Flash and Ambient Balance Quiz: Off-Camera Lighting

The flash pulse is far shorter than the shutter speed, so below sync speed the shutter speed has almost no effect on flash exposure. What it does change is how long the sensor collects the ambient light, so it sets how bright the background looks.

The aperture limits all light passing through the lens, whatever its source. Closing it one stop darkens the ambient part of the exposure and the flash part equally, unlike shutter speed, which mainly affects ambient light.

The background in a flash portrait is usually lit by ambient light, which depends on shutter speed. Slowing the shutter, often called dragging the shutter, lets in more ambient light and brightens the background, while the flash exposure on the subject stays about the same. A faster shutter would darken it further.

When the flash is the main light on the subject, lowering its power directly reduces how bright the subject is. Changing shutter speed below sync speed mostly affects the ambient light, not the flash, and raising ISO would make the subject even brighter.

The inverse square law says light intensity falls with the square of the distance. Double the distance from the source and the subject receives one quarter of the light, a loss of two stops. That is why light fades quickly with distance.

Moving the flash closer puts more light on the subject because of the inverse square law. It also makes the source larger relative to the subject, so shadow edges become softer.

Softness depends on the size of the light source relative to the subject. Bouncing a small flash off a wall or ceiling turns that surface into a much larger source, which wraps light around the subject and softens shadow edges.

High speed sync lets the flash work at shutter speeds faster than the camera's normal sync speed, often around 1/200 or 1/250 second. That allows wide apertures in bright light, or a darker ambient background, while still using flash.

In high speed sync the flash fires a rapid series of weaker pulses instead of one strong burst, so it can cover the whole shutter travel. Spreading the output out this way reduces the effective power, often by a stop or more, so the flash has less reach.

Flash exposure compensation tells the TTL system to make the flash brighter or dimmer than its own metering suggests. It changes only the flash output, leaving shutter speed, aperture and ISO as they are.

Below sync speed, a faster shutter cuts the ambient light reaching the sensor while the flash, which fires in a fraction of the exposure, stays just as bright. The result is a darker background and an evenly lit subject. A slower shutter, higher ISO or wider aperture would brighten the ambient light instead.

With manual flash, the output stays the same from shot to shot, as long as the distance and settings do not change. TTL reevaluates every frame and can vary with subject tone or framing, which makes results less consistent.

Flash is roughly daylight balanced, so under warm indoor lights it looks blue by comparison. A colored gel, such as a CTO (color temperature orange) gel, warms the flash to match the room light, so one white balance setting can neutralize both.

A grid is a honeycomb of small cells over the flash that blocks light traveling at an angle. That narrows the beam into a controlled spot so the light can be put exactly where it is wanted, such as on a face or a patch of background, without spilling.

Fill flash or mixed-light setups require treating ambient exposure and flash output as two independent variables. The shutter speed controls ambient brightness; the flash power controls subject brightness, and balancing them is a separate decision from metering either alone.