Once you have mastered the natural light exposure triangle (ISO, shutter speed and aperture), taking the shot becomes a fairly simple task. Off course you compensate up or down in some situations depending on the contrast, colors and mood but it is fairly a simple thing specially if you are are aperture priority, shutter priority, or program modes.
Now when an artificial light comes into play like a speedlite in the middle of the day sun or at sunset and you want to well balance the tones between your subject and the background, it becomes a big challenge. In this post I will show you by example that this is easier than you think.
Light, is the main ingredient in a photograph. There are of course other ingredients: time, composition, subject, emotion and your unique style, but light is the thing that makes photography work. Now, first things first; what are the facts that you need to know about artificial lighting:
- There are 2 exposures in artificial lighting, one for the ambient light and another for the light source itself.
- Shutter speed controls ambient light whereas aperture controls the amount of artificial light hitting your sensor. Use a faster shutter speed to dim the ambient and a slower shutter speed to collect more ambient. ISO affects the “volume” of the signal therefore affects both flash and ambient equally.
The closer the light source is to your subject, the softer the shadows and moving the light source further away, creates more harsh shadows.
Speedlite, unlike permanent studio lights, fire at the same intensity i.e. there is no way to make it brighter. It is the duration of the explosion that produces more light.
Artificial illumination is affected by distance. This is known as the inverse square law leading to light “falloff”. The subject-to-light distance only matters; not the distance between the subject and your camera.
Every DSLR has a maximum flash sync shutter speed (1/200 or 1/250). At shutter speed faster than flash sync, flash will expose only a slice of the image.
Now the challenge is balancing natural and ambient lights to have a correct exposure when your your photo has varying light levels like a backlit subject or a portrait at dusk. Take the below scene of my son Jad taken at mid-day sun. Exposing for Jad, the background will be washed out. Exposing for the background, and Jad would be too dark. With me adding flash to the equation, I have exposed "correctly" for both. By adjusting the shutter speed and aperture to get exactly the desired tone in the background and then filling my subject with flash sufficient to have a balanced tonal range. Off course I like to lighten mu subject and darken the negative space, so I increase shutter speed to reduce the ambient light hitting my sensor.
I shot this photo at 1/200th at f/13 at ISO 100. Off course I could have shot it at the correct exposure as suggested by camera metering but when using flash there is no correct exposure. I all depends on how you want the background to look.
I could have dragged the shutter to 1/125 and lighten the background. I could have also increased the power of the flash or close the aperture to f/5.6. Well I didn't because I wanted the background to appear dark. Should I wanted it to appear darker, I could have increased the shutter speed to 1/250th.
Which is correct? Well, they all are. It all depends on you and what you would like to achieve.