How to measure magnification

The magnification of any macro photography rig can be determined by using the rig to photograph a metric ruler such as the one shown below.

Plastic 15 cm (6″) ruler from the Natl. Science Teachers Assn. (NSTA).

The following photograph was taken using an AmScope 4x microscope objective mounted on my Fujifilm X-T3 digital camera with a plastic lens adapter designed and 3-D printed by Nicholas Sherlock. Notice that only a tiny part of the ruler is shown in this “full frame” (uncropped) macro photo!

Segment of an NSTA metric ruler.

The formula for magnification is as follows.

length of camera sensor, in mm / #mm visible in photo frame

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

The APS-C digital sensor featured in the Fujifilm X-T3 is 23.5 mm long. The annotated image shows 5.35 mm of the small plastic ruler is visible in the photo frame.

23.5 mm / 5.35 mm = ~4.4x

The actual magnification of the AmScope 4x microscope objective is greater than 4x due to the design of the lens adapter.

What are the take-aways?

As a result of photographing the ruler, subject selection should be easier. Now I know ~5 mm is the size limit for subjects to fit entirely within the photo frame. That’s actionable intel.

Related Resource: How to Calculate Your Camera’s Magnification in Macro Photography, by Stewart Wood (12:02).

Tech Tips

The “Ruler Tool” in Adobe Photoshop was used to measure the length (in pixels) of 5 mm along the double-headed red arrow superimposed on the plastic ruler shown above. That value was used to set a “Custom Scale” for the ruler, in millimeters. (See Setting a Custom Scale and Measuring in Photoshop for more step-by-step instructions.)

Select the “Ruler Tool.” From the Menu bar, select Image / Analysis / Set Measurement Scale. 60s ‘shop: Using the ruler tool to measure distances in Photoshop CC, by Photoshop for the Scientist (1:00) provides a clear and concise explanation of how it’s done.

Then the “Custom Scale” for the “Ruler Tool” was used to measure the entire length along the ruler that’s visible in the photo frame: 5.35 mm.

Post Update

Photopea” is a free Web-based clone of Adobe Photoshop — Photopea doesn’t do everything Photoshop does but it can be used to measure length (in pixels) using its version of a ruler tool.

Right-click on the “Eyedropper Tool” — located in the left sidebar of the main window — and select the “Ruler Tool.” Click and drag a line segment; record the length of the line, in pixels. Click the “Clear” button (optional) and repeat the same process for more line segments, as needed.

As far as I know, the Photopea “Ruler Tool” doesn’t allow the user to set a custom scale. No problem. Make measurements similar to mine and set up a proportion of two similar ratios.

x mm / 5 mm = #pixels for photo frame / #pixels for 5 mm

Solve for x by cross-multiplying and dividing.

x mm = #pixels for photo frame x 5 mm / #pixels for 5 mm

Remember that similar units above and below the dividing line cancel (pixels, in this case) so the final answer is in millimeters (mm).

Copyright © 2022 Walter Sanford. All rights reserved.


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