Flash enables sharp photos and creative lighting in low-light conditions. Built-in flashes are weak and produce harsh light, while external speedlights are more flexible and powerful. The flash sync speed must be observed to avoid exposure errors. Bounce flash and off-camera flash provide softer light and more creative options. The red-eye effect can be reduced using various techniques.
Category: Photography Basics > 3. Exposure & Camera Settings Article ID: 3.13 Created: May 2026
Key Points in Brief
- Flash enables shooting in low light and can be used deliberately for image design
- The built-in camera flash is convenient but weak and produces harsh, frontal light – an external speedlight offers significantly more flexibility
- The flash sync speed must be observed with focal-plane shutters – exceeding it results in partially exposed images
Explanation
When Do I Need Flash?
Flash is useful in the following situations:
- Dusk / Darkness: Exposure time without flash is too long → camera shake or motion blur
- Filling in Shadows: Backlight or harsh shadows on the face → fill flash
- Moving Subjects: Very short flash duration (~1/10,000 s) freezes motion that the camera alone cannot freeze
- Creative Lighting: Targeted lighting in the studio
When to Avoid Flash:
- Museums, theaters, animal exhibits (often prohibited)
- Subjects behind glass (reflections)
- When you want to preserve the existing light atmosphere
The Guide Number – Flash Range
The guide number (GN) describes the light output of a flash unit. Formula:
GN = distance (m) × aperture value
Rearranged: distance = GN ÷ aperture value
Example: Flash with GN 32, aperture f/8 → range = 32 ÷ 8 = 4 meters
Important: The formula applies at ISO 100. At ISO 400 (two stops higher), the range doubles → at ISO 400: 4 m × 2 = 8 meters.
Typical guide numbers:
- Built-in camera flash: GN 8–15
- Entry-level speedlight: GN 20–32
- Professional speedlight: GN 50–60+
Flash Sync Speed
Cameras with focal-plane shutters have a maximum flash sync speed – typically 1/125 s to 1/250 s. When exposing faster, the sensor is never fully open at once, so the flash only illuminates a strip of the image.
Solution for faster speeds: High-Speed Sync (HSS) – many modern system flashes can synchronize at 1/1000 s and faster using multiple short pulses, but with reduced power.
The Built-In Camera Flash
Almost all compact cameras and many DSLRs / mirrorless cameras have a built-in flash. It is convenient but has clear disadvantages:
| Feature | Built-In Flash | External Speedlight |
|---|---|---|
| Guide Number | 8–15 (weak) | 20–60 (much stronger) |
| Light Direction | Frontal, harsh | Swiveling, indirect possible |
| Red-Eye Effect | Common | Rare (distance from lens) |
| Battery Load | Camera battery | Own batteries |
| Flexibility | None | High |
The Red-Eye Effect
Occurs when flash light hits the retina along the lens axis and reflects back – especially with large pupils in the dark.
Solutions:
- Place external flash far from the lens axis
- Pre-flash (Red-Eye Reduction): Constricts the pupil before the actual shot – but people often blink at the pre-flash
- Post-processing correction in image editing (quick and easy)
- Turn on additional ambient lighting
Bounce Flash
With bounce flash, the swiveling reflector of the speedlight is aimed at the ceiling or wall. The light is diffused and falls softly and evenly on the subject.
Advantages:
- Softer, more natural light
- No harsh shadows
- No red-eye effect
Disadvantages:
- Range is greatly reduced
- Ceiling or wall should be white (otherwise color cast in the image)
Off-Camera Flash
A flash does not have to be mounted on the camera – it can be positioned freely in space with a cable (sync cable), infrared, or radio. This allows creative lighting from any direction and is the basis of studio and portrait photography with multiple flashes.
TTL Flash Control: Many systems support Through-The-Lens (TTL) automatic exposure metering. The camera sends a test flash just before the shot and calculates the optimal flash output – manufacturer-specific, not standardized.
Practical Tip
When using fill flash outdoors (backlight, shadows on the face), reduce flash power by −1 to −2 EV – the flash should brighten the subject, not overpower it. If the light seems too harsh, simply hold a white sheet of paper in front of the flash – this diffuses the light and softens it. A step toward a studio look without studio equipment.