The principal function of ISO and its influence on effective exposure is described in the introduction to ISO and requires no repeating. The purpose of this chapter is to acquaint you with the correct method of using ISO to manipulate the effective exposure of a photograph, the practical consequences of raising ISO values, and, lastly, some means for minimizing the effects of image noise in software.
Correct ISO values
In digital photography, the best practice for choosing the right ISO value is to keep it as low as practicable in each scenario. In other words, avoid setting an ISO value that’s higher than necessary to achieve the effect you desire from the aperture and/or shutter speed.
Aperture Priority. If you’re using Aperture Priority mode and the camera is indicating a shutter speed that you know is two stops too slow for handheld photography, raise the ISO by two stops to achieve a faster shutter speed.
Shutter Priority. If you’re using Shutter Priority mode and the camera indicates that it can’t obtain a correct exposure because it requires a larger aperture than your lens can achieve—indicated by the maximum aperture value blinking in the viewfinder—progressively raise the ISO until the f-number stops flashing.
Reciprocity law and ISO
In the Reciprocity Law section, the relationship between aperture and shutter speed was described as an inversely proportional compromise between pictures that exhibit a great depth of field and slow shutter speeds or shallow depth of field and fast shutter speeds. The ability to raise ISO values in digital photography allows you to realize photos showing both great depth of field (via small apertures) and frozen motion (via faster shutter speeds).
ISO priority mode
Except for Pentax, no camera manufacturer offers an explicitly named ISO priority mode. Nevertheless, Program mode has existed as the de facto ISO priority mode on digital cameras for many years. Program mode is an automatic exposure mode in which the photographer selects a desired ISO value, and the camera attempts to achieve ideal effective exposure by varying both the aperture and shutter speed. Program mode is commonly indicated as P on most cameras’ mode dials.
In most digital cameras, the ISO value is automatically set in all fully automatic modes. In other exposure modes, it’s possible to designate a range of ISO values, from minimum to maximum, from which the camera automatically selects the most appropriate to achieve optimal effective exposure.
Manual mode. In Manual mode, the aperture and shutter speed values are set by the user. With ISO set to Auto, the camera will attempt to achieve optimal effective exposure by automatically varying the ISO within the preset Auto ISO range. Pentax is the first and only camera manufacturer to offer the functionality described above as a dedicated exposure mode, known as Shutter & Aperture Priority (TAv).
Aperture Priority. In Aperture Priority mode, the aperture value is set by the user, and the shutter speed value is determined by the camera. When configuring Auto ISO, many cameras allow photographers to designate a desired minimum shutter speed. If the camera determines that the ideal effective exposure requires a shutter speed value that is slower than the set minimum shutter speed, the camera will increase ISO within the Auto ISO range to reach the optimal effective exposure.
Some cameras offer two possibilities for determining the minimum shutter speed, either determined automatically by the camera or set manually by the user. When done automatically, the camera sets the minimum shutter speed to the reciprocal of the focal length of the lens, which follows the minimum shutter speed formula for handheld photography. (Some cameras offer further refinements to the method.) When set manually, the photographer selects a desired minimum shutter speed, which the camera will maintain regardless of the focal length of the lens. In general, the camera will only increase the shutter speed above the set minimum when the scene becomes too bright to achieve ideal effective exposure using the minimum ISO value of the auto ISO range.
Shutter Priority. In Shutter Priority mode, the shutter speed is set by the user, and the aperture value is determined by the camera. When the camera determines that the ideal exposure requires an aperture that’s larger than your lens can achieve, the camera will increase ISO within the Auto ISO range to reach the optimal effective exposure. Since the aperture range of a lens is much narrow than the camera’s shutter speed range, the Auto ISO function doesn’t permit setting an aperture limit.
Native ISO and expanded ISO
Most cameras offer two types of ISO values, native and expanded. Native ISO consists of the ISO values generated through amplification, or gain, of the image sensor’s analogue signal. The majority of your camera’s full-stop ISO values—such as 100, 200, 400, and so forth—are native. In cameras that offer Expanded ISO, the Expanded ISO values, often marked as high or low, are achieved by simulating changes to exposure after the signal has been digitized, typically via software adjustments to brightness. For example, when taking a photo at ISO 12,800 on a camera whose highest native ISO is 6,400, the camera will underexpose an ISO 6,400 image, and subsequently lift its brightness by one stop in software; this process of inversely proportional underexposing and brightening is known as pushing. Conversely, when taking a photo at ISO 50 on a camera whose lowest native ISO is 100, the camera will overexpose an ISO 100 image, and then reduce its brightness by one stop in software; this process of inversely proportional overexposing and darkening is known as pulling. Pushing an image using Expanded ISO producers images with increased noise and less shadow detail. Pulling an image using Expanded ISO nominally reduces image noise at the risk of losing details in the bright areas of the image to overexposure.
Whether the intermediate ISO values are a product of analogue gain (native) or pushing and pulling (expanded) is the cause of much online speculation. Every camera that permits user selection of the intermediate ISO values includes them as part of the Native ISO range. If the speculators are correct and the intermediate ISO values are derived by some variation of pushing or pulling of adjacent Native ISOs, then the difference is only a maximum of one-third stop, which should neither result in a perceptible increase in noise or noticeable loss of highlight details.