Tuesday, 7 August 2012

Understanding TTL Flash – Flash Terminology, Anatomy, Functions and Features

Flash units are one area of photography where the most advancement has been achieved. Modern day flash guns are nothing short of miniature super computers that could do a number of things like determining subject to camera distance, automatically setting the zoom level, auto exposure, communicate and control other flash guns etc. Apart from these they also possess a number of features like high speed sync, slow sync with first and second curtain, stroboscopic flash, focus assist beam, infra-red transceiver etc. Here in this article we will have a look of the important functions and features of the two major camera brands, Canon and Nikon.

Components of TTL Flash
Components of TTL Flash

Flash Terminology


Flash is a short-duration, brilliant burst of light.

Auto Flash Metering

The camera's light sensor meters the light reflected by the subject after the flash begins firing. The flash output is then controlled on the fly so as to obtain proper flash exposure.

Bounce Flash

Bouncing flash off surfaces like walls ceiling etc is a popular method of diffusing the light.

Clip on Flash

Any flash unit that can be attached to the accessory shoe of a camera.

Guide Number

Guide number indicates the amount of light a flash emits.

G No. /Aperture (f) = Distance for optimal exposure
G No. /Distance = Aperture (f) for optimal exposure

Pre Flash

A low-output flash fired before the main flash is fired in synchronization with the shutter. It is used for measuring the subject distance and evaluative metering. A pre-flash is also fired when using FE lock.

Multiple Flash

A set-up in which more than one flash is used to light the subject or scene. Multiple flashes could be set-up either via a wired connection or wireless.

Red Eye

Red eye occurs when using on camera flash to take pictures of people in low light situations. It happens because the angle of light is very near to that of angle of the lens. A pre flash is fired before the actual flash exposure (which shrinks the subject’s pupils) when red eye reduction is activated.

Slave Unit

Slave unit is a flash that fires at the command of a master flash unit.

Components of Flash

1. Pilot Lamp

Pilot lamp tells the photographer that his flash unit is ready to fire. It is a small light situated on the back of the flash unit which glows when the flash is ready. The photographer will also get another confirmation in his viewfinder. Once the flash unit fires it takes time to recharge fully before it can be used again. The time it takes for flash to recharge between shots is called recycling time and it varies as per the model and make of flash. The type of batteries used in the flash units and also their charge level affects recycle times. There are special battery packs available for use with most modern flash units that considerably reduce recycle time enabling you to shoot more frames. This is one criterion which photographers use when deciding if a flash will suit their needs. To find out the recycle time of your flash refer the instruction manual that came along with it. Recycle time – faster the better.

2. Exposure Confirmation Lamp

Exposure confirmation lamp lights up when the camera and flash in TTL mode has arrived at a suitable exposure for the scene.

Canon – in canon flashes exposure confirmation lamp is located near the pilot lamp and it glows green when the camera confirms exposure and if for any reason exposure is unable to achieve it does not glow.

Nikon – the pilot lamp glows steady when exposure is confirmed else it blinks rapidly.

Tackling situation where insufficient exposure is indicated:-
  •     Increase ISO
  •     Open up Aperture
  •     Move closer to the subject.

3. Flash Head

The head of the modern flash units have the capability to rotate 360 degree and also tilt up and down. This allows precision control over where the light should be directed. Tilting the flash down is a very useful thing especially in macro photography; refer the user’s manual for distance recommendations when tilting down.

The 360 degree rotation feature is very helpful when multiple flash units are being used in wireless mode. In such cases the flashes are positioned with the receiver (located at the front in case of canon and at the side in case of Nikon) pointing towards the master and the flash head towards the subject.

4. Built in Wide Panel

Latest flash units come with a built in wide panel which is a piece of Fresnel plastic that could be pulled out over the front of the flash. When pulled out and placed over the flash head it scatters the flash beam increasing the spread or coverage. The flash instruction manual contains detailed information regarding focal lengths that require the use of a wide panel. There is a common misconception that the wide panel could be used to soften the light, but in reality it does not; it just spreads the beam wider.

5. Built in Bounce Card

Latest flash units also has a built in bounce card often referred to as the catch-light panel. It pulls out along with the wide panel, and then the wide panel is pushed back into position. Care should be taken while pulling out the bounce card and wide panel as it is a delicate set-up and use of excess force could easily damage it. It is mainly used to create catch-lights in the eyes of the subjects in outdoor portraits. While shooting indoors with bounce flash off the ceiling pulling out the bounce card helps reflect some light strait to the subject there by filling in shadows forming under the eyes. Detailed information regarding the use of the bounce card is available in the user’s manual.

6. Zoom Head

Flash head could be zoomed in and out within their zoom range (refer user’s manual for range) in TTL mode the flash automatically adjusts the angle of illumination to match the focal length being used. The photographer could also manually set the zoom level if he so desires. Zooming allows more control over how the light is delivered making it more efficient. When zoomed out it reaches wide and when zoomed in it travels further in a more focused beam. Zooming along with wide panel and bounce card and tilt and swivel capability makes the flash unit a very versatile light source.

7. AF Assist Beam

AF (Auto Focus) assist is a couple of infra-red LED’s located in front of the flash units which emits infra-red light to allow the auto focus system of the camera to operate efficiently in low light conditions.

Canon – AF assist could be turned ON and OFF with custom function settings. When used as a slave in wireless ttl mode the red panel blinks to indicate that flash has recycled.

8. Modeling Flash

In modelling flash the flash fires short bursts in rapid succession lasting approximately a second. This is intended to give the photographer an idea about the affect the flash has on the subject. Modelling flash can be activated by pressing the depth of filed preview button on the camera. However the point to be remembered is that modelling flash lasting a second is a huge strain on both the flash and the batteries and excessive use could easily cause the flash to overheat and batteries to run down.

Nikon – has a dedicated button on the flash for modeling flash.

9. Flash Button

To test fire the flash.

10. Light Sensor Window for Wireless Remote Flash

Aids working of wireless TTL and should be kept clean. In Canon flashes the sensor Is located in the front and in Nikon flashes the sensor is located on the side.

11. LCD Panel

LCD panel Is located on the back side of the flash units and it displays flash settings and enables adjustments to various functions which are discussed below.

a. Flash Exposure Compensation

Flash exposure compensation is different from the normal exposure compensation. In flash exposure compensation only the output of the flash is adjusted; shutter speed and aperture values remain untouched. Most flash units allow compensation values up to 3 stops and can be set either on the flash or on the camera. For situations that demand adjustments of more than 3 stops it is recommended to use the flash in Manual mode.

Canon – when flash is operating at minimum or maximum power, setting flash exposure compensation may have no effect on flash exposure.

Nikon - when flash is operating at minimum or maximum power on limited exposure compensation settings are allowed.

b. Flash Exposure Bracketing

Flash exposure bracketing feature is only available in Canon flash units and it enables the photographer to shoot three images in succession with varying flash power (could be adjusted in 1/3 stop increments).

c. Flash Exposure Lock

Flash exposure lock feature is very useful when photographing small objects that only cover a small part of the frame or when the subject is in front of a reflective background. It allows locking the flash metering values and recomposing the image.

Canon – AE lock (*) serves as the Flash Exposure Lock button in most models while some cameras feature 

a dedicated F.E Lock button.

Nikon – Option in camera menu, some models allow program the function button to act as FE Lock.

d. Slow Speed Synchronization

Use of flash in combination with slow shutter speeds, where flash exposes the subject and the slow shutter speed exposes the background.

Nikon – allows shutter speeds lower than 1/60 th of a sec with flash.

e. High Speed Sync (FP Flash)

High speed sync flash otherwise called Auto FP (Focal Plane) flash allows the use of flash at shutter speeds higher than the camera’s sync speed. In High speed sync mode the flash fires many short bursts in rapid succession during the entire period of the exposure. It is mainly used when a wide aperture need to be used with bright lighting conditions. High speed sync may be set either through the flash or through the camera depending on the model of the camera.

f. Rear or Second Curtain Sync

By default the flash is set to fire immediately after the shutter opens, but when set to rear curtain sync mode the flash only fires at the end of the exposure. When photographing moving subjects with rear curtain sync and slow shutter speeds the motion trail appears behind the subject thus making the shot look more real. Depending upon the make of the camera and flash, this function should be set either at the flash or the camera.

g. Stroboscopic Flash or Repeating Flash

In Multi stroboscopic flash mode the flash will fire multiple times during a single exposure. It is mainly used in low light conditions when the moving subject is illuminated only by the flash creating multiple images in the frame. This technique is best applied with light subjects against a dark background

Just as in using modelling flash, stroboscopic use is a strain on the flash; so use it judiciously.

h. Manual Mode
In manual mode the photographer has full control over the flash settings like zoom, power output etc. Flash could be set to work in manual mode irrespective of the fact that the camera is working in auto modes or manual mode. The LCD panel on the flash shows the correct exposure settings, seasoned photographers could also do their math using the flash guide number.

In the next article we will discuss about Photography - Lighting - Flash - Understanding TTL Flash – Fill Flash

Related Reading

  1. High Speed Flash Synchronization
  2. How to Trigger your Off Camera Flash
  3. Slow Sync Flash
  4. Focal Plane Shutter
  5. Types Of Flash Synchronization