How to calculate magnification

By definition, a true macro photo is one with a magnification of at least one-to-one (1:1, or 1/1), that is, one unit on the camera sensor is equal to at least one of the same units in the real world. Magnification (in-camera) can be calculated using the following formula.

Mc = size of subject on camera sensor / size of camera sensor

Both measurements must be expressed in the same units in order for the units to cancel during division.

For example, let’s look at the following “full-size” image of a Corduligastridae erronea exuvia. “Full-size” means the image is uncropped (4896 x 3264 pixels).

The photograph shown above was taken using a Fujifilm X-T1 digital camera, Fujifilm MCEX-11 extension tube, and Fujinon XF80mm macro lens. The specifications for the macro lens lists the maximum magnification as 1x. Question is, what is the actual magnification of the subject?

The X-T1 features an APS-C sensor (23.6 mm x 15.6 mm). The dragonfly exuvia is approximately 35 mm in length, or 3451 pixels out of 4896 pixels across the entire image.

Using two equivalent ratios, the following proportion can be used to solve for the length of the exuvia on the camera sensor, in millimeters (mm).

x mm / 23.6 mm = 3451 pixels / 4896 pixels

x = ~16.6 mm. In other words, the exuvia is ~16.6 mm wide on a camera sensor that is 23.6 mm across.

Calculate the in-camera magnification using the following formula.

16.6 mm / 23.6 mm = ~0.7x

The magnification of the subject is ~0.7x, meaning the size of the subject on the camera sensor is slightly smaller than the width of the camera sensor. Expressed another way, the in-camera image is ~7/10 life size.

1.19x is listed as the theoretical maximum magnification using an MCEX-11 extension tube mounted on the 80mm macro lens. If we round the spec’d magnification to ~1.2x, then it’s clear that the actual magnification of ~0.7x is less than advertised, meaning the lens/extension tube combination is capable of focusing more closely on the subject than my sample photo.

Related Resources

Copyright © 2018 Walter Sanford. All rights reserved.

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