Mastering Exposure in Photography & Video Part 4: ISO

The acronym stands for International Organization for Standardization, the body which in 1974 combined the old ASA (American Standards Association) units of film speed with the German DIN standard. That’s why you’ll often hear the terms ISO and ASA used interchangeably.

Two different cameras filming the same scene with the same filters, aperture and shutter settings will not necessarily produce an image of equal brightness, because the ways that their electronics convert light into video signals are different. That is why we need ISO, which defines the relationship between the amount of light reaching the sensor (or film) and the brightness of the resulting image.

For example, a common ISO to shoot at today is 800. One way of defining ISO 800 is that it’s the setting required to correctly expose a key-light of 12 foot-candles with a lens set to T2.8 and a 180° shutter at 24fps (1/48th of a second).

If we double the ISO we double the effective sensitivity of the camera, or halve the amount of light it requires. So at ISO 1600 we would only need 6 foot-candles of light (all the other settings being the same), and at ISO 3200 we would need just 3 foot-candles. Conversely, at ISO 400 we would need about 25 foot-candles, or 50 at ISO 200.

Note that I said “effective” sensitivity. This is an important point. In the photochemical world, ISO indeed denotes the light sensitivity of the film stock. It is tempting to see digital ISO as representing the sensitivity of the sensor, and changing the ISO as analogous to loading a different film stock. But in reality the sensitivity of a digital sensor is fixed, and the ISO only determines the amount of gain applied to the sensor data before it is processed (which may happen in camera if you’re shooting linear or log, or in post if you’re shooting RAW).

So a better analogy is that altering the ISO is like altering how long the lab develops the exposed film negative for. This alters the film’s exposure index (EI), hence some digital cameras using the term EI in their menus instead of ISO or ASA.

We can take this analogy further. Film manufacturers specify a recommended development time, an arbitrary period designed to produce the optimal image. If you increase (push) or decrease (pull) the development time you will get a lighter or darker image respectively, but the quality of the image will be reduced in various ways. Similarly, digital camera manufacturers specify a native ISO, which is essentially the recommended amount of gain applied to the sensor data to produce what the manufacturer feels is the best image, and if you move away from that native ISO you’ll get a subjectively “lower quality” image.

The most obvious side effect of increasing the ISO is more noticeable noise in the image. It’s exactly the same as turning up the volume on an amplifier; you hear more hiss because the noise floor is being boosted along with the signal itself.

I remember the days of Mini-DV cameras, which instead of ISO had gain; my Canon XL1 had gain settings of -3dB, +6dB and +12dB. It was the exact same thing, just with a different name. What the XL1 called 0dB of gain was what we call the native ISO today.

At this point we need to bring in the concept of dynamic range. Let’s take the Arri Alexa as an example. This camera has a dynamic range of 14 stops. At its native ISO of 800, those 14 stops of dynamic range are equally distributed above and below “correct” exposure (known as middle grey), so you can overexpose by up to seven stops, and underexpose by up to seven stops, without losing detail.

If you change the Alexa’s ISO, those limits of under- and overexposure still apply, but they’re shifted around middle grey. For example, at 400 ISO you have eight stops of detail below middle grey, but only six above it. This means that, assuming you adjust your iris, shutter or filters to compensate for the change in ISO, you can trade-off highlight detail for shadow detail, or vice versa.

Imagine underexposing a shot by one stop and bringing it back up in post. You increase the highlight detail, because you’re letting half the light through to the sensor, reducing the risk of clipped whites, but you also increase the noise when you bring it up in post. This is basically what you’re doing when you increase your ISO, except that if you’re recording in linear or log then the restoration of brightness and increase in gain happen within the camera, rather than in post with RAW.

We can summarise all this as follows:

Doubling the ISO…

• increases overall brightness by one stop, and
• increases picture noise.

Then adjusting the exposure to compensate (e.g. closing the iris one stop)…

• restores overall brightness to its original value,
• gives you one more stop of detail in the highlights, and
• gives you one less stop of detail in the shadows.

Alternatively, halving the ISO…

• decreases overall brightness by one stop, and
• decreases picture noise.

Then adjusting the exposure to compensate (e.g. opening the iris one stop)…

• restores overall brightness to its original value,
• gives you one less stop of detail in the highlights, and
• gives you one more stop of detail in the shadows.