From the course: Photo Gear Weekly

Focal length and magnification

From the course: Photo Gear Weekly

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Focal length and magnification

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- I think it's fair to say that basically every photographer is familiar with the basic relationship, or at least the basic concept of focal length as it relates to essentially magnifying a scene. With a longer lens, you can reach further out to get a subject, to a capture a photo of a subject, making that subject appear larger in the frame. But what does the millimeters of the lens really mean in terms of how much more magnification? I was thinking about this recently when I considered a 70 to 200 millimeter lens and then, from the same company, a 70 to 210 millimeter lens. Why might I choose a 210 millimeter maximum on that 70 to 200 or 70 to 210 versus just 70 to 210? What will that extra 10 millimeters of focal length really get me in terms of magnifying the scene that I'm photographing? You can use a very rough formula to get a sense of what that magnification is. It is really a rough formula because ultimately, magnification would relate to how the image is finally presented. For example, the print that you might produce from a photo. But as a general rule, if you divide the focal length by 50, then you'll come up with a magnification factor. At the shorter end of the range here, the 70 millimeter range on a 70 to 200 millimeter lens, you can think of that as about a 1.4X magnification factor. And all the way out at 200 millimeters, that gives us a 4X magnification factor. What does the extra 10 millimeters on that 70 to 210 millimeter lens give us? Well that gets us up to 4.2, so it's just a 5% increase, but it's increasing magnification and that will be impactful in the final image. Of course, I'm not sure I would choose a 210 millimeter version of the 70 to 200, a 70 to 210, over 70 to 200 just based on trying to get that little bit of extra reach. But the key is to understand that focal length really does translate into how much the scene is being magnified and essentially how much we're cropping that scene right in the camera so that we can get a far away subject to appear a bit closer in our photos.

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