What Does The MM Mean On A Lens? :: Digital Photo Secrets
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What Does The MM Mean On A Lens?

by David Peterson 114 comments

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.

Most people think this post is Awesome. What do you think?


  1. Nevin Tom says:

    Hi David,

    I have recently bought a Canon Rebel T6(1300D) with the kit lens 18-55mm only. I am more interested in clicking portraits. So which lens should I add to my camera? A 55-250mm Tele lens or 50mm f1.8 Prime lens?

    Many Thanks :)

    • David Peterson says:


      For portraits, I'd definitely go with the Prime lens because it has a wide open aperture that means you can get a much more pleasing 'blurry background' look than is possible with a telephoto.


  2. Ryan Gainer says:

    Yeah I'm just starting out again with amateur photography got a Canon Rebel t6i and I was thinking about getting a 50-millimeter single Focus lens what the amateur 1.8 would this be good to use in a church setting for a baptismal please respond back as quickly as she can as this will be next Sunday thank you and have a great day

  3. Leonard Banks says:

    I have a 70-300mm sigma lens, you said to take portraits it is best taking we a 70 or 100 mm focal length. But could you please tell me the best camera settings for portrait photography please, I enjoyed reading about the way you explain about different types of lenses. Thank you. Len.

  4. Cynthia says:

    Hi David,
    I'm getting into concert photography, what is the best lens for this. I want to be able to go wide or zoom in while in the photographers pit and then when out of the photographers pit really zoom in. I'll most likely have to go behind the audience. Is there one lens to get for this or will I have to get two lenses. I also need low aperture for low light.

  5. Mo says:

    Hi David

    Just starting out and I'm working with portraits with blurred back grounds and of close up imagines with blur back ground.
    I have a Sony a5100 with a 3.5-5.6 15-50mm lens.
    im thinking of getting a second lens for portrait pictures. Should I get a prime lens 35mm with a f1.8 or a 50mm with a f1.8?

    • David Peterson says:

      HI Mo,

      For portraits, definitely go for the 50mm prime lens, not the 35mm lens. The more wide angle you go, the more you'll see big noses and large foreheads, which aren't flattering. In fact, I'd actually recommend a 100mm lens for portraits because it further reduces the size of your subject's nose.

      I hope that helps.


  6. Liwen says:

    I own a 18-55mm lens and a 70-300mm. What's the lens in the middle for an amateur photographer?

  7. Pat Serio says:

    Mr. Peterson: I use a Canon EOS Rebel T5 1200D. The kit lens is 18-55mm; I just bought a telephoto 55-250mm - EF-S, by mail-order. I am taking my first vacation since 1999, in one week, to the Pacific Northwest. I hope to use the 55-250mm to be able to capture distant waterfalls and the awesome rock structures that the Pacific Ocean is known for. Will I be OK with this lens? Also, I know Depth of Field as narrow in the foreground, becoming wider in the distance. Can the 55-250mm do that? If so, are you able to instruct me in how to do this? So many thanks!

    • David Peterson says:

      Hi Pat,

      Congrats on your holiday!

      Yes, the 55-250mm will work fine for zooming into rock structures on the Pacific Ocean. Just make sure you take a tripod too because unless you have fast shutter speeds (faster than 1/250 sec) you're likely to get blurry images at maximum zoom.

      Depth of Field is narrower closer to the lens, but when zooming into objects in the distance, it won't matter too much.

      Make sure you take some nice landscapes while you're there as well! I have a course on landscape photography if you're interested: http://www.digital-photo-secrets.com/courses/landscape


      • Pat Serio says:

        Thank you so much, Mr. Peterson! I have signed up for your 5-day Landscape photography course. I'll see what I can absorb in the short time left before leaving for my trip.
        For me, a tripod is essential. I love capturing waterfalls and making white water along the surface with a slow shutter speed. I was able to make a lovely video of a waterfall, yesterday. My new tripod has a ball on the head, so I was able to slowly pan L-R and Up and Down the falls - just lovely!

  8. Paul Spears says:

    To me a beginner , your about as awesome as it gets. At the age of 64 years old I finally have the time to explore the things I always really want to explore. Photography has always been at the top of the list. I love the way you explain things. I have always been told keep the ISO as low as possible and have NEVER explored high ISO's. 200 ISO has been my largest leap . Thank you for posting these things. I now have your website on my computer. Again Thank you for sharing it to us newbies even at 64 years old.

  9. Mike Evaristo says:

    I recently purchased a Sony Alpha a350. It's a 3.5-5.6/18-70 zoom lens. Just now getting into digital and would like to know what the mm of the lens is? Any assistance would be appreciated.

    • David Peterson says:

      Hi Mike,

      A 3.5-5.6/18-70 lens has an 18-70mm focal length. The other two numbers (3.5-5.6) means your lens can support an aperture of f/3.5 when zoomed out and f/5.6 when zoomed in.

      I hope that helps.


  10. Dewey DOVE says:

    Back when I was using the SLR film camera I loved playing and looking something very different in my photos. This one trick that gave really different photo. I took my 55mm mounted on the camera set the opening wide open, then using my 135mm
    I opened it wide open then placed them face to face using the lens shade to cover any opening. This is the fun part, you can not focus on the subject in the normal way you must get really close on your subject slowly rocking back and forth then shot it. You get really super close to your subject.
    A friend of mine back when painted Medieval characters, mounted them on pennies. My job was to make them look larger..

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