Understanding your aperture and how to use it is a primary skill that every shooter should understand.
Aperture is the opening that lets light pass through your lens to your sensor. The iris is the series of blades that open and close to make your aperture larger or smaller.
The iris can be controlled by a ring on your lens, a dial on the side of your camera, or by accessing a menu system on your camera.
Opening your iris will allow more light to pass through, giving you a brighter exposure. It will also reduce the depth of field in your shot. The depth of field refers to how much area in front and behind your point of focus appears perceptibly sharp.
Closing your iris down will let less light in, giving you a darker exposure. It will also increase your depth of field, meaning a larger area in front and behind your point of focus will look perceptibly sharp.
Alright, what about auto iris?
It’s tempting to set your iris to automatic, but this is not the best technique for most situations.
Letting the camera decide what portions of your scene are relevant often results in overexposure or underexposure of your main subject. Another issue is that the exposure can shift rapidly, especially if your shot is moving and bright highlights and dark shadows are passing through your scene. In general, you want to set your iris manually.
Auto iris may be a valid option if you’re moving from one extreme lighting scenario to another, for instance, outside on a sunny day to a dimly lit interior, However, even in this case, the preferred method would still be to change your iris manually via the ring or dial as you’re shooting to maintain control.
In video production, we want to quantify everything we can, and the amount of light hitting your sensor is no exception.
The amount of light that hits your sensor is affected by both the size of your aperture, and the focal length of your lens. The focal length of your lens is a measurement from the optical center of your lens and your sensor. The f-stop, or f-number takes both of these factors into account to arrive at a standard reference.
In mathematical terms, the f number is the ratio of the lenses focal length to the diameter of the aperture. Unfortunately. the resulting numbers from this ratio are totally counterintuitive.
Lower fstop numbers represent more light reaching your sensor, while higher fstop numbers represent less light reaching your sensor.
Common f-stop numbers are 1.4,2,2.8,4,5.6,8,11,16 and 22.
Opening your iris a full f-stop represents a doubling of the amount of light being let through. Closing your iris a full stop represents halving the amount of light.
Cinema lenses will often use t-stops in place of f-stops. While f-stops are a mathematical formula that ignore light lost as it passes through your lens, t-stops account for this loss, and are a real-world measurement that is precisely accurate.
Using your iris to control your aperture is one of the three main camera controls in the exposure triangle. While beginning shooters will use this as the primary means to expose a shot, professionals use it as a tool to control their depth of field, and use the amount of light in a scene or filters to get their exposure correct.