens.
Without considering the sensor at all any lenses or telescope have an angle of view they can deliver. In the link above the angle of view of the lens on the left is larger than the angle of view on the right, however they have about the same focal length. In other world you will be able to see a larger portion of the world with the lens on the left (a larger angle of view).
Coming back to the cell phone, imagine how big would be a 5mm lens for a FF to deliver a projected circle of the size of the detector it has to deliver an angle of view of more than 160degree. Your 5mm cell phone lens is not that big because it does not need to cover such an angle of view (->smaller projected circle for a smaller sensor).
Cheers,
sly.
Great article, Elizabeth.
Your comment that telephotos are fast catching up with primes gave me a smile. I first heard that when a salesman convinced me to buy my very first telephoto lens, using the second paycheck I ever earned. I was sixteen, and the year was 1969.
But I’ll give you this, telephotos really are better now than they were a half-century ago.
Lol! Your story reminds me of my first zoom purchase, which I bought after saving all my summer pay cheques working when I was in grade 10! It was a Tamron (can’t even remember the focal length), for my Canon A1. Its optics were terrible! Yes, we have come a long ways!
Cheers!
Thank you Elizabeth for the article and the great photos. One subject I missed a bit while talking about focal lengths: depth of field and the circle of confusion leading to sharpness and bokeh.
May I drop in that the ‘angle of view’ is not the same as the ‘angle of coverage’, the latter of which I think, Sly refers above.
When I bought the Nikon FA along with a 50mm f/1.8 second hand back in 1986 I spent much for a Tamron SP 60-300mm f/3.8-5.6. The optics are not that bad, but I used the 50mm and rarely the zoom. Money spent for nothing. Today I use the Nikon AFS 80-200mm f/2.8 which, I think, is on par with my AFD 180mm f/2.8. The big advantage of fixed lenses is that they generally have much less distortion and vignetting than zooms.
Quite honestly, with all do respect, this is quite useless information. Yes, it’s all true, but how does the technical knowledge like this really aid the photographer? Just my 5 cents… it was very accurate article…
Some of us like to learn more about our craft that doesn’t necessarily lead directly to better pictures. It is just fun to learn something new. Not everything has to be written with a specific goal of teaching us how to take better pictures. It is okay just to enjoy our hobby or profession. My 5 cents…
Thanks for your comment JJ.
As I said in the conclusion, what is important is not all the jargon, but to know what telephoto, standard and wide angle lenses are, and when to use them. We have a lot of folks new to photography that use Photography Life to learn the basics. What you and I take for granted now, may be foreign to someone starting out.
Thanks again for contributing to the conversation.