What mirrorless lens (video) equates to a camcorder 20X optical zoom?

Started 5 months ago | Discussions thread
Mark Scott Abeln
Mark Scott Abeln Forum Pro • Posts: 18,166
Zoom ratio is not helpful

Zendit_Outdoors wrote:

What size (focal length) lens would a guy need on a mirrorless camera (say a Sony A7Siii) to be able to 'reach out' and film video at the same distance as a prosumer camcorder boasting a 20X optical zoom?

Find all of the zoom lenses available for your Sony. Divide the longest focal length by the shortest focal length and that gives you the zoom ratio. Whatever lens you find that beats 20x, then you'll win. However, this doesn't answer your question below.

Real world scenario:

Say there's a elk standing 250 meters away in a meadow and your buddy has a camcorder with a 20X lens and you want to meet (or, preferably, beat!) the quality of his video, what focal length lens would you need? In other words, you want to elk to take up as much of the screen (assume the same image ratio is being used) as it will in your buddy's camcorder? Even better if you could get a little closer and clearer.

The zoom ratio tells you nothing about either the longest or the shortest focal length, and hence it will tell you nothing about what angle of view you'll get on the elk at the longest focal length.

This formula should help:

Focal length / sensor width = subject distance / field of view at the subject.

This is the "law of similar triangles" from geometry.

If you know how far away the elk is, and if you know how tall the elk is, and if you know the size of your sensor, then you can algebraically determine what focal length you'll need. Whatever the zoom ratio of the lens you have, it must have a focal length that is somewhere within the zoom range.

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