Off Camera Flash - How To Control Exposure

Canon Dedicated Flash - very capable, very complicated (£400)

We have talked a lot about the "Exposure Triangle" that we use to set the visual attributes of our non flash photographs.  It gets a little more complicated if we want to use flash to illuminate our images.

I suspect most of our flash photography up to now has been with the built in camera flash or an on-camera flash; in which case the camera has probably worked everything out for us using Through The Lens (TTL) metering and we have not had to change our way of thinking.   Take the flash off the camera and things start to get more complicated unless we stick with high end (expensive) dedicated flashes.   We can continue to use TTL for off camera flash if we use flashes that are dedicated to the make of our camera but these flashes cost £200 - £400 so are extravagant if we want to use multiple flashes.  A non dedicated slave flash can be purchased for around £70. 

Yongnuo flash, still complex, but easier to use
and only £75.

Why Take our Flash Off Camera?

On camera flash has its role but is not ideal for portraiture or close up photography where we want to use shadows and texture to add interest.  We create those shadows by moving the flash off the camera/subject axis.  Then, having deliberately added shadows we might want to reduce them by adding in more light on a different axis.  So we can end up with 2 (or 3) light sources away from our camera.  Unless we are very well off, we are unlikely to have 3 dedicated flashes so need to consider manual control of our Flash light. 

What Controls our Flash Image

If we work in manual mode to cater for non dedicated flash units, there are now 5 factors that can be adjusted to change the flash results:

3 are the same ones we are already familiar with:

• ISO - the sensitivity of our photographic sensor
• Aperture - the determinant of depth of field in our images.  The impact is greater because, if we are using flash, we are closer to the subject.
• Exposure, 
• for flash photography we need to be aware of minimum and maximum constraints (more in a moment)

Flash photography introduces 2 new factors:

• Flash Power - how bright the flash is.
• Light to subject distance
• The light reaching our subject varies according to an inverse square law.  If we double the flash to subject distance we reduce the light reaching the subject by a factor of 4.
• At close distances, the fall off in illumination is very quick and has to be considered when photographing groups or a head to feet image, for example.

Exposure Limitations

Upper speed limit

The light produced when the flash fires has a very short fixed duration.

Although we can set very short duration exposure on our cameras, e.g. 1/8000th second, the shutter itself cannot move at that speed.  Our camera overcomes this limitation by using 2 shutters, one to expose the sensor and a second coming up close behind to shut off the exposure.  At very high shutter speeds the shutter is never fully open; only a small strip of the sensor is exposed at a time.  If a flash is fired during this time only that strip of the sensor that is exposed will receive the light.  A dark strip will cover the rest of the image.

Most modern cameras will automatically limit the exposure when the internal flash is activated or a flash attached.  It is not possible to set a speed faster than the synchronisation speed.

Lower Speed Limit

There is no physical limit on the lower speed I can set on my camera but you need to remember that the flash only fires once and for only a very short duration.  Increasing the length of the exposure does not increase the amount of flash light reaching the sensor, it only increases the ambient light.

Baseline Settings and Do We Need a Flash Meter?

A flash meter was essential in the days of film photography as there was no quick way to see the results.  The cost of flash meters meant that this style of photography was the preserve of professionals.  Digital photography allows us to use trial and error; instant review and no limit to the number of images we can take.

Start from settings that match the style of image you are after, for example:

• ISO 200 - for maximum quality, though modern cameras create excellent images at faster than this.
• 1/250 sec - or alternative camera maximum synch speed.
• f4.5 - to minimise depth of field

Then adjust the flash power and flash to subject distance to give a correctly exposed image.