The Aperture (Iris)
The word “aperture” means “opening.” In the world of cinematography, this opening is located in the camera, more specifically on the lens. The size of the aperture is regulated by the lens’ diaphragm, which opens to admit light and closes to block it. The figure to the right shows the diaphragm, which is formed by several blades that leave a circular hole in the center – the aperture.
When the camera is in auto mode, the diaphragm achieves a certain level of autonomy (self-control) and behaves according to the camera’s sensors, which determines the amount of light entering the lens.
In auto mode, the camera uses its built-in reflected light meter to average the quantity of light falling on the subject in frame. If too much light is reaching the subject, causing the image to be overexposed or washed out, the aperture closes to shield some of the light. When this happens, the image darkens and exposure is corrected according to the camera’s judgment.
The size of the aperture is measured in f-stops. In technical terms, the f-number is the focal length divided by the diameter of the aperture. The figure to the right shows the same lens open to two different f-stops. In the top picture, the f-stop is 2.8. In the bottom picture, the f-stop is 16.
Ergo, the greater the f-stop number, smaller the aperture is, resulting in less light entering the lens and the production of dark images. Conversely, the smaller the f-stop number, bigger the aperture is, meaning more light admitted through the lens and the production of bright images.
Also note that the variation from f/2.8 to f/16 is quite drastic. In the figure, several f-stops were skipped to better illustrate the point. Between f/2.8 and f/16, there are four whole stops, as the f-stop scale below demonstrates:
The f-stop scale can begin at f/1 (the widest possible for most cameras) and end at f/32. Most video or film cameras, however, don’t go that narrow; though some still cameras can stop down all the way to f/64 and beyond.
Also note the scale above only shows full stops, disregarding fractional stops that exist between one full stop and the next. Whole stops are important because they represent the admittance or blockage of half or double the light.
For instance: f/1 lets in twice as much light as f/1.4. Likewise, f/1.4 lets in twice as much light as f/2. On the other hand, f/16 lets in half as much light as f/11. And f/5.6 lets in half as much light as f/4. And so on.
Aperture and Depth of Field
The aperture has a major role in determining not focus per se, but depth of field. The greater the f-stop number, the deeper the depth of the field is, meaning that more objects are likely to be sharp in frame. Likewise, the smaller the f-stop number, the shallower the depth of field is.
The diagram below demonstrates the relationship between aperture and depth of field. Assume that the camera is on the left side of the image, where lens and aperture readings are located.
The area in red represents what’s in sharp focus. With a 100mm lens, the plain of critical focus is at 4 meters (approximately 13 feet) from the camera. Observe the variation as the f-stop number increases.
To illustrate, consider the following example. The photo below was taken with the aperture at f/2 :
Note that the background is in soft focus and not much detail can be recoderd. Only the lenses in the front and intermediate grounds can be identified.
Now, the following photo was taken with the aperture at f/11 and the very same settings (aside from the shutter speed, adjusted to keep decent exposure.)
Compare both pictures. In the second, we have far more detail on the background, which seems to contain a VCR deck with VHS tapes on it. The object on the right-hand side looks like a helmet. On the left there is a bag.
The Language of Cinematography
The forward slash or dash is silent when the f-stop is said. F/16 would be read as “f sixteen.” F/2.8 would be read as “f two point eight.”
The word “iris” and “aperture” are interchangeable. However, you could be asked to do an “iris rack,” while you probably will never hear someone say “aperture rack.”
When the DP wants you to close the aperture, he may ask you to “stop down.” If the DP wants you to open the iris, he may simply say “open up.” There’s no such thing as “stop up.”