I've read How do I convert lens focal length (mm) to x-times optical zoom?, and learned that the times-zoom number doesn't tell you much. So, how do you determine the magnification — the amount a lens let's you "zoom in" — from focal length data? Why don't the lens manufacturers just tell you? Maybe there is another factor involved?

For example, 15-45mm and 100-300mm are both "3x zoom", but doesn't the 300 offer more "zoom in" capability or magnification than the 45mm lens?

If so then how does one measure the magnification abilities of a lens?

  • \$\begingroup\$ Oooh, I knew I saw a question similar to this somewhere before. :) \$\endgroup\$
    – mattdm
    Jun 6, 2015 at 0:07

3 Answers 3


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

Zoom is zoom and magnification is magnification.

Apples and oranges.

Optical Magnification is the ratio between the apparent size of an object (or its size in an image) and its true size.

It is calculated with the following formula:

M = (di - f) / f

with di as found in the following image: enter image description here

Zoom is the ratio of focal length as you mention it in your question:

zoom = maximum focal length / minimum focal length.

  • \$\begingroup\$ Ho = height of the object ? \$\endgroup\$
    – Jdams
    Jun 5, 2015 at 15:02

A 3x is not a mesure. It is a proportion, a ratio of the minimum and the maximum.

Taking the minimum focal lenght as a base you start making multiplications.

But this proportions have no meaning regarding the size of the picture either becouse it does not take into acount the sensor size.

In your first example, the ratio 3x dosen't imply that the photo is 3 times "larger" all of them are actually smaller than the normal 50mm lens on a full frame camera. So they are not 3x. It has no magnification at all.

This type of convention is just for compact cameras.


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