Natural light is beautiful, but you cannot always choose it. Sometimes it is too weak, too harsh or coming from the wrong direction. A flash lets you create your own light, whenever and wherever you need it. But flash behaves very differently from sunlight or a lamp, and many beginners find it confusing. This first lesson of the course explains what flash is, what makes it special, and the key ideas you need before using it.
What is a flash?
A photographic flash is a light source that produces a very brief, bright burst of light at the exact moment the camera takes the photo. It stores electrical energy and releases it in a flash that usually lasts only a small fraction of a second.
Light that stays on all the time, such as the sun, a window or a lamp, is called continuous light, or ambient light when it is the existing light in a scene. Flash is the opposite: it exists only during the exposure. That one difference explains most of what makes flash special.
What flash changes
| Continuous light | Flash | |
|---|---|---|
| When it is on | All the time | Only during the exposure, for a tiny moment |
| Can you see the effect before shooting? | Yes | Not directly; you check the photo afterwards |
| Effect of shutter speed | Longer shutter records more of it | Within the sync speed, shutter speed does not change the flash part of the exposure |
| Motion | Freezing depends on shutter speed | The very short burst can freeze motion on its own |
| Power | Limited by brightness of the source | Very bright for its size; can overpower the sun at close range |
Flash freezes movement
A flash burst is extremely short, often from around 1/1000 of a second to much shorter at low power settings, depending on the flash. When flash is the main light, this short burst acts like a very fast shutter speed. That is why flash can freeze splashing water, a jumping dancer or a moving child, even when the camera's shutter speed is relatively slow. Freezing motion with shutter speed alone was covered in motion blur and freezing movement.
Flash sync speed
Most cameras have a shutter made of two curtains: one opens to start the exposure, the other closes to end it. At fast shutter speeds, the second curtain starts closing before the first has fully opened, so only a moving slit of the sensor is exposed at any moment. A flash fired during that time would light only a stripe of the frame, leaving a dark band.
So there is a fastest shutter speed at which the whole sensor is uncovered at once. This is called the flash sync speed, typically around 1/200 or 1/250 of a second on many cameras, though it varies by model. Your camera manual lists yours.
| Shutter speed | What happens |
|---|---|
| At or slower than sync speed | The whole frame is lit by the flash; normal flash use |
| Faster than sync speed (normal flash) | Part of the frame may be dark |
| Faster than sync speed with High-Speed Sync (HSS) | The flash pulses rapidly to cover the moving slit; works, but with much less flash power |
Remember:Many cameras automatically limit the shutter speed to the sync speed when a compatible flash is turned on. High-Speed Sync, called Auto FP by Nikon, is useful in bright sun with wide apertures, as you will see in fill flash in daylight.
Flash is a small, close light
A bare flash is a small light source, usually close to the subject. You know what that means from the light course:
- Small source: hard light with sharp shadows, unless you make it larger with modifiers or bouncing, as in hard and soft light.
- Close source: it falls off quickly with distance, following the inverse square law from understanding light. A subject 2 metres away gets about a quarter of the light of one at 1 metre.
Most flashes produce light close to daylight in color, which matters when you mix flash with other lights, covered in mixed lighting.
Types of flash
| Type | Description | Strengths | Limits |
|---|---|---|---|
| Built-in (pop-up) flash | Small flash built into some cameras | Always with you | Weak, harsh, fixed direction |
| Speedlight (hot-shoe flash) | Battery-powered flash that mounts on the camera's hot shoe | Portable, bounceable head, can be used off camera | Limited power compared with studio lights |
| Studio strobe / monolight | Larger flash, mains or battery powered | Much more power, fast recycling, many modifiers | Bulkier, usually on stands |
Many photographers also use continuous LED lights, especially for video. They are easy to see and adjust, but usually much less powerful than flash for still photos. Studio lights are covered in studio lights and common modifiers.
Common mistakes
- Pointing the flash straight at the subject every time: it gives flat, harsh light.
- Setting a shutter speed above sync speed: dark bands across the photo.
- Expecting flash to light far-away subjects: it falls off quickly with distance.
- Forgetting the existing light: every flash photo is a mix of flash and ambient light, explained in flash exposure and ambient light.
How professionals think about flash
Professionals treat flash as just another light source with special properties: very brief, powerful, small and close. They shape it to be soft or hard, choose its direction, and decide how it mixes with the existing light. Good flash photography often does not look like flash at all; it simply looks like good light.
Practical examples
Water splashing in a kitchen
- Situation
- You want to freeze water splashing from a glass indoors, but the room is dim.
- What to do
- You use a flash as the main light at low power, with the shutter at 1/200.
- Why it works
- The very short flash burst freezes the water; low power makes the burst even shorter.
- Result
- Crisp, frozen droplets that a dim room could never freeze on its own.
Dark bands in your photos
- Situation
- With a flash on, you set 1/1000 to freeze action and get a dark band across the photo.
- What to do
- You return to 1/200, your camera's sync speed.
- Why it works
- Above sync speed, the shutter curtains block part of the sensor when the flash fires.
- Result
- The whole frame is evenly lit.
Key points
- Flash is a brief, bright burst of light; continuous light stays on.
- The short flash burst can freeze motion.
- Flash sync speed, often about 1/200–1/250, is the fastest normal shutter speed with flash.
- High-Speed Sync allows faster shutter speeds but with much less flash power.
- A bare flash is small and close: hard light that falls off quickly with distance.
Frequently asked questions
What does a camera flash do?
What is flash sync speed?
What is high-speed sync?
Why does flash look harsh?
Conclusion
Flash is a brief, powerful, small light that can freeze motion and light any scene, within the limits of sync speed and distance. Understanding how it differs from continuous light is the foundation for everything in this course. Next, you will learn about built-in and on-camera flash and how to get better results from them.
Finished reading? Track your progress through Flash and studio lighting.