A Viewfinder Darkly

Photography tips and tutorials

Flash Photography Introduction

June 27 2016

by Philip Northeast

Demystifying the jargon is a starting point for mastering modern flash guns, the mainstay of wedding portrait and photojournalists.

Flash photography evolved over the years from igniting flash powder, through single-use exploding glass bulbs to the latest electronic marvels. A flash is no longer just a source of light when it is too dark for normal photography.   The improving high ISO performance of digital cameras means adding light is not essential for low light scenes. The versatility of modern flash units and their integration with the camera’s electronic exposure systems makes them useful in a wide range of lighting conditions.

flash on a camera

Flashgun mounted on a camera

While flashguns have a limited range they are essential for photographing where there is little or no control over the timing of an event or the lighting. Typically, this is the situation confronting wedding photographers and photojournalists.

Daylight Fill Flash

Daylight Fill Flash for a wedding photo

Flash Synchronisation

The flash synchronisation speed is the fastest shutter that a camera with a focal plane shutter can use for normal flash photography.

Electronic flash is a brief burst of light that poses difficulties for focal plane shutters at faster shutter speeds.  The focal plane shutter is a common type in SLR and dSLR cameras. They have two curtains, the first opens to allow light to the film or digital sensor  and the second follows, closing the shutter to finish taking the photograph. At faster shutter speeds the second curtain starts closing before the first curtain is fully open. This means the shutter is never fully open at fast shutter speeds. Because of the short duration of the light from a normal electronic flash, at higher


The second curtain of a focal plane shutter

speeds part of the sensor is hidden by the moving focal plane shutter curtains.  This means you would get black areas in the photograph.

In available light photography the exposure is correct as each part of the photograph is exposed for the same period of time, even if not at exactly the same time.

The key is after the shutter release button is pressed, the flash must wait until the digital camera’s shutter is wide open before it fires, and must fire before the shutter starts to close. Depending on the camera, this can be a quite specific shutter speed or a limited range of the camera’s shutter speeds. This does lead to more specialised flash modes with fast and slow shutter speeds.


A variety of names for the same thing. Program Through The Lens (P-TTL), Electronic TTL according to Canon or iTTL from Nikon. This is the system where the camera’s through The Lens (TTL) metering system controls the  flash unit and determines the correct exposure, taking into account the effect of the flash. There is a short pre-flash burst for the camera’s normal exposure sensors to measure the light reflected from the scene to set the amount of flash power to use.


This is a Through The Lens (TTL)  system that seems similar to the previous term. It is a hangover from the film era where the camera measured the light reflected from the surface of the film. This determined the amount of flash power required to take the photograph. It carried over to the digital era, but now camera manufacturers prefer the using the pre-flash method. One aspect of the TTL method is that it requires additional sensors in a DSLR to measure the light reflected off the surface of the sensor.  The additional sensors are needed because when the reflex  mirror moves to allow the light from the lens to reach the shutter it blocks the normal exposure sensors located in the optical viewfinder system.

Guide number

This is a measure of the light output power of a flash unit and is used in calculating the  settings for lens aperture and camera ISO. It is based on the distance from the camera to the subject Normally the manufacturers quote the guide number as meters of illumination at a specific ISO setting. Now, all this is set by modern computer-controlled flash systems that vary the flash output based on the actual lighting conditions. However, it is a useful comparison of their relative light output between flash units from the same manufacturer.

normal flash

Normal flash setting only lights a short distance with a dark background

Red Eye 

In dim light the pupils of  the subjects eyes tend to enlarge to let in more light. The bright direct light from a flash mounted close to the digital camera lens reflects from the red blood vessels at the rear of the eye, giving the famous red-eye effect.

Red-Eye Reduction

One way to reduce the red-eye effect is by automatically firing the flash before taking the picture. The eyes react to the light, reducing the size of the opening in the pupil. The main flash burst follows and the smaller pupil opening allows less reflected light, reducing the red-eye effect.

Daylight Sync/ Fill Flash

Yes, using the flash in daylight. This fills in the shadows on people’s faces in or shady locations. Also helps to lift the subject in a portrait out of a dimly lit background.

fill flash

Using daylight flash to fill in the shadows created by strong backlighting

Slow Speed Sync Flash

This is the same as the previous technique. At a slow shutter speed, the flash will illuminate the near subject while the dark background, which is out of the flash’s range , will benefit from the slow exposure to show details using natural light.  Also known as Leading Curtain sync.

see the background

The slow speed sync setting helps keep the background exposed

Rear Curtain Sync flash

This is a variation of slow speed sync, that gives a slightly different view of moving subjects. A slow shutter speed allows details of the background in the image and capturing any motion of the subject in the low light portion of the image. The flash fires just before the shutter starts closing to properly show the subject in its final position. The overall image gives the impression of the subject moving out of the dimly lit area and into the bright light.

High Speed Sync (HSS)

The combination of the flashgun and camera firmware allow the use of shutter speeds faster than the normal flash synchronisation speed for cameras with focal plane shutters. This mode compensates for the fact that the shutter is never fully open at fast shutter speeds. This is accomplished by firing the flash longer than the normal one brief burst.

Wireless Flash

This is a function for advanced photographers where the camera fires multiple flash units by remote control. This is common in professional portrait photography.


All these are general terms and how individual equipment manufacturers implement these features varies. This may cause problems when using a flash from an independent manufacturer. Even if the third party flash is sold as compatible with a particular camera it might not support all the features in exactly the same way as the camera manufacturer intended.  Alternatively, a flash or camera  may use different names to describe the same function, mainly as a marketing tool.

Most DSLRs come with a popup flash. While it is of limited usefulness it does provide a good tool for learning your DSLR’s basic flash functions.


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One response to “Flash Photography Introduction”

  1. Sadeeq Jibreel says:

    That was a great photography lesson I’ve ever across with.I really impressed and understood some of the major things that I should have supposed to have known as a photographer. Thanks a lot. Really appreciate!

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