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Lens Focal Length Explained

| November 13, 2009 | 9 Comments
Upgrading from a compact digital camera to a dSLR ? Or if you are just trying learn more about lens focal length and how it relates to zoom factor look no further. In this guide we will briefly explain various focal lengths (and how to interpret them) and lens types like wide angle and telephoto lens, as well as zoom and prime lens.


Pentax dSLR with lens

Pentax dSLR with lens

When you are on a lookout to buy a new compact digital camera, one of the many features that we are bombarded with is optical zoom. Today’s compacts pack from 2x optical zoom up to a whooping 26x zoom. Simply put optical zoom is a parameter indicating the ability of the lens to visually “make” subjects appear closer or further away from the camera. And I must admit, having a decent optical zoom level certainly gives you a nice advantage when taking photos. So it only comes natural that when you decide to purchase your first dSLR, it is one of the parameters that you would be looking out for. The interesting part is that in dSLR’s this parameter is nowhere to be seen. If you ask a salesman about it, he starts throwing some numbers at you such as 18-55mm, 50-200mm, etc. We will come to the meaning of these numbers in a moment, but remember this: You can find out optical zoom level (if any) if you divide these 2 numbers (e.g. 200/50 = 4x). That’s it. Period.


How to interpret focal length ?
Focal length of a lens is a technical aspect of camera design that you do not have to worry about: you just need to know what effect it has. Simply put the higher the focal length number, the more magnification a lens has. Check out the diagram below for better understanding of focal lengths and how they relate to magnification level. Note that focal length equivalent of a human eye is 50mm. That means if we observe an object through 50mm lens, we will see it exactly the same as with naked eye.

Focal Length Comparison Diagram

Focal Length Comparison Diagram

This sequence of 5 photos was taken from the exact same location, using lens with different focal lengths. Observe how the cameras angle of view decreases with an increase in focal length, resulting in the main subject being “zoomed in”. According to the focal length, lens can be divided into:

  1. 10-35 mm – Wide, Ultra Wide and Fisheye lens. Lens with large angle of view, and are as such suitable for architecture, landscape, as well as indoor photography. (Hint: Use them to make the rooms look more spacious).
  2. 35-70 mm – Normal lens. Considering that human eye equivalent of the focal length is around 50mm, those lens cover pretty same angle of view as human eye. These lens are mostly used in street and documentary photography.
  3. 75-135mm – Medium telephoto lens. These lens have 1,5-2,5 magnification factor and are mostly used also for portraits (Hint: longer focal length makes it easier to blur background – see article on DOF, and aperture).
  4. 135-300 mm and over – Telephoto lens. Lens with big magnification factors (2,5x up to 10x), very useful for wildlife photography where keeping distance from birds and other wild animals is essential to a successful photography. They are also very common in sports and also favored by paparazzi ;-).

In addition, some lens have fixed focal length, while focal length on other lens can vary their which leads us to another important division:

Prime and Zoom lens example

Prime and Zoom lens example

Zoom lens (left on the photo)- lens with variable focal length, and most common starter lens (also called kit lens). Their focal length is usually expressed as range (e.g. 18-55mm, or 70-300mm). As we mentioned earlier in the guide, by dividing these 2 numbers, you can come up with zoom factor of the lens. The main advantage of this type of lens is the creative flexibility given to the photographer, since he can use zoom as a powerful compositional tool without the need to move around. This flexibility comes with some cost in quality, which brings us to…

Prime lens (right on the photo) – lens with fixed focal length, expressed as a single number (e.g. 50mm). Although these type of lens do not have the flexibility of the zoom lens, they more than make up their worth in terms of image quality. Prime lens are usually sharper, and come with higher quality optics, optimized for their focal length. Most popular primes revolve around 50mm. Some purists say that these type of lens are the best ones for photographers eager to learn, since they force the photographer to move around and explore new perspectives when composing an image.

Both types of lens have their usage, and their supporters. While a single ultra zoom lens e.g. (18-250mm) is ideal for travel photographers, since it substitutes for a heavy bag full of lens, its image quality cannot match the sharpness and the picture quality of a good prime lens. So where to start ? That depends on you and your choice of subject. If you are just buying your first dSLR, a kit lens (often 18-55mm zoom) is a good place to start, since it is inexpensive, and yet flexible enough until you figure out which type of lens do you need.

Let us know what your favorite type of lens is. Happy shooting.


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Category: Beginners Photography, Photo Equipment, Photography Lessons

About the Author (Author Profile)

Alan Graf is the editor and founder of Digital Photography Student. He lives in Stockholm, Sweden, and is also editor of CRO-Wallpapers.com – a Croatian wallpaper archive, and his own photo gallery at http://www.alangraf.com.

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  1. […] lens, and a decent set of features. Some compacts might even have a decent zoom that covers broader focal length range than an initial (kit) lens of a DSLR. Diagram below illustrates the lens focal range for an […]

  2. […] you would like to learn about lens’ focal length, check out our Lens Focal Length Explained article. This entry was posted in Beginner Photography, Video Tutorials and tagged focal length, […]

  3. […] among the beginners in macro photography. Your subject will appear the same size regardless of the focal length you are […]

  4. […] sharper than zoom lens. This is understandable since the prime lens are optimized for a single focal length, and are much simpler to produce with great […]

  5. […] have used the following settings on my Pentax K200D (manual mode): 1. ISO set to 100 2. Lens 18-55mm kit lens set to 18mm 3. Aperture set to minimum (f/22) 4. Shutter set to 30sec Tuk Tuk […]

  6. […] effects are even more pronounced with a telephoto zoom lens. So, our photography tip of the week is: “Try using your kit zoom lens for flower […]

  7. […] Lens focal length […]

  8. […] Graf | Published April 19, 2010 Owning a DSLR means that you are no stranger to changing the lens. And more often than not, the change occurs somewhere in the field (unless you are a studio […]

  9. […] Lens Focal Length Explained — by Alan Graf […]

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