ISO - Understanding Exposure
- Tea L
- Aug 5
- 4 min read
Updated: Sep 4
Let's cover the basics of ISO when it comes to working with cameras. We're going to start with shorter to-the-point explanations and at the end we will dive a bit deeper, so you can use this as a quick reference if needed.
Utilitarian Explanation of ISO
ISO is one of the three methods by which you control exposure, along with Shutter Speed and Aperture (Iris).
A lower ISO results in a darker exposure (i.e. ISO 250).
A higher ISO results in a brighter exposure (i.e. ISO 6400).
The side effect of ISO is it’s the introduction of visible noise (aka. grainyness) into the image.
A lower ISO minimizes noise.
A higher ISO may increase noise.
Exactly what ISOs are deemed to be clean (noise free) or noisy exist on a spectrum and depend on the camera and sensor. This is ususally based on a cameras “native iso” or “native gain stages” which are the ISO values the camera was designed to perform best at.
This is also often affected slightly by shooting in Logarithmic (Log) modes, which are a more advanced technique which will be covered in a separate post.
Suggestions for using ISO when shooting
ISO is largely a concern based on your personal taste towards the quality of your image. (This is where the "pixel peepers" are separated from the "utilitarian documentarians. ;)
If you simply MUST have the cleanest, most immaculate image, you will likely stick to the base ISO.
However in documentary, the shot you did get is always better than the shot you didn’t because you didn’t want it to be noisy. So this is all relative.
Additionally a noisier shot is often more useable than one with excessive motion blur (from a very low shutter speed) or focus issues (from fast of an aperture.)
(DISCLAIMER: The following on advice on base ISOs for specific cameras is subjective, your mileage may vary. Don't come at me.)
For the cameras we have available currently, here are their ISO values:
A7sII / A7rIII (CDA)
Base: 800 (without Picture Profile) or 1600 (minimum for S-Log).
Useable: 250 to 3200
Noisy: 3200 to 6400
Un-usuable: 12800
*A7sII has a single base iso or gain stage, ~ISO 800.
FX30
Base: 800 and 2500
Useable: 250 to 6400
Noisy: up to 12800
Un-usuable: Above 12800+
*The FX30 has two gain stages at 800 and 2500 so both will look very clean. Many people claim it’s lowlight to be better than the older A7sII.
Note: Base ISO is tricky because EVERYONE disagrees on this for basically every camera in existence. The manufacturer will say one thing, fiction cinematographers with lighting and time will say another and documentary cinematographers working with available light will say another. Your mileage may vary.
Detailed Explanation of ISO
ISO is an international standard for describing a sensors sensitivity to light.
Sensors are made up of individual photo receptors (pixels) that convert light into electrical current and then pass that through a processor in the camera to record that analog signal as a digital value.
Sensors vary wildly on exactly how they do this; what technology they use, how many color receptors make up each pixel, how large each individual pixel is, what size the overall sensor is, etc.
What really makes sensors different to their predecessors, motion picture film, is that their sensitivity can be modified by amplifying the signal before it is turned into digital data, thereby allowing you to change it’s sensitivity. (With film the sensitivity of the film was fixed.)
However much like with amplifying sound this comes at a cost; noise. So why do some cameras have more noise and some have less at the same ISO?
Generally, a physically larger sensor pixel will take-in more light than a smaller one. (More on sensor sizes in the sensor size post.)
So, if two sensors have the same number of pixels, but one sensor is physically larger, its pixels will be physically larger and therefore will take-in more light. By doing this, it will require less amplification to match the same exposure, resulting in a less noisy image.
This is why Full Frame cameras with larger sensors typically outperform “Crop Sensor” S35mm / APS-C cameras with smaller sensors. Advances in technology also typically make newer cameras outperform old ones in this regard.
All of this leads to the concept of Base ISO or Gain Stages, the ISO values at which a camera is designed to perform its best; producing the least noise, but also the greatest dynamic range.
Dynamic range is the range between the darkest and brightest shades properly reproduced by the camera. (See post on dynamic range.)
From this point on things get murky, as users and manufacturers consistently argue over what ratings certain cameras should have, how “clean” their images are and at what ISOs they function best at, acceptably at, or at which point their images "break down."
Plus these values are different if you know you intend to use noise reduction software like that found in Davinci Resolve or Neat Video as part of your workflow in post-production.
Suffice it to say this is a matter of personal taste to a large extent and you will be able to see it in the camera while you shoot especially if you use focus magnification.
Comments