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As you know, exposure is a delicate balancing act that requires the alignment of four distinctly different yet interrelated variables that include Illumination, Sensor Sensitivity, Shutter Speed, and Aperture. The first factor that we're going to consider is brightness or luminance. Luminance describes how much light--either from a natural source like the sun or an artificial source like a small strobe-- is falling on the subject. The second variable, Sensor Sensitivity, determines how much light is required to record an exposure.
The ISO setting determines the sensor's sensitivity to light. The setting controls the amount of amplification that is being applied to the digital data that is collected by the sensor. When you are shooting in low light, or with an underpowered flash, you can increase the numerical value of the ISO to increase the gain or amplification. As this gain increases, less illumination is needed to achieve the same exposure. Together, luminance and sensor sensitivity combine to determine what in available light is known as the Exposure Value.
Once you know how much light you have and how much you need, the photometric system identifies a unique number called the Exposure Value. The Exposure Value is linked to all of the combinations of shutter speed and aperture that will create a normally exposed image. A similar process occurs when shooting with flash. But instead of beginning with a continuously burning light source like the sun or a tungsten bulb, all of the light is produced in a very short burst of energy from a flash tube. When we're dealing with small strobes, the power output of the flash unit and the sensor sensitivity combine to determine what is called a Guide Number.
I'll get into this in a lot more detail in a later chapter, but once we know the guide number for any given flash, then we know all of the combinations of flash to subject distance and aperture that will create a normal exposure at any given ISO. Learning the principles behind the guide number is the key to unlocking the mysteries of small strobe flash exposure. That's what I hope to do with you in the rest of this course.
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