What Does The MM Mean On A Lens?

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What Does The MM Mean On A Lens?

In a word, MM shows the focal length. But I’m sure you wanted to learn much more than that. You want to know what focal length means and how an understanding of it can improve your photography. As you have probably noticed, a lot of different lenses have different focal lengths. Some of them have a range of focal lengths, like 18mm to 55mm while others have a single fixed focal length. We call the lenses that cover a range of focal lengths “zoom” lenses, because you can easily change the focal length and “zoom” in or out on your subject. Other lenses are called “fixed-length” lenses because they only have one focal length and thus can’t do the zoom.

The Nerdy Definition

If we were to be as nerdy and mathematical as we could get, the focal length would be the distance between the lens and the back of the camera where the image is formed. In a 55mm lens, there are 55 millimeters between the lens and the camera’s image sensor. You can’t see this distance because the lens is housed in a protective shell, but if it weren’t, you would be able to measure it.

There are also 55 millimeters between the lens and front of area of the camera where light focuses. Try the following. Take your hand and place it a foot away from your face. Focus on your hand and make sure it looks perfectly crystal clear. Now, slowly your move your hand toward your face as you continue to pay attention to it.

No matter how hard you try, your hand will eventually become blurry and out of focus at a certain distance. When this happens, you have passed the focal point of your eyes. The focal point for your eyes is a point exactly one focal length away from the lens in your eye. Most human eyes focus near 50 millimeters, so the focal point is 50 millimeters away from your face.

But We’re Not Nerds, We’re Photographers!

I know. I’m getting to that. Here’s how you can use a knowledge of focal length to your advantage. Have your ever overheard the guy at the photography shop talking about his wideangle and telephoto lenses? Have you ever wondered what makes a wideangle a wideangle and a telephoto a telephoto? The answer is focal length.

Wide Angle and TelePhoto Focal Lengths

A wideangle lens has a smaller focal length while a telephoto lens has a longer focal length. Typically, most wideangles start at 35mm and move down to 1mm. Telephoto lenses start at 70mm and go as high as 1700mm. With a wideangle, everything appears “zoomed out”, and telephoto lenses allow us to “zoom in” as much as we want.

Each lens and focal length have different photographic purposes. Sometimes you need to get close to a subject without disturbing it, like in wildlife photography, so you would use a telephoto lens. At other times you want to incorporate an entire scene, so you would use a wideangle.

A lens with a 75-300mm focal length

When To Use A Wideangle Lens

Because a wideangle lens has a focal length closer to your camera’s image sensor, the lens itself has to be wider across to capture enough light to fit the image into the frame. This means wideangle lenses have more peripheral vision and can “see” more of what is to their side.

Many people use wideangle lenses for landscape photos because the lens pulls more of the landscape into the picture. While this is a great use, it doesn’t work if you don’t already have a great composition in mind. In other words, purchasing a wideangle lens won’t make you an overnight landscape photography success.

I personally love to use wideangle lenses to get really really close to my subjects. Try it out. Get a 24mm lens, or a zoom lens that includes 24mm within its range, and get as physically close to your subject as you can before the image goes blurry. When you take the picture, your subject will appear to have more depth. Sometimes this effect can be creepy, and at other times it is immersive.

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 wideangle lens to bring more outside elements into a photo, try to include one “up close” subject.

When To Use A Telephoto Lens

Taken with a telephoto lens

Telephoto lenses are especially handy when you can’t get any more physically close to your subject, but they have other uses as well. Did you know, for example, that a 70mm lens is excellent for portrait photography? Here’s why.

There is something known as the “telephoto effect”. Whenever you zoom in on something that’s very far away, the image you get back is kind of flat. It’s very similar to what you experience when you go on a long hike, and there is a mountain in the distance. It doesn’t seem very far away until you spend hours trying to get to it. Far away things are flat until you get within a certain range.

Okay, back to people. There’s a reason some people get nose jobs, and unless you’re Micheal Jackson, it’s probably because your nose is a little too long. Instead of using plastic surgery, you can use a telephoto lens to flatten your subject’s face. Even normal people look a little strange very close up. Have you ever gotten really close to a mirror and looked at yourself? The extra depth is very unflattering.

The telephoto effect can be applied to a lot of other situations where you want to create a flattening effect. One famous photographer used it to flatten a faraway crowd and make the people seem impersonal. There are many other uses, and only experimentation will give you a photo you want.

Now that you know about focal length, try out some of these different lens uses. What different effects can you achieve?

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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 (86)

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  1. 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.

  2. 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

  3. 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

  4. 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!

  5. 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.

  6. 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/


  7. Deepali says:

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

  8. 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!!

  9. 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

  10. Ambreen says:

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

  11. 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

  12. @Bruce,

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


  13. 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! :)

  14. 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??

  15. soni says:

    thanK you a ton:)

  16. 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

  17. 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.

  18. @shwetank.

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


  19. 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

  20. ajithaa says:

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

  21. Genevieve says:

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

  22. 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.

  23. 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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