Small Girl, Big Ideas

Photo Tip :: Understanding Lenses

During this post, we were giving away a Canon Rebel T3i with a 18-55mm f/3.5-5.5 lens. But, what do those numbers mean?!?

First, let's tackle focal length. For example, the lens below has a focal length of 50mm.

Focal length is the numbers that end in mm. They measure the space between your sensor and your lens. These basically translate into how wide your lens sees. A lower number is a wider lens, a higher number is a more zoomed in lens. For reference, on a full frame camera (a 5D), a 35mm is about your eyes' viewing. On a camera like a Rebel or Nikon 3000 series with a APS-C crop sensor, this would have to be a wider angle lens. What happens on most cameras you all probably own, is that you have a crop sensor. Meaning, with all lenses, your camera actually multiples that focal length by 1.6. So a 35mm lens on my camera ends up 56mm. So a 50mm is a much closer crop lens on a Rebel or 60D than it would be on a 5D.

Generally, anything wider than 20mm is considered wide angle. You'll notice some distortion with the lens, almost a 'barrel' effect, where if you were to shoot a wall the wall would not appear straight, but slightly curved. 30-85mm is a standard lens - a pretty normal viewing angle. Getting above 85mm and into about 150mm is a medium telephoto lens. Generally these lens are a bit larger but nothing too crazy. Above 200mm is a telephone lens and is quite zoomed. Personally, I have never found much use for my telephoto lens, a 70-300mm lens, and am selling it. For most purposes, the kit lens like the one that comes on the Rebel T3i is great for most things. I like having a good, standard, prime (meaning only 1 focal length) lens on my camera.

Next - aperture. Aperture is what determines your depth of field. The lower the number, the less depth of field (meaning blurry backgrounds). The higher the number, the higher depth of field, for landscapes usually. So why does the 18-55mm lens have 2 numbers?

The f/# represents the widest stop that the aperture can be set to. If you see 2 numbers, it means that when set to 18mm, the lowest focal length, the lens can be opened up to an aperture of 3.5. At 55mm, the highest focal length, it can be opened up to an aperture of 5.5. Prime lenses, like a 50mm, will have one number, for example 1.4.

If you shoot a lot of low light photography or want blurry backgrounds, having a lens with a wide aperture like 1.4 will make a huge difference. If you shoot mostly in daylight, then pretty much any lens will work well for you.

Hopefully that helps decipher lenses a bit more for you! Now go enter to win a DSLR yourself!


  1. I've never used a Canon camera other than my point and shoots. I've typically been a Nikon SLR/DSLR, but still hoping to win :)

    Rebecca {at} Preppy Panache

  2. Thank you! These posts of yours are always so enlightening for me.
    Major ShenANNIEgans

  3. Thanks for the tips, I have my kit lens and the 50mm f/1.4.....I love it. :)

  4. Def bookmarking this for future reference! Hoping to get a DSLR soon!


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