Flash and Ambient Light

A quick one today about flash and how to use ambient light. One thing that some people forget about the flash is that it goes of at about 1/2000s to 1/8000s. Some can go even faster. In most cases, the shutter speed have little to no affect on the light from the flash. This can be confusing since we normally think of the shutter speed as something that controls the light getting to the sensor.

The shutter does affect the light going to the sensor. So, why did I just say the shutter have little to no affect on the light from the flash. It's because the burst of light from the flash is very very fast. Like I said, some will burst in as little as 1/8000 of a second, or even faster! The difference is the intensity of the light from the flash compared to the ambient light. Imagine a semi dark room. If I take a picture at 1/200s, ISO 200 and f/5.6, the picture will be almost completely dark. Now, with the flash, at the exact same setting, my picture is perfect...well...not so quick...it will only be perfect if the intensity of the flash is set correctly. If I set it too high, then it will be over exposed, if I set it too low for my subject, it will be underexposed.

This is also where TTL comes in. TTL stands for Through The Lens. There are various version of TLL now, unlike the old days of film. The latest Nikons are iTTL. The flash will fire off a test burst of light and measure the light bouncing back from the subject through the lens and adjust accordingly. iTTL, Auto mode, Manual mode flash will be another time...today, we're talking about how ambient light can be used to flash lighting.

First example, a picture using a flash at ISO200, f/5.6, 1/30s


Notice the ambient light in the back ground? Because the shot was taken at 1/30s, the shutter was slow enough to let in the ambient. The flash was set to expose the subject only.

Now, basically the exact same shot, except this time, the shutter was set at 1/800s.


As you can see, the flash's light intensity was still the same to correctly expose the subject, but because the shutter was set to 1/800s, it allowed a lot less ambient light through. So, if the shutter has little effect on the light from the flash, what does? Other than changing the flash setting, the aperture has a distinct direct effect on the light from the flash. Of course, the ISO also has a pronounced effect on the light from the flash as well.

Again, have fun with it and shoot shoot shoot...take picures, make a note of your settings and experiment! :)

This is just a quick start. More strobist stuff to come...especially when we get to off camera flash! That's when the real fun begins.