Enlarging

Part of Photography

Enlarging projects a negative image through a lens onto light-sensitive paper at greater than 1:1 scale, producing prints larger than the original negative.

Why This Matters

Contact printing produces excellent prints, but they are the same size as the negative. A 9 × 12 cm glass plate makes a 9 × 12 cm print. This is adequate for many purposes, but sometimes a larger print is needed: a medical photograph for teaching, a land survey map for a communal wall, an engineering detail for a workshop. Enlarging solves this.

The physics is identical to the camera: a lens projects an image. The difference is that the enlarger runs the camera’s process in reverse — the negative (small) is the “object,” and the sensitized paper (larger) is the “film.” The lens projects the negative’s image onto the paper at any desired scale by adjusting distances. A 9 × 12 cm negative can produce a 30 × 40 cm print — a 3× linear enlargement — simply by adjusting the lens and paper distances.

For a rebuilding civilization, enlarging enables photographic communication at human scale. A 9 × 12 cm print passed between individuals is a private document. A 40 × 60 cm print mounted on a wall teaches fifty people simultaneously. The same negative can serve both purposes.

The Physics of Projection

The thin lens equation governs the enlarger: 1/f = 1/u + 1/v

Where:

• f = focal length of the lens
• u = distance from lens to negative (object distance)
• v = distance from lens to paper (image distance)
• Magnification M = v/u (ratio of image size to object size)

Example: Using a 100 mm focal length lens to produce a 4× enlargement (a 9 cm negative becomes a 36 cm print):

• M = 4, so v = 4u
• 1/100 = 1/u + 1/4u = 5/4u
• u = 5 × 100 / 4 = 125 mm
• v = 4 × 125 = 500 mm

So: the negative must be 125 mm from the lens, and the paper must be 500 mm (50 cm) from the lens for a 4× enlargement with a 100 mm lens.

Building an Enlarger

A functional enlarger needs:

1. A light source above the negative
2. A holder for the negative
3. A lens below the negative, focused on the paper
4. A means of adjusting the lens-to-paper distance
5. A means of adjusting focus (lens-to-negative distance)
6. A support that holds all of this stable and aligned

Simple stand-and-arm design:

Materials: Heavy wooden post or board, horizontal arm, lens board, lens.

1. Erect a vertical post 120-150 cm tall, firmly anchored to a heavy base board
2. Attach a horizontal arm that slides up and down the post and can be locked at any height with a wing nut or wooden wedge
3. Mount the negative holder and lens assembly to the underside of this arm
4. The base board (where the paper lies) is the flat surface below

The negative holder:

1. Build a sandwich of two wooden frames, each with a central cutout slightly smaller than the negative
2. The negative sits glass-side up in the lower frame; the upper frame presses it flat
3. A lip or registration stop positions the negative in the same place every time

The light source: Above the negative holder, mount a bright light source. Options in order of brightness:

• A candle or oil lamp: works for contact printing but is marginal for projection — requires very long exposure times
• Multiple candles or lamps: improved; 3-4 lamps together enable practical exposures
• A lime light (calcium oxide in a flame): very bright, usable for projection work
• A magnesium ribbon or magnesium flash: extremely bright flash exposure; practical for single prints but not adjustable

Position the light so it illuminates the negative as evenly as possible. A curved reflector (polished metal bowl) behind the lamp improves efficiency.

Diffusion of the light source: Direct point-source lighting produces sharp but harsh results and shows every dust speck on the negative. Placing a sheet of white paper or translucent oiled cloth between the light and the negative (a diffuser) produces softer, more even illumination at the cost of some brightness.

The lens: Any well-corrected convex lens can project an enlargement. Longer focal length lenses require more vertical height for the same enlargement but are easier to use because the angles are smaller and optical aberrations are reduced. For enlargements up to 5×, a 100-150 mm lens works well.

Focusing mechanism: Attach the lens to the negative holder board on a sliding sleeve so the distance between them can be adjusted. A rack and pinion (a gear rack on the sleeve and a knob driving a pinion gear) gives fine control. In the simplest design, a friction-fit sleeve that slides and locks is sufficient — move by hand, lock with a wedge or clamp.

Enlarging Procedure

In the darkroom:

1. Raise the enlarger arm to the height that gives the desired enlargement. Use the distance formula above to calculate, or use a trial strip.
2. Place a scrap of plain white paper on the base board where the print will be
3. Turn on the light source and open the lens to full aperture
4. Adjust the focus (lens-to-negative distance) until the image of the negative appears sharp on the white paper. Check sharpness by looking at fine detail — a hair, a thin line in the scene
5. Confirm the image size is correct
6. Stop the lens down 1-2 stops from maximum for better image quality
7. Turn off the light source or close the lens cap
8. Replace the white paper with a sheet of sensitized paper, correctly positioned
9. Expose for the calculated time

Calculating exposure: Make a test strip first. Expose a strip of paper in 10-second increments across the image. Develop and fix the strip. Choose the best section and use that exposure time for the full print.

Enlargement affects required exposure: a 4× linear enlargement (16× area increase) requires approximately 16× more exposure than a contact print with the same light source. This is why bright light sources are essential for practical enlarging.

Exposure Adjustments for Different Enlargements

When you change magnification, the exposure time must change because:

1. The image area is larger (16× area for 4× linear enlargement)
2. The lens-to-paper distance is greater (light intensity falls as the square of distance)

Both effects reduce the light per unit area. The combined effect is that exposure scales as (M+1)^2 / (M_ref+1)^2 when changing magnification from M_ref to M, holding all else constant.

Practical table (relative to 2× magnification = 1.0):

Magnification	Relative Exposure
1× (contact)	0.44
2×	1.0
3×	1.78
4×	2.78
5×	4.0
8×	9.0

If a 2× print takes 30 seconds, an 8× print of the same negative with the same aperture requires about 270 seconds (4.5 minutes) — a very long exposure that demands a bright, stable light source.

Print Quality in Enlarging

Sharpness: Enlarging magnifies all defects in the negative: grain, dust, focus errors. A negative that looks acceptably sharp at contact size may appear soft at 4× enlargement. Use your best negatives for large prints.

Grain: Silver grains in the emulsion become visible at high magnification. Coarser emulsions (higher sensitivity, heavily ripened) show more visible grain in enlarged prints. Contact prints on the same emulsion show no visible grain. This is one reason large-format negatives are preferred for large prints — you can achieve the same final print size with less enlargement from a larger negative.

Dodging and burning: During exposure, you can hold a small card below the lens to shade selected areas of the print (dodging — lightening those areas in the final print), or hold your hand to shield the paper selectively during extra exposure of specific areas (burning — darkening those areas). These techniques allow you to balance prints from negatives where some areas are more or less dense.

Practical Applications for Enlarging

Application	Negative Size	Print Size	Magnification
Map for wall display	9 × 12 cm	30 × 40 cm	3×
Medical teaching	6 × 9 cm	18 × 27 cm	3×
Engineering detail	9 × 12 cm	18 × 24 cm	2×
Portrait for identification	6 × 9 cm	12 × 18 cm	2×
Document archival	9 × 12 cm	9 × 12 cm	1× (contact)
Botanical illustration	9 × 12 cm	36 × 48 cm	4×