Aperture beyond exposure
We have established that controlling exposure is the aperture’s primary function. Beyond that, the aperture influences several technical and aesthetic effects that are related to its central role. In this section, you’ll learn about how to determine your maximum aperture, aperture priority exposure mode, the difference between f-stops and t-stops, lens sharpness and diffraction, bokeh, and vignetting.
How is aperture indicated?
The nomenclature of most modern photographic lenses follows some variation of the following formula:
(brand + model line) (focal length)(max. aperture) (versions + features)
Lens name examples:
- Canon EF 85mm f/1.4L IS USM
- AF-S NIKKOR 14–24mm f/2.8G ED
- Sony E 18–135mm F3.5–5.6 OSS
- smc PENTAX-DA★ 200mm F2.8 ED[IF] SDM
Amidst the alphabet soup you’ll find the two most important details, the focal length and maximum aperture, expressed in that order (indicated in bold above). The conventions that describe the lens names and features are less consistent when analyzing the descriptive markings printed on the lenses. For instance, Canon, Nikon, Olympus, and Fujifilm all indicate the focal length followed by the maximum aperture, with the latter written as a ratio of one to the f-number (for example 1:2.8 is ƒ/2.8 and 1:2.8–4.0 is ƒ/2.8–4.0). Pentax and Leica mark their lenses with the maximum aperture as a ratio followed by the focal length. Sony’s lenses indicate the aperture as an undefined number followed by the focal length. Very generally, it’s helpful to remember that in the absence of any unit of measurement, numbers with single digits or decimal points indicate the maximum aperture and double-digit numbers refer to the focal length.
The aperture inscribed on a lens is typically the fastest possible for that lens, and known as its “lens speed.” Lenses can be “fast,” and they can be “slow.” In this context, speed refers to the other half of the exposure equation: the duration. Given identical light conditions, a fast lens with a large maximum aperture permits using faster shutter speeds. Since a slower lens gathers less light, an equal exposure is attained with slower shutter speed. On most small format cameras (with image sensors up to 24×36 mm), a lens whose maximum aperture falls in the range of ƒ/1.0–ƒ/2.0 is considered fast. Fast lenses tend to cost more than slow lenses because they require more glass for their larger glass elements, the inclusion of unique types of glass to minimize aberrations and because integrating both increases design and manufacturing complexity.
Aperture priority auto-exposure mode
Aperture priority mode is an automatic exposure mode in which the photographer selects the desired aperture, and the camera attempts to achieve ideal exposure by varying the shutter speed. Aperture priority mode is commonly indicated as A or Av (for aperture value) on most cameras’ mode dials. Aperture priority mode is different from other automatic exposure modes because it allows photographers to control the depth of field.
F-stops and T-stops
The chapter about Aperture and exposure stated that all lenses set to a specific f-stop will, in theory, transmit the same amount of light to the image sensor. It was an over-simplification; in practice, two different lenses set to the same f-stop will transmit slightly different amounts of light. Recall that f-numbers are derived from the focal length divided by the diameter of the entrance pupil. This sets a maximum theoretical upper limit on light transmission. Unfortunately, this equation doesn’t consider the light loss incurred during its transmission through the lens. A compound lens is composed of multiple glass elements that both absorb and reflect light. Since no glass is both 100 percent transmissive and 0 percent reflective of light, lenses will always transmit less light than the theoretical maximum implied by the f-stop.
The t-stop (t for transmission) indicates the measured light transmission value of a lens. Two different lenses set to the same t-stop will always give the same exposure. T-stops are used as the standard aperture markings on lenses designed for cinematography. In filmmaking, it’s common to set camera exposure and light intensity using external light meters. For these settings to remain correct in a scene filmed with multiple lenses, the lenses must be calibrated in t-stops. T-stops are calculated using the formula:
t-stop = (f-stop)/(lens transmittance %)
T-stop values are mostly obsolete in modern photography. With through-the-lens (TTL) light metering, cameras are using transmitted light values to determine exposure settings. Furthermore, t-stops pervert the calculated values for depth of field and hyperfocal distance, which are directly related to the actual f-stop.
Aperture and sharpness
Sharpness, or acutance, describes the ability of a photographic lens to resolve fine image detail of a subject that’s in focus. In technical circles, it’s determined by photographing test charts to measure how many distinct lines per millimetre a lens is capable of resolving. In practical photography, it’s defined by sharp edges in the scene being rendered as sharp edges in the photograph. A sharp lens reproduces details precisely across the frame, while a lesser lens may produce images with a loss of acutance towards the corners, where details may appear smeared, blurred, or split into their constituent colours, as if by a prism. Such loss of sharpness is caused by the presence of lens aberrations, to which no lens is immune.
Your choice of aperture has a strong influence on lens sharpness. Optical aberrations are most pronounced when a lens is set to its largest aperture. The severity of aberrations decreases as the aperture is stopped down. In general, modern lenses achieve their peak optical performance, their “sweet spot,” in the range of ƒ/4 to ƒ/8 (or about 2.5 to 3 stops down from the largest available aperture). Common sense would dictate that aberrations should continue to decrease as a lens is stopped down beyond this range, but the effective increase in sharpness never transpires; in fact, once the sweet spot is surpassed, sharpness starts to decline due to diffraction of light.
In photography, diffraction is the phenomenon of light “bending” slightly around the sharp edges of the diaphragm blades, which causes it to spread and diffuse marginally more than the light passing through the aperture’s centre. Although diffraction is present at all aperture sizes, it becomes most pronounced with smaller apertures because a higher proportion of the total light striking the image sensor is diffracted. As aperture sizes decrease, diffraction increases and the result is reduced image sharpness.
Aperture and vignetting
Every photographic lens produces a circular projection of light. This so-called image circle is always large enough to adequately cover the image sensor format for which the lens is designed. The image circle’s boundary, being where the projection ends and shadow begins, isn’t sharply delineated, but instead, shows a gradual darkening. It’s this zone of transition from light to dark that causes most lens vignetting.
Lens vignetting, or light fall-off, manifests as the progressive darkening of the image towards the corners of the frame (the corners are closest to the image circle’s boundary). Most lenses exhibit the highest amount of vignetting at their largest apertures, and stopping down the aperture reduces light fall-off dramatically. This occurs because vignetting is caused by the physical obstruction of light by the lens barrel, whose shadow is rendered out of focus—and hence as a gradual transition to black—due to its proximity to the frontmost lens element. Stopping down a lens reduces the depth of field and creates an increasingly sharper image circle boundary—although it never becomes solid—thereby reducing vignetting.