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.
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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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