ISO Invariance and Exposure Strategy

Started Jan 31, 2018 | Discussions thread
gollywop Veteran Member • Posts: 8,284
I'll try.

Somewhat oversimplified, image noise consists of shot-nose and read-noise.

The shot-noise comes with the light (signal) during exposure and cannot be controlled. However, the ratio of shot-noise to signal decreases as the signal increases. This has two implications: (1) overall image noise is relatively less the greater the overall exposure* to light, and (2) the darker parts of the image have greater relative shot-noise than the brighter parts. As a further implication of (1), when shooting raw, one is best off maximizing exposure before adding ISO beyond base ISO. If one can achieve a maximal exposure at base ISO just shy of clipping desired highlights, one has achieved exposure to the right (ETTR).**

The read-noise is due to the camera's electronics and software and, depending on the camera, can vary with the camera's ISO setting. As a further implication of (2), read-noise is swamped by shot-noise in the brighter parts of the image and becomes important in overall image quality only in its effects in the darker shadows.

A truly ISO-invariant camera would have very low read-noise at all ISO settings. The image noise for such a camera is, therefore, essentially due to the shot-noise inherent in the light itself. If, then, one were unable to achieve ETTR (because of shooting contraints imposed by desired DoF affecting aperture or motion-blur/camera-shake considerations affecting shutter speed), the image noise would be effectively the same regardless of whether one increased the image brightness using the camera's ISO control or later during the raw processing. In other words, with an ISO-invariant camera, one would not have to use the camera's ISO, but could instead do all needed brightening afterwards during the raw processing. The latter has the advantage that it removes the possibility of inadvertent and unrecoverable highlight clipping through the application of excessive in-camera ISO.

There are, however, no truly ISO-invariant cameras.

Instead, virtually all cameras are ISO-variant to some extent. ISO-variant means that the read-noise decreases as the camera's ISO control is increased. For some cameras (Canons) this effect is significant over a wide range of ISO. For others, the effect is less pronounced and, indeed, may disappear altogether with low read-noise at a relatively low ISO (such as ISO 400, or 800, or 1600, depending on the camera). At this point, such cameras become essentially ISO-invariant for further increases in ISO. Let us denote this ISO, if it exists, as the point of ISO-invariance. You can check the nature of your camera's ISO-behavior at Bill Claff's web site.

If, then, one is unable to achieve ETTR (max exposure to avoid unwanted highlight clipping at base ISO) with an ISO-variant camera, one should first maximize exposure subject to shooting considerations at base ISO (to minimize relative shot noise), and then apply in-camera ISO as needed, up to the point of ISO-invariance, to achieve reduced read-noise. At this point the camera becomes ISO-invariant, and so any additional brightening that may be needed can be done equally well either using in-camera ISO or later during raw processing. As before, the advantage of doing any such brightening afterward during raw processing is that it helps to avoid inadvertent clipping resulting from excessive ISO.

The preceding is primarily aimed at raw shooters. If you're shooting JPEG, do as much of your in-camera image brightening as you can via exposure (f-ratio and shutter speed) and use the minimally needed amount of in-camera ISO.

* As determined for the light from a given scene by the f-ratio and shutter speed.

** Some cameras, such as the Nikon D810 and the Sony A7Rii, employ a sensor technology (DR-Pix) that effectively has two base ISOs.

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