How do focal length and sensor size relate to a lens’s actual magnification?
Asked 6/3/2015
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I understand that a lens’s “3x zoom” only describes the ratio between its shortest and longest focal lengths, so it doesn’t tell me how much closer a subject will appear. For example, 15–45mm and 100–300mm are both 3x zooms, but the 300mm lens clearly reaches much farther than 45mm.
So what specification actually describes how much a lens “zooms in”? Is it focal length, sensor size, or true optical magnification? Why don’t manufacturers usually state this as a simple magnification number?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
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I think the answer you seek is in terms of "angular magnification" as it is used with binoculars.
In the world of binoculars there is a number that is referred to as "magnification". For example a 10×45 set would offer 10× magnification (and a 45mmØ ocular). This means that the subject appears 10 times closer than with the unaided eye. This isn't the same as optical magnification used in photography.
With this understanding, a 50mm lens (or 50mm equivalent if you have a sensor that is not fullframe) sees roughly what the unaided eye sees so it offers 1× "magnification".
With this interpretation in mind:
- A 300mm lens would offer 300/50 or 6× magnification.
- A 45mm lens would offer 45/50 or 0.9× magnification
Originally by user17634. Source · Licensed CC BY-SA 4.0
user17634
11y ago
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Generated from our catalog & community — verify before relying on it.
“3x zoom” is only a range ratio: max focal length divided by min focal length. So 15–45mm and 100–300mm are both 3x, but they do not give the same view.
What most photographers use to describe how “zoomed in” a lens looks is focal length together with sensor size (or 35mm-equivalent focal length). Sensor size matters because the same focal length gives a narrower or wider field of view on different sensors.
As a rough viewing comparison, a 50mm lens on full frame is often treated as about 1× relative to normal human vision. By that convention, 300mm is about 6× and 45mm is about 0.9×. On smaller sensors, use the equivalent focal length instead.
Strictly speaking, true optical magnification is a different concept: it is the ratio of image size to subject size, often discussed for close-focus or macro work. So zoom ratio and magnification are not the same thing.
That’s why manufacturers usually quote focal length (and sometimes equivalent focal length), not a single magnification number: magnification depends on how you define it, and on sensor size and shooting distance.
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