Flash Photography Basics

Flash transforms what you can shoot on film. Indoor events, harsh midday shadows, evening portraits—flash handles situations that would otherwise mean unusable images or pushed film with compromised quality.

This guide covers flash fundamentals for film photographers: sync speed, guide numbers, fill flash, bounce technique, and mixing flash with ambient light.

Why Use Flash?

Flash provides:

• Light where there isn't enough: Shoot indoors or at night without pushing film
• Fill for harsh shadows: Balance bright sun with shadow detail
• Controlled quality: Consistent colour temperature and intensity
• Freezing motion: Flash duration is much faster than most shutter speeds

The trade-off is learning the technical requirements—sync speed, guide numbers, and flash-ambient balance.

Sync Speed Explained

What is Sync Speed?

Your camera's shutter has two curtains. During exposure, the first curtain opens, the film is exposed, then the second curtain closes. At fast shutter speeds, the second curtain starts closing before the first finishes opening—only a slit travels across the film.

Flash fires instantaneously. If the shutter is only a slit, flash only exposes that slit—the rest of the frame is unexposed.

Sync speed is the fastest shutter speed where the entire frame is open simultaneously when flash fires.

Typical Sync Speeds

Nikon FM2: Famous for 1/250 sync—fast for a mechanical SLR.

Canon AE-1: 1/60 sync.

Contax T2/T3: Leaf shutter, syncs at all speeds up to maximum.

Leaf Shutters

Leaf shutters (built into the lens, not the body) open and close from the centre like an iris. At any speed, the entire frame is open at some point. This is why Hasselblad, Mamiya TLRs, and many compact cameras sync at all speeds.

Guide Numbers and Manual Flash

What is a Guide Number?

A guide number (GN) describes flash power as a relationship between distance and aperture:

GN = Distance × Aperture

If your flash has GN 36 (meters at ISO 100) and your subject is 4 meters away:

Aperture = GN ÷ Distance = 36 ÷ 4 = f/9

Round to the nearest available aperture: f/8 or f/11.

ISO Adjustment

Guide numbers are usually rated at ISO 100. For other ISOs, the effective GN increases:

• ISO 400: Effective GN doubles (film is 2 stops more sensitive)
• ISO 800: Effective GN quadruples (film is 3 stops more sensitive)

Example at ISO 400 with a GN 36 flash at 4 meters:

• Effective GN = 36 × 2 = 72
• Aperture = 72 ÷ 4 = f/18

The higher ISO lets you use smaller apertures (more depth of field) or shoot at greater distances with the same flash.

Quick Reference for GN 36 (ISO 100)

Working with Manual Flash

1. Set your shutter speed to sync speed or slower
2. Estimate flash-to-subject distance
3. Calculate aperture from guide number
4. Fire the flash

Manual flash is consistent and predictable once you understand the maths. No batteries in the camera body affect exposure—what you set is what you get.

TTL Flash Metering

Many cameras and flash units offer TTL (Through The Lens) flash metering. The camera fires the flash, measures light reflecting from the film plane, and quenches the flash when exposure is correct.

Advantages

• Automatic exposure adjustment for distance
• Works with bounce flash
• Handles changing distances (parties, events)

Limitations

• Requires compatible flash and camera
• Can be fooled by bright/dark subjects (like reflected metering)
• Batteries in camera or flash affect function
• Harder to predict and control than manual

For predictable, repeatable results, many film photographers prefer manual flash.

Fill Flash Outdoors

Why Fill Flash?

Bright sun creates harsh shadows—dark eye sockets, unflattering contrast. Fill flash adds light to shadows without overpowering the sun.

The Technique

1. Meter for ambient: Find exposure for the bright areas (sun-lit skin, background)
2. Set shutter to sync speed: e.g., 1/250 at f/8
3. Reduce flash power: Flash should fill shadows, not dominate. Start at -1 to -2 stops below calculated exposure.
4. Fire and check: With negative film's latitude, you have some margin for error.

Example:

• Ambient exposure: 1/250 at f/8
• GN 36 flash at 3 meters would need f/12 for full flash exposure
• For fill, use f/8 on camera (1 stop less flash than ambient)
• Result: flash opens shadows, sun remains the dominant light

Balancing Ratios

For natural-looking portraits in daylight, 1:2 to 1:4 ratio usually works best.

Bounce Flash

Why Bounce?

Direct flash creates harsh shadows and unflattering, flat light. Bouncing flash off a ceiling or wall creates larger, softer light source.

The Technique

1. Find a bounce surface: White or neutral ceiling is ideal. Avoid coloured surfaces (they tint the light).
2. Angle the flash head: Point at the ceiling/wall, not the subject.
3. Increase exposure: Light travels further and spreads. Add +1 to +2 stops from direct flash calculation.
4. Watch ceiling height: Very high ceilings may not return enough light.

Bounce Flash Exposure

Bounced light is harder to calculate precisely. With TTL flash, the camera adjusts automatically. With manual:

• Estimate total light travel distance (flash to ceiling to subject)
• Add +1 to +2 stops for absorption and spread
• Bracket if possible

Many photographers develop intuition through experience rather than calculating exactly.

Mixing Flash and Ambient

Dragging the Shutter

Use a slower shutter speed than sync to record ambient light, then flash freezes the subject. This creates atmosphere—background glow, visible room lighting—while the subject is sharp.

Example: Indoor party

• Sync speed only (1/60, f/5.6): Sharp subject, black background
• Slow shutter (1/15, f/5.6): Sharp subject + ambient room light visible

First Curtain vs Second Curtain

• First curtain sync: Flash fires when shutter opens. Motion blur trails in front of the subject.
• Second curtain sync: Flash fires just before shutter closes. Motion blur trails behind the subject (more natural).

Most cameras default to first curtain. Second curtain is available on some units and cameras—check your manual.

The Trade-off

Slower shutters risk blur from camera shake or subject movement before/after the flash. The flash-lit instant is sharp, but ambient-lit portions may blur. Sometimes this is desirable (artistic motion); sometimes it's not (accidental camera shake).

Flash with Different Films

Colour Negative (C-41)

Very forgiving. Overexposed flash is recoverable; underexposed shadows can be lifted. Portra 400 is a popular choice.

Black & White

Also forgiving. Flash provides consistent exposure that simplifies development.

Slide Film (E-6)

Unforgiving. Flash exposure must be accurate. Overexposed highlights are lost. Bracket if possible, and test your flash/film combination before important shoots.

Common Flash Mistakes

Dark Background

Flash exposed the subject but ambient was underexposed. Use slower shutter to capture background, or get closer to the background, or use more powerful flash.

Partial Frame Dark Band

You exceeded sync speed. Use sync speed or slower.

Red-Eye

Flash too close to lens axis; light reflected from retina. Solutions: bounce flash, off-camera flash, have subject look away from camera, use red-eye reduction mode.

Harsh Shadows

Direct flash. Bounce off ceiling/wall, or use a diffuser.

Flash Didn't Fire

Batteries dead, flash not charged (wait for ready light), flash not in hot shoe correctly, or PC sync cord not connected.

Summary

• Sync speed is the fastest shutter where the whole frame is exposed when flash fires
• Guide Number = Distance × Aperture (at ISO 100)
• Fill flash: expose for ambient, reduce flash 1-2 stops
• Bounce flash: softer light, add +1-2 stops for distance and absorption
• Drag the shutter (slow shutter + flash) to capture ambient light
• Colour negative film is forgiving; slide film requires precise exposure
• Test your equipment before important shoots

Flash photography adds another dimension to what's possible on film. The learning curve is steeper than available-light shooting, but the control and consistency it provides opens creative possibilities that would otherwise be unavailable.