Working with Flash

Okay so I attended a CPW workshop on flash photography -part one. Many photographers consider flash as the enemy and where ever possible avoid using it. Oh so lucky to have the right light all the time! No doubt, the pop up flash and the flash you place on your DSLR can be unflattering however correct placement and use of this can reap rewards. So too can you improve your photos by taking the Flash off Camera. The thing is there is a bit of Maths involved and you need to know your aperture (f stops). So here we go;-

1.0,1.4, 2.0, 2.8, 4, 5.6, 8, 11, 16, 22, 32 

f/stops are a bit confusing because the numbers appear so arbitrary. This is the standard sequence of f/stops from f/1.0 to f/32. Although it may not seem intuitive at first, in this sequence the f/1.0 setting lets in the most light while the f/32 setting lets in the least. Also, each of these f/stops has precisely the same halving/doubling of light let in.


As Glynn from CPW said think of Aperture as the dimmer  and the higher the number the less light on the sensor of your camera. Your shutter speed controls the ambient light (the Scene) and of course the flash lights your main subject.

Camera flashes normally provide specifications for Guide Number (GN) as a guide to the flash power and its distance capability. Specifically GN is a tool for manual flash power levels.  Typically the aperture for any particular shot can be worked out by dividing the GN by the distance from the object you are shooting. Aperture = Guide Number / Distance

So by rearranging the variables we are also able to determine the following;

Distance = Guide Number / Aperture

Guide Number = Aperture x Distance

If for example, if  the GN is 32 and you place the Flash 8 Metres from the subject then the Aperture will be f/4.   The advantage of knowing this Guide Number constant is that if we then move the light to be 2 metres from subject, then it tells us that GN32/2 metres= f16 will give us correct exposure there too. All Guide Numbers are based on ISO 100. So if you increase ISO what are you doing? You are increasing the sensors sensitivity to light and by each step up in ISO you are effectively increasing by a stop. It is at this stage that you realise a calculator would be handy and this is where a handy app on an iPhone is useful. Thankfully most Photographers own an iPhones and there is a  handy app PhotoCalc which allows you to do the sums changing the variables and also if you are wanting to decrease flash power, increase ISO or use incremental fstops. Whilst this app is handy it is best to understand the concepts. So you know the concepts but nothing beats actually doing it and experimenting with the placement of the flash and dialling up or down your f-stop. Having been on  a couple of workshops I have also noticed subtle differences between Nikon and Canon and from Canon to Canon. Some of this may be to do with the LCD display settings on the back of the camera. One thing you have to watch is the placement of the flash making sure you do not get the light reflected back at the camera. It would seem in most situations placing the flash 90 degrees to the subject works the best. Start practicing with one flash then move up to two or more.