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What Does The MM Mean On A Lens?

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What Does The MM Mean On A Lens?

Most photographers have a pretty basic understanding about lenses. But if that mm number printed on your lens barrel is still a bit of a head-scratcher, it’s time for a crash course in focal length. Here’s everything you need to know.

What does the mm mean?

All lenses have an “mm” number printed on them somewhere. The short definition of mm is “focal length,” which is measured in millimeters. Some lenses have a range of focal lengths, such as 18mm to 55mm, while others have a single, “fixed” focal length. We call variable focal length lenses “zoom” lenses, because you can “zoom” in or out to get closer to a subject or further away. Fixed focal length lenses are called “prime.” Prime lenses have only one mm designation for a simple reason: they don’t zoom. If you want to get closer or further from your subject with a prime lens, you have to use your feet.

If you’re like many photographers, you don’t really spend a lot of time pondering focal length, or thinking about how you can use it to improve your photographs. Focal length falls into that realm of technical details, and a lot of people just don’t want to go there. But understanding focal length is important to get the best from your lens.

Wide angle and telephoto lenses

A wide angle lens has a smaller focal length, while a telephoto lens has a longer one. Most wide angle lenses are 35mm or smaller. Telephoto lenses start at 70mm and go up from there. Wide angle lenses include more of a scene, while telephoto lenses include only a small part of a scene. You probably refer to this as “zoomed out” and “zoomed in,” respectively.

Different focal length lenses have different purposes. If you’re shooting wildlife, for example, you want a longer focal length lens (100mm is the minimum, but at least 300mm is optimum) so you can get optically closer to your subject without fear of scaring it away. If you’re shooting architecture, on the other hand, you might want a wide angle lens (35mm or smaller), because with a wide angle lens you can include all of your subject in a single shot without having to back up a quarter mile.



A lens with a 75-300mm Focal Length

When to use a Wide Angle Lens

Because the distance between the lens and camera sensor is a lot shorter on a wide angle lens, the lens itself has to be wider in order to capture enough light to make an exposure. So the peripheral vision of a wide angle lens is broader, which means it “sees” more of what is in a scene. That means wide angle lenses are very well suited to any type of photography where your goal is to photograph something that is physically large, such as that building we talked about earlier or a big, sweeping landscape.

My favorite use for my wide angle lens is kind of the opposite of shooting big sweeping landscapes or impressive buildings—I like to get really close to smaller subjects. This gives the object the illusion of being a lot deeper or longer than it actually is. That can result in a photo that seems either creepy or immersive, depending on the subject. To make this work you have to pay close attention to focus—remember that your lens can only focus to a certain point, so don’t get so close that your subject blurs.

I took this picture on the coast of Oregon. I set my lens to 18mm, got on my stomach, and focused on the footprint in front of me. Even though my lens was right next to the footprint, it still appears “normal” sized. If you are going to use a wide angle lens to bring more outside elements into a photo, try to include one “up close” subject. This helps give your image a feeling of depth and dimension.

When to use a Telephoto Lens


Taken with a telephoto lens

Telephoto lenses are especially handy when you can’t get physically close to your subject, but they have other uses as well – portrait photography, for example. In fact my favorite lens for shooting portraits is a 70mm. Focal length is important for portraits because lenses tend to cause different types of distortion at different focal lengths. A wide angle lens may exaggerate your subject’s features, which makes it the wrong choice for portraits. But at 70mm, your subject’s features start to flatten out a little. That’s because the distance between, say, your subject’s nose and her cheekbones appears less at longer focal lengths. So those longer focal lengths are actually flattering, because they can make features look smaller. Once you get past 100mm, the effect starts to become a little strange, and your subject’s face will start to look wide and kind of pancake like, which is less flattering.

You can use this effect to optically decrease distances between objects as well, and also to flatten features of inanimate objects. It’s hard to get your mind around what you can do with a telephoto lens until you actually get out there and start experimenting, so if you own a lens with a longer focal length, I encourage you to spend a day playing around with it and comparing images shot at different focal lengths. Do this with your wide angle lens, too. The only way you can really start to get a good feel for focal length and how it can change a scene is if you experience those differences in your own work.

The Nerdy Definition

Now that we’ve delved into some of the uses for different focal lengths, let’s get into some of the nerdy technical stuff. Technically speaking, the focal length of a lens is the distance between the lens and the back of the camera, where the image is formed. For example, in a 55mm lens, there are 55 millimeters between the lens and the camera’s image sensor. Now, this is actually pretty meaningless information to you because the lens is housed in a protective shell, so you couldn’t actually measure that distance even if you had a reason to (which you don’t). But that’s where that number comes from.

There are also 55 millimeters between the lens and the spot where light focuses in front of the camera. This works in a similar way to the way that your eyes work. Try this: place your hand a foot away from your face and focus on it with your eyes until it looks crystal clear. Now, slowly move your hand towards your face. Keep trying to focus on your hand as you move it.

No matter how hard you try to keep your hand in focus, at a certain distance you just won’t be able to do it anymore. When this happens, you have passed the focal point of your eyes. Most human eyes focus at or near 50 millimeters, so that means that the focal point of your eyes is 50 millimeters away from your face.

But we’re not nerds, we’re photographers!

Well, photographers kind of are nerds, but that’s beside the point. Anyway after you’ve absorbed the nerdy part of this knowledge, you can use your more general knowledge about focal length to help improve your images. The key is to think about size and distance.

  • When you’re close, go wide. Use a wide angle lens with a smaller mm focal length like 18mm.
  • When you’re at a distance, go long – a telephoto lens with a larger mm focal length number like 200mm or 300mm.

And don’t forget to consider perspective, too—use your knowledge about focal length to visually exaggerate or reduce the distances between objects. It’s a kind of magic, when you think about it.

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About the Author ()

David Peterson is the creator of Digital Photo Secrets, and the Photography Dash and loves teaching photography to fellow photographers all around the world. You can follow him on Twitter at @dphotosecrets or on Google+.

Comments (88)

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  1. siva says:

    great article, I got knowledge about camera lens

  2. Mr.Vijay Kumar says:

    Hi David, I will be on holidays traveling to Europe soon in Dec 2015. I am planning to buy a camera, preferably Nikon DSLR to take photos during my visit. I normally would like to take photos of historical buildings, monuments and landmarks in the background with the subjects ( myself and my family) in the forefront.

    What type of lens would you recommend, the focal length( the range in mm) and the type of lens which would capture larger area of the background. Please reply with simple explanation at your convenience. Thank you.

  3. Mr.Vijay Kumar says:

    Thank you for your generous knowledge sharing about photography. Your explanation about photography for beginners like me is very easy to understand, you made it very simple. Thank you

  4. kaddu Emmanuel says:

    This is so so so so informative. Am planning on starting up a photo studio. So I needed some break down of it. Thanks buddy

  5. Priscilla Mitchell says:

    This was just the article I needed, my fellow photographer friend said that he used a 18mm lens to get a shot of something, and I had to know what the mm meant. I had learned before, but the whole lesson escaped me. This described it simply!

  6. Darshan says:

    Very informative post. Person like me wants to learn very basic then the rocket science. I believe, if we are perfect in basics, then we can do the wonders.

    • The Spider says:

      I felt the same way. I didn’t want to go to Harvard to learn the basics that were offered in this article. I wish more compositions were done with the author’s bent towards technical writing so we can understand what is being said.

  7. Bob.Pearce says:

    Recently in a secondhand shop I saw a lense which the owner though was from an old projector but he did not know ? It marked 150.300 which I assume is the focal length and the actual DIAMETER of the lens was around 150 mm .As a beginner photographer the diameter of the lens took me some time to realise that it was seldom part of the discussion .
    As a participant in the recent dashes which required taking a picture using an external lense and having several times tryed to photograpg the moon this lens set to thinking about diameters of lenses .and what would happen if I could somehow mount this large lense onto my camera . Yes there would be extra focal length of between 150 to 300 on top of the focal length of the existing mount , but what would an increase in diameter do ?

    • Hi Bob,

      The width of the lens won’t make much difference, as your camera will still just be using the middle part of it. The only time consumer camera lenses get larger is usually for special effects like wide angle lenses.

      You won’t be able to mount the lens along with your existing lens – the optics won’t work together. However, you might try ‘free lensing’ – take your current lens off and hold this lens in front of your camera. See more here: http://www.digital-photo-secrets.com/tip/3594/the-art-of-freelensing/

      David.

  8. Deepali says:

    Gr8 post !! good ones for starters,it gives good knowledge of lenses.

  9. Michael says:

    Great article! Very helpful for me, a novice who doesn’t know much. One thing though… [Micheal Jackson -> Michael Jackson]*** That’s the late, great King of Pop!!

  10. walt says:

    Great article Dave, I didn’t understand the jargon although I have owned a DSLR kit for more than a year. This will definitely have an effect on my future shoots. I was really moved by the telephoto effect, its an amazing trick. Thanks

  11. Ambreen says:

    Hi,
    Thanks for a great article. Can you suggest me whether a telephoto lens or a wide one would be suitable for capturing fireworks?

  12. Megan Arce says:

    i always thought, too, that cameras work the same way our eyes do. i feel glad to have stumbled upon this article, which also increased my understanding about photography in general. thank you for making it easy for aspiring photographers like me. :D

  13. @Bruce,

    Yes, anything less than 24mm can be considered a wide angle.

    David.

  14. Bruce says:

    Thanks, great article. One question though, at what point or distance is a wide angle lens considered a wide angle lens? Like as in as defined lower than 24mm for example? I am new to all this and I must admit, some of these camera geeks are really confusing me! :)

  15. Manzar Alam says:

    Thank you for the article. I want to undetstand the difference between a 300mm lens vs a 55-300mm lens. Does 300mm lens cannot be zoomed out to less than 300??

  16. soni says:

    thanK you a ton:)

  17. Karl Baker says:

    not wanting to confuse people, but I also like taking landscapes with a 70-200 zoom, obviously I’m shooting from a long way away but it gives a great feel to the photo and great dof

  18. Jen says:

    Thank you so much for your explanation. It was clear and easy to understand. I am considering a new lens for my nikon d80 and this article really answered a lot of questions that I had. I can’t wait to learn more for you.

  19. @shwetank.

    See this article for an explanation of f-stops and which to use for portraits with the background out of focus.

    David.

  20. shwetank says:

    hi thanks for a nice explanation. Can you please explain what is F-stops (f/4 etc) and which is best to get a nice flat portrait photo. Also which will be the best lens to get a flat face and a good Bokeh with Sony SLTa58, should I go for a 17-70mm f/2.8-4.5 lens or 70-300mm f/4-5.6 lens

  21. ajithaa says:

    Thank you David, as to be expected, your explanations are thoroughly educative and useful.

  22. Genevieve says:

    I am just wondering what would you suggest for a lens size for taking product pictures.

  23. Mir Bashir says:

    Hello Mr. David, Thank you very much for this very useful post. After a very long time I came to understand what is a telephoto & wide angle lens. I now understand what is 17-55 etc. written on the lenses.

  24. greg says:

    You beat my professor explanation, now I understand what 55mm is all about. thank you.
    would be nice if you could share another simple explanation about what’s the meaning on the conversion factor, from having full frame and the DLSR cameras that are not full frame.
    The explanation you just gave on 55 mm. I like both explanation it took a mixture of both to really understand it, that is the nerdy and the simple one.
    If you do both for the conversion it will be great.
    I understand that nikon is 1.5 and canon is 1.6 but i have no idea of what they are talking about.

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