THE LIGHT-GATHERING POWER OF THE LENS
F-STOPS AND T-STOPS
F-Stops, the lens gathers light from the subject and projects its image on the film or on the digital camera’s sensor. The maximum light-gathering efficiency of the lens is called the speed of the lens. It is expressed as an f-number (f-stop or relative aperture), which is the ratio between the focal length of the lens and its diameter (maximum aperture):
F-stops do not take into account light lost internally in a lens, whereas T-stops do (see p. 153). When you use a professional lens marked in T-stops (sometimes a red scale on the opposite side of the iris ring from the f-stops), use the T-stops instead of the f-stops for all exposure calculations. Ignore the fact that light meters are marked in f-stops. T-stops should also be used whenever you are filming with more than one lens.
Insert image the FStop formula on page 144
The f-number essentially tells us the light-passing power of a lens. As the lens diameter increases, so does the amount of light that passes through the lens. As the focal length increases, the light is dispersed over a greater area and the amount of light available for exposure to the film or video sensor decreases. The faster a lens, the more light it lets through; the slower a lens, the less light. Lenses of about f/2 (that is, the diameter of the lens is one-half the focal length) are usually considered fast.
Inside most lenses is the iris diaphragm, which can close down to control the amount of light that the lens lets through. The iris is a mechanical device, usually made of overlapping blades (see Fig. 4- 8), that can be set to form a large or small hole or aperture (lens aperture). It functions similarly to the iris in the eye. In dim light, the eye’s iris opens to admit more light, and in bright light it closes down to let less light pass through.
Insert image of the aperture on page 144
Fig. 4-8. Iris diaphragm. As the blades of the iris open, more light passes through the lens.
A formula similar to the one that describes the speed of the lens is used to express the light- gathering power of the lens at any iris diaphragm opening. The f-number, or f-stop, is the focal length of the lens divided by the diameter of the aperture. The standard series of f-stops, or f-numbers, is:
1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32
The distance between consecutive numbers is called a stop. On most professional cine (movie) lenses, the f-stops are engraved on a ring on the lens barrel. Each stop represents the halving (or doubling) of the amount of light that the lens passes. At high f-numbers, the iris is more closed and less light passes through. As the ring is turned toward the lower numbers (opening up), the iris opens; conversely, as the ring is turned to the higher numbers (stopping down or closing down), the iris closes. For a lens set at f/4, for instance, opening up a stop would mean setting the lens at f/2.8 (which doubles the amount of light for exposure), and closing down two stops from f/4 would mean setting the lens at f/8. Remember, opening the lens three stops lets eight—not six—times more light in (each stop doubles the previous amount). See Fig. 7-14 for intermediate f-stops.
Lens manufacturers generally engrave the focal length, serial number, and speed of the lens (widest relative aperture) near the front element. The speed is sometimes written as a ratio; 1:1.4 would be an f/1.4 lens.
A standard technique for setting the iris is to open the lens wider than the f-stop you want, then stop down to the selected opening without passing it. This avoids any play in the iris that, in some lenses, can cause errors at the smaller apertures.
On some digital cameras, the iris is not adjusted on the lens itself but through controls in the camera. With certain cameras, the iris adjustment is made in discrete steps instead of continuously, which is a real disadvantage when making changes during a shot (adjustments are sudden, not smooth).
T-stops
Particularly when using a light meter to calculate exposure (either for film or digital), it’s important to know how much light is actually passing through the lens to the film or sensor. The f- stop is a geometric relationship between focal length and aperture and doesn’t take into account how much light is lost within a lens. Each air-to-glass surface within the lens reflects some light. A zoom lens may have more than fifteen elements and can lose a significant amount of light to internal scatter. A T-stop accounts for this loss (T is for “true” stop or “transmission”). A T-stop is defined as the equivalent to an f-stop of a perfect lens (a perfect lens transmits all the light it gathers with no internal losses). Thus, on a perfect lens, the f-stop and T-stop are identical. On a zoom lens that loses a full stop internally (that is, a 50 percent loss), setting the lens to f/8 would result in the same exposure as T11. Note that the T-stop is always a higher number than the f-stop.
Some cine lenses are calibrated in both f-stops and T-stops (sometimes the f-stop in white on one side of the iris diaphragm ring and the T-stop in red on the other side). Many lenses are marked only in T-stops; others only in f-stops. Be sure to check the lens you’re using. With prime lenses, the difference between f-stops and T-stops is usually less than one-quarter stop—not enough to upset exposure calculations. With zoom lenses, the difference is usually greater. One zoom might have the widest aperture marked as f/2, but the T-stop could be T2.5, which means this lens loses two-thirds stop in transmission.
If the lens is marked with T-stops, use them to calculate film exposures even though the light meter is marked in f-numbers (see Chapter 7). For depth of field calculations (see below), use f-stops.