Enlarging & Printing

Focus Finder

A magnifying scope that lets you focus on the grain instead of guessing.

4Enlarging All enlargersNegative inspectionLight boxes

Wear PPE & Ventilate Nitrile gloves Eye protection Cross-flow air Label every bottle Dispose of fixer properly

Enlarge stage

Focus Finder — line drawing with key specifications

Key specifications
Magnification	10× or 25×
Screen	Ground glass, sometimes with a crosshair
Body	Metal or plastic, with a base that sits flat on the baseboard
Adjustable eyepiece	Dioptre adjustment for glasses wearers
Use	Place in a mid-tone area with visible texture
Focus at	The printing aperture, not wide open
Alternative	A 50 mm lens used as a loupe on the baseboard
Storage	Capped, dry, away from dust

Overview

Focusing an enlarger by eye on the baseboard is unreliable. At f/8 with a 50 mm lens, the depth of focus is small enough that a fraction of a turn of the focus control moves the image out of critical sharpness, and the human eye cannot reliably judge that at working distance.

A focus finder is a small telescope with a ground-glass screen that sits on the baseboard and magnifies a patch of the projected image ten or twenty-five times. At that magnification you can see the film grain itself, and grain is the only reliable focus reference — an edge can look sharp while the grain is still soft.

The technique is to focus for maximum grain definition in a patch that contains a mid-tone with visible texture, then stop down to the working aperture and check again. Stopping down shifts focus slightly, so always focus at the aperture you will print at.

What it does well

Makes critical focus possible instead of approximate
Magnifies film grain so you focus on the right thing
Dioptre adjustment suits glasses wearers
Also useful for inspecting negatives on a light box
Cheap compared with the improvement it delivers

Safety notes

Read before you use itNo hazard. Keep the base clean — a speck of grit under it will scratch the baseboard and tilt the scope.

Working notes

Focus at the aperture you will print at; stopping down after focusing moves the plane of focus.
Pick a mid-tone area with visible texture — a clear sky has no grain structure to focus on.
Focus at the centre and then check the corners; if they disagree, your enlarger needs aligning.
Re-check focus after any change of magnification. It always shifts.

Compatible with

All enlargersNegative inspectionLight boxes

Educational reference. Specifications and working figures on this page are drawn from published manufacturer documentation and are stated for a small inversion tank at 20 °C unless noted otherwise. They are starting points, not guarantees. Always confirm against the data sheet supplied with your own film, developer, paper and equipment, and follow the manufacturer's safety instructions.