How can I explain a lens’s focal length to non-photographers?
Asked 3/30/2016
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Friends and family often ask how far a lens like 300mm can "reach," but focal length numbers don’t mean much to them. Is there a simple way to describe what a 300mm lens shows? For example, is there an easy rule of thumb for what size subject will fill the frame at a given distance, especially compared with normal human vision or binoculars?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
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I have often been asked this question and I use a "rule of thumb" explanation.
On a full-frame (or 35mm) body, a 50mm lens is about the same field of view as a human eye. So you could say that 300 mm is about 6x your normal vision - or similar to a 6 power rifle scope.
If you have a crop sensor, you would multiply that by, say, 1.6... so 6x1.6=9.6 or about 10x your normal vision. The quick (and not entirely accurate) answer is, "It's about 10x your normal vision, it'd be close to a 10 power scope or binoculars."
The other question I get a LOT is "how many times zoom is that lens"... I usually answer it the same way, but you could add that a fixed 300mm is about 6x your normal vision, while a 70-200 is about a 3x zoom (200/70 ~ 3). Does that make sense?
Originally by user48223. Source · Licensed CC BY-SA 4.0
user48223
10y ago
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Generated from our catalog & community — verify before relying on it.
A simple, non-technical way is to explain focal length in terms of field of view rather than “reach.” On a full-frame camera, a 50mm lens is commonly treated as roughly similar to normal human vision. That makes a 300mm lens about 6× narrower in view, so you can describe it as roughly like a 6-power binocular or scope. On a crop-sensor camera, multiply by the crop factor, so 300mm can feel closer to about 10× “normal view” on a 1.6× crop body.
For non-photographers, visual examples usually work better than formulas. Showing side-by-side photos or using an interactive focal-length simulator is often the clearest explanation.
If you want a rough framing rule: to keep the same subject size in the frame, doubling the distance requires roughly doubling the focal length. So an 85mm lens frames a subject from about twice as far away as a 40–50mm lens, and 170mm does so from about four times as far. These are approximations, but they’re easier for most people to understand than exact math.
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