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How to calibrate the magnification

Task-oriented. "40×" is a claim — here you measure it, and afterwards you can give real sizes in micrometres.

What you need​

  • Any working microscope build (best: the smartphone microscope)
  • The calibration ruler / scale target from the box
✏️ TODO — Benedict

State the exact calibration target shipped in the current CoreBox (stage micrometer with 0.1 mm divisions? printed ruler?). The archive photos show a div = 0.1 micrometer scale.

Step 1 — Photograph the scale​

Put the calibration ruler in the sample holder instead of a sample, focus, and take a photo without digital zoom (zoom changes the calibration!).

The 0.1 mm scale through the 4× objective.

Step 2 — Count pixels per division​

Open the photo, zoom in, and measure how many pixels one division (0.1 mm = 100 µm) covers. Most gallery apps show pixel coordinates when you crop; or transfer the image to a computer.

pixel size in the sample=100 µmpixels per division\text{pixel size in the sample} = \frac{100\ \text{µm}}{\text{pixels per division}}

Example: one division spans 250 px → every pixel corresponds to 0.4 µm in the sample plane.

Step 3 — Use it​

From now on every photo from this unchanged setup can be measured:

real size=size in pixels×µm per pixel\text{real size} = \text{size in pixels} \times \text{µm per pixel}

Measure an onion cell, a hair, a printed halftone dot. A human hair should come out at 50–100 µm — if it doesn't, something changed (zoom, eyepiece distance, different build).

Measuring the optical magnification itself (Sek II)​

If you know your phone's physical pixel pitch pp (look up the sensor; typically 1.0–1.6 µm, mind pixel binning!), the total magnification of the optics is

M=psensorµm per pixel in the sampleM = \frac{p_\text{sensor}}{\text{µm per pixel in the sample}}

Compare this measured MM with the predicted objective × eyepiece value — the discrepancies (phone lens, eyepiece distance) are worth a classroom discussion.

Rules that keep the calibration valid​

  • No digital zoom between calibration and measurement (or calibrate at that zoom).
  • Don't move the phone relative to the eyepiece.
  • Re-calibrate after every rebuild or objective change (4× vs. anything else).