Does a lens’s zoom ratio match the change in subject size on the sensor?
Asked 2/14/2022
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2 answers
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If a subject stays at the same distance and I zoom from the shortest to the longest focal length, should its size in the image scale by the same ratio as the lens’s zoom range? For example, with a 70–210mm lens, would a distant subject appear about 3× taller in pixels at 210mm than at 70mm?
I tested this idea on a surveillance camera advertised as having 5× zoom by photographing the same distant object from the same position at minimum and maximum zoom, then comparing the object’s pixel height in the two images. I only measured about a 3× increase, and the manufacturer replied that “zoom number is not equal to magnification.” Is my understanding wrong, or could something else explain the discrepancy?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
4y ago
2 Answers
2
I don't see an error in your basic understanding.
However, a lens' stated focal length is only accurate when focused to infinity; which is another way of saying, when the lens is focused so that everything beyond a particular distance is in reasonable focus (i.e. maximum depth of field). This distance is called the hyperfocal distance and it varies with the lens' focal length, aperture setting, and sensor size. In this case it would need to be set for the longer zoom position and (resulting) aperture... the hyperfocal distance can be found with most DoF calculators.
If the lens is focused to less than infinity it can introduce focus breathing; which is an effective focal length (magnification) less than indicated... that could be part of what you are seeing. But usually this is not as significant and requires the lens to be focused far short of infinity.
And manufacturers tend to round numbers; some out of convenience (e.g. 59.5mm is labeled 60mm), and some because they think it sounds better (e.g. long effective focal lengths due to crop factor). I would guess that at least some of what you are seeing is because of something like that.
Originally by user70370. Source · Licensed CC BY-SA 4.0
user70370
4y ago
0
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Your basic understanding is correct. For the same camera position and a sufficiently distant subject, the subject’s image size on the sensor is approximately proportional to focal length. So a true 5× zoom range should give about a 5× change in subject size, and a 70–210mm lens should make a distant subject appear about 3× larger at 210mm than at 70mm.
Why “approximately”? Because stated focal lengths are most accurate at infinity focus. At closer focus distances, some lenses change effective focal length as they focus (“focus breathing”), which can reduce the apparent magnification at the long end. But at distances like yours, that usually should not account for a drop from 5× to only 3×.
So if your test was done from the same position on the same distant subject, your method is reasonable, and a large mismatch suggests the camera may not be delivering a true 5× optical zoom across the captured image.
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