A Photographic Enlarger: Two Lenses for a Real, Bigger Image
1 · Predict
Unlike a compound microscope's typically virtual final image, a photographic enlarger needs to project a REAL, enlarged image onto photo paper. Can two converging lenses in sequence still produce a real (not virtual) final image?
- Yes — with the right spacing and focal lengths, the two-lens chain's final image lands as real, ready to project onto a surface.
- No — chaining two lenses always produces a virtual final image.
- It depends only on the object distance, not the lens arrangement.
2 · Set Up
- Open the compound-enlarger preset and press Reset. L1 has f = 8 cm; L2 has f = 12 cm, separated by 34 cm.
- Enable L1's image-distance and system-magnification readouts.
- Set the object distance from L1 for each trial and record L1's image distance and the overall system magnification.
3 · Collect Data
| Object distance from L1 (cm) | L1's image distance (cm) | System magnification M |
|---|---|---|
| 12 | ||
| 16 | ||
| 20 |
Plot system magnification M (y-axis) against object distance from L1 (x-axis) for your three trials.
4 · Analyze
- For one trial, compute L1's image distance d_i1 from 1/f₁ = 1/d_o1 + 1/d_i1 (f₁ = 8 cm), then L2's object distance d_o2 = 34 − d_i1, then its image distance d_i2 from 1/f₂ = 1/d_o2 + 1/d_i2 (f₂ = 12 cm). Finally M = (−d_i1/d_o1)·(−d_i2/d_o2). Compare M to the table.
- This preset's default object distance (16 cm, L1's 2f point) gives M = 2 with a REAL final image. Explain why checking d_i2's sign (positive = real) matters for whether this system could actually project onto photo paper.
5 · Extend
- For an enlarger to work, its final image must land at a positive (real) distance beyond L2, at the exact height where the photo paper sits. Explain why an enlarger's design must carefully control both lenses' focal lengths and spacing to guarantee this.
- Compare this experiment's default M = 2 (real final image) to compound-relay's M = 1 and compound-microscope's M = 4 (typically viewed as virtual through an eyepiece). Explain why the SAME two-lens chain math (product of magnifications) can produce such different practical outcomes depending on the specific focal lengths and spacing chosen.
The Physics Behind This Experiment
Two-Stage Enlarger Magnification
Like any compound lens chain, the enlarger's system magnification is the product of its two lenses' individual magnifications: M = m₁ × m₂. Careful choice of focal lengths and spacing ensures the final image lands real (not virtual), ready to project.