Focal Length in Plain English

I’ve talked about lenses so much but -can you believe it- I’ve never actually done a post about focal length.

Focal length is those main numbers you see on your lens. Like: 50mm or 24-105mm or (in my current dreams lens) 70-200mm. You may already know the basics of these numbers and that 50mm is ‘wider’ than 200mm. And 24mm is even wider than 50mm. And 200mm is ‘zoomy’. But do you actually know what it means? Well, then, I’ll tell ya!

Focal length is the amount of space {in millimeters} from the optical center of your lens to the camera’s sensor. Much like a magnifying glass, the further away the glass is stretched from your sensor, the more magnified the subject will be in your viewfinder and on the sensor when the image is captured. So when you look through a 24mm lens, you may be seeing the same field of view that you see through your naked eye, but a 200mm zoom lens magnifies just one portion of that scene; wherever you have it pointed at the moment.

So you can see now why what you see through the same lens on a crop sensor camera and a full frame camera are very different. It’s not the lens making the difference {because the measurement from the center of the lens to the sensor is the same} but, rather, the sensor making all the difference. Because one sensor (the cropped sensor) is still magnifying the subject the same – it’s just cropping the edges (like cutting the crust off the bread).

  • HI Elizabeth, Thank you for this, this is clear and simple to understand. I have a question though, there are obviously different kinds of lenses, eg. for the Nikon range, the DX and FX – for crop sensor and full-frame respectively. If, as in your article, the lens makes no difference, then why do we have these 2 types of lenses? Any answer in plain English would be appreciated! :)  Thanks. Lily

  • Soly

    Hi Elizabeth…
    why are some lenses with the same focal length so different in price? they can double some times.

  • I just love your blog!

  • Jagparrish

    It’s becasue of the aperture.  The wider the aperture the more expensive the lens.  For example a 50 mm 1.8 is around $125, a 50 mm 1.4 is around $400 but a 50 mm 1.2 is almost $1300.  The wider aperture let in more light and are always more expensive.  Hope that helps a little.