The Aperture (Iris)

apertureThe 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.

F-Stopsf-numbers

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:

f-stop scale

This f-stop scale begins at f/1 (the widest aperture) and end at f/32 (the narrowest aperture). Most lenses, however, don’t go to those extremes, but the professional ones might.

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.

 

Lens Speed: How Fast is That Lens?

When you hear the pros talking about lenses, they may ask something like, “How fast is that lens?” In this case, they are most likely referring to how wide the aperture can get, or the maximum aperture diameter. This is valuable information because a wide aperture means that more light enters the camera optically, which is good because cinematographers needs less production lights to obtain the exposure desired.
To illustrate: above you saw a hypothetical chart with the widest possible aperture being f/1. Most consumer cameras don’t even go that much. I don’t know the average, but based on my experience, I would say that f/2 is realistic average for most consumer lenses.
However, when shooting the period feature film Jane Eyre (2011, Cary Joji Fukunaga), cinematographer Adriano Goldman used lenses that were f/0.6 fast. That is fast! So fast in fact that it facilitates candlelight exposure, which is exactly what the movie required.
This explains why fast lenses are expensive. They are  simply too awesome! But to be fair, they are only indispensable in extreme situations, which is why most consumers can get away without them.

 

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.

aperture-depth-of-field

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 of the red area as the f-stop number increases.

To illustrate, consider the following example. The photo below was taken with the aperture at f/2 :

aperture

 

 

Note that the background is in soft focus and not much detail can be recorded. 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 and  ISO, adjusted to preserve exposure.)

aperture

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. All this new elements are now visible due to a simple aperture change, which is why photographers and filmmakers worry too much about it.

How the Pros Talk

The forward slash or dash is silent when the f-stop is said aloud. F/16 would be read as “f sixteen,” F/2 would be read “f two,” and so on. In numbers with a decimal, the decimal mark or point is usually excluded in conversations, so that F/2.8 would be read as “f two eight.”

When the DP wants you to close the aperture, he may ask you to “stop down.” If he wants you to open the iris, he may simply say “open up.” There’s no such thing as “stop up.”

Aperture, iris, and f-numbers refer to the same opening in the lens. They can all be used interchangeably.

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