Exposure...Part 3 - ISO

ISO - Sensor Sensitivity to Light ISO stands for the "International Organisation for Standardization". The ISO was founded in early 1947 and is headquartered in Geneva, Switzerland and is composed of representatives from various of it's 158 members national standards organisations.

One of the standards defined by this organization is photographic film's sensitivity to light. An image sensors 'film speed', is often referred to as its "ISO number."

ISO - and Stops of Exposure

Just like aperture and shutter speed ISO is set up as multiples of 2. ISO 100 allows twice as much light as

Exposure.....Part 2 - Shutter Speed

Today's DSLR cameras use a two curtain focal plane shutter.

At left is an image of a two curtain focal plane shutter part way through the sequence of making an exposure. The image sensor is immediately behind the curtains.
This shutter was set to 1/500th of a second. Both curtains move from the top to the bottom. The front curtain is released first and in this image is near the bottom of it's range of travel below the black horizontal gap low in the image. The rear curtain follows close behind due to the the 1/500th of a second shutter speed.
The gap between the two curtains travels down and is spaced to expose each part of the image sensor for the required 1/500th of a second.
Once the exposure has been made both curtains get reset to the top of the frame so they are ready for the next exposure.
If the shutter had been set to 1/60th of a second the front curtain would travel all the way down before the

Exposure...Part 1 - Aperture

While you're fresh we'll tackle the math stuff first. If you skip it I can guarantee you'll be lost further on. In fact you'll probably have to go over this first part several times. Believe me, you won't be the first, nor the last to do so but it's part of the essential knowledge you need to have for making creative exposure adjustments.


Inside the lens is a mechanism called a diaphragm that adjusts the size of the lens opening (aperture), larger or smaller. Controlling that lens opening is the first step in controlling the exposure. The photograph on the left shows a 50 mm lens aperture set to f/2.8 (#1) and then adjusted to an aperture of f/16 (#2).