There are two key elements to the apparatus of the camera that control the way it exposes light; Iris (aka Aperture) and Shutter. Understanding what these two camera mechanics do is fundamental to getting not just a ‘good’ image but also sculpting the particular ‘look’ you’re after.

The Iris controls the size of the hole through which light will pass to the camera’s sensor. The size of the iris opening is measured in f-stops where each ’stop’ is twice the amount of light of the previous ’stop’. The wider open the Iris is (a smaller f-stop) he more light let in. The smaller the Iris (shown as larger f-stop numbers) the less light let through to the sensor.

Simple enough?

There is a second element of control the Iris introduces on top of the control of light. The size of the Iris directly effects the Depth of Field of the image.

DoF is of course the area in focus within the image; a deep depth of field has both foreground and background in focus. A shallow depth of field has a narrow point of focus with the background and foreground beyond the point of focus blurry.

A small Iris letting in less light creates deep depth of field. A wide open iris creates a shallow depth of field.

This concept of the Iris and depth of field is commonly understood in both still photography and moving image cinematography. What is often less well understood in the context of cinematography is Shutter speed.

The shutter is effectively an oscillating blade system to control how the image is exposed. The shutter speed indicates the intervals of time that the Shutter is open to expose light to the sensor. It is most common in digital cinematography to set the Shutter speed to an even multiple of the frame rate to create smooth image motion. For example with standard 25fps PAL-based video the shutter would often be set to 1/50th of a second.

The fundamental concept of the shutter speed is that the faster it is the sharper the image motion will be. The slower the shutter speed is the more motion-blur will be in each frame when either subject or camera move.

This factor of using shutter speed to control motion-blur becomes a very powerful tool for creating a particular look or style. Slower shutters can give a more film-like and organic smooth motion where the image blurs as it moves or subjects move within the shot. Faster shutters give a crisp dynamic motion that can suit action sequences. The battle scenes from Spielberg’s Saving Private Ryan are an example of very fast shutter to create staccato flickering motion.

The important thing to remember is that shutter and iris are interconnected in terms of exposure. Slow down the shutter to get more motion blur and you will let more light in. This means you’ll have to close down the iris to limit the light to avoid over-exposure. But when you close down the iris you increase Depth of Field and loose the precision of shallow focus.

In the camera everything is connected.