Exposure Time

Part of Photography

Exposure time is the duration the shutter remains open, controlling how much total light reaches the plate alongside aperture — the two variables that determine correct exposure.

Why This Matters

Of all the variables in photography, exposure time is the one you have the most direct control over. Unlike aperture (which requires switching stops) or emulsion sensitivity (which is fixed when you coat the plates), time can be continuously varied from a fraction of a second to hours. Understanding how to judge, measure, and control exposure time is the most practical skill in getting consistently usable photographs.

With hand-coated emulsions in the field, many subjects are lit by conditions you cannot change — sunlight is sunlight, shade is shade. The aperture you choose is determined partly by the depth of focus you need. Time becomes the fine adjustment that balances these fixed inputs against your plate’s sensitivity to yield a correctly exposed negative.

Historically, photographers in the early 19th century used exposure times measured in hours. By the 1880s, gelatin dry plates had reduced this to fractions of a second. Hand-coated plates sit in the middle — 1 second to several minutes for most outdoor work. Understanding how to measure and control these times, and how to verify you made the right choice, is what separates systematic photography from guesswork.

Measuring Time Without a Clock

Most outdoor exposures with gelatin bromide dry plates fall between 1 second and several minutes. Counting seconds aloud (“one-and-two-and-three…”) is accurate enough for exposures under 30 seconds. Above that, use a pendulum or sand clock.

Counting method: Say “one-and-two-and-three” aloud at an even pace. Each complete count (“one-and” is approximately 1 second; “one-and-two-and” approximately 2 seconds). Practice against the sun’s shadow movement to calibrate your count. A person who counts at a consistent rate will be accurate to ±20% — sufficient for photography.

Simple pendulum: A weight on a cord, swinging. A 25 cm cord from pivot to weight-center has a period of approximately 1 second. A 100 cm cord has a period of approximately 2 seconds (period scales as the square root of length). Count complete swings (one full back-and-forth = 1 period = 1 second for the 25 cm pendulum). Make a pendulum the day before fieldwork and calibrate against sunrise or known star positions.

Sand glass: A sealed vessel with a small opening through which sand flows at a calibrated rate. Not precise but repeatable. Useful for longer exposures — a sand glass that empties in 5 minutes is a reliable 5-minute timer. Make from two ceramic vessels with a small hole in one, sealed together with clay.

Candle marks: For darkroom timing (development, fixing), mark a candle at measured intervals. One candle of known diameter and composition burns at an approximately constant rate. Measure how much it burns in 5 minutes with a ruler and mark accordingly. Consistency matters more than absolute accuracy.

Shutter Operation

The shutter controls the start and end of exposure. The simplest shutter is a hand-held cap over the lens. This works for any exposure longer than about half a second — the error of 0.1-0.2 seconds in hand timing represents less than half a stop of inaccuracy on a 1-second exposure.

Lens cap technique:

1. Camera on stable support — tripod, wall, or heavy stone
2. Compose and focus with the cap off
3. Replace cap
4. Load plate
5. Take a slow breath; settle
6. Remove cap smoothly — do not jerk or bump the camera
7. Count the exposure time
8. Replace cap in one smooth, firm motion
9. Do not attempt to move the camera until the cap is fully in place

The most common error with cap exposures is camera movement from the act of removing or replacing the cap. Use a lightweight, well-fitted cap. Practice the motion many times with an unloaded camera before using it for real exposures.

Drop shutter: For shorter exposures (0.1-0.5 seconds), a wooden flap hinged at the top of the lens board, held by a pin, and released to drop past the lens:

1. Cut a wooden flap slightly larger than the lens opening
2. Hinge at top with leather or cord
3. Hold up with a removable pin or hook
4. Pulling the pin releases the flap; it swings past the lens in 0.1-0.3 seconds depending on fall height
5. A stop peg prevents it from swinging back into the light path

Vary exposure time by adjusting the height from which the flap falls (greater height = faster; less height = more time). Test with unexposed plates, developing each to check density.

Exposure Time Tables

Derived from field testing with gelatin bromide plates in a box camera. Use as starting points; calibrate against your specific emulsion batch.

Outdoor exposures (lens at f/8, medium aperture):

Subject and Lighting	Estimated Exposure
Landscape, direct noon sun	2-4 seconds
Landscape, 9 AM / 3 PM direct sun	4-8 seconds
Landscape, thin overcast	8-20 seconds
Building, direct sun, front-lit	3-6 seconds
Building, overcast	15-40 seconds
Portrait, outdoor shade	10-30 seconds
Portrait, direct sun (squinting)	3-8 seconds
Foliage, direct sun	5-15 seconds
Snow scene, bright sun	1-2 seconds (bright subject)

Indoor exposures (lens at f/5.6, large aperture):

Subject and Lighting	Estimated Exposure
Near window, bright day	1-4 minutes
Near window, overcast day	5-15 minutes
Interior lit by multiple lamps (1 m distance)	10-30 minutes
Interior, single candle at 1 m	30-90 minutes
Very dim interior	1-6 hours

These times assume hand-coated gelatin bromide plates with moderate ripening (40-60 min digestion). Shorter-ripened plates may need 2-4× more time. Longer-ripened or bromine-sensitized plates need less.

Effect of Exposure Time on Image Quality

Correct exposure: Full shadow detail; midtones at medium density; highlights dense but showing some texture.

Underexposure (too little time):

• Thin, pale negative with low density overall
• Shadow areas have no useful detail
• The scene appears washed out in the print — flat and lacking contrast
• Can be partially rescued by extended development, but at the cost of increased grain and contrast

Overexposure (too much time):

• Dense, dark negative with blocked-up highlights
• Bright areas in the scene lose all texture and detail
• The scene prints with blown-out highlights — large areas of pure white
• Cannot be rescued by development adjustment — the image is simply missing

The asymmetry: It is generally better to err toward slight overexposure than underexposure with silver emulsions. An overexposed negative can still be printed down (shorter printing time, or with a filter). An underexposed negative has simply not recorded the shadow information — it is gone forever.

Special Exposure Situations

Night and low-light photography: With a bright oil lamp (equivalent to perhaps 15 candlepower), a well-prepared subject 1 meter away, and your fastest emulsion at maximum aperture (f/4): Approximate exposure: 20-45 minutes.

Practical uses: documenting a workshop operation, recording a birth or surgery, photographing stored records by lamplight.

Set the camera on a rigid support. Have everyone in the scene remain absolutely still for the exposure. Warn them beforehand.

Copying documents: For flat subjects evenly lit by sunlight through a window or outdoors:

1. Mount the document flat on a wall or board
2. Position camera perpendicular to the document at a known distance
3. Illuminate with even sunlight from both sides if possible (reduces shadows from paper texture)
4. Use medium aperture (f/8-f/11) for maximum sharpness across the entire flat document
5. In bright sunlight: 3-8 seconds. In overcast: 15-30 seconds.

Moving subjects: Water, animals, people in motion require short exposures to freeze movement.

• Walking person at 5 m: needs at least 1/10 second to prevent blur
• Children playing: 1/30 second or faster
• Running: 1/50 second or faster

With hand-coated plates in bright sun at f/4 maximum aperture: 1/5 to 1/2 second is achievable. This can freeze slow walking but not faster movement. Position subjects against bright backgrounds for maximum contrast; choose moments of minimum movement (the top of a wave, the pause at each step in walking).