Photography Masterclass Magazine
What Is ISO Invariance & When Should You Use It?
ISO Invariance is an important topic to know about, yet very people have even heard of it!
There are a lot of misguiding generalizations out there and the way that your digital camera performs at different ISO settings has implications to consider when taking images in low light scenes.
When photographing in low light (astrophotography for example) it is a delicate balancing act of several different factors to be able to achieve a high-quality image. It is important to know that you are making trade-offs, likely without realizing it, when selecting specific settings and affecting the final image.
Knowing about ISO invariance and variance, and how your own model of camera performs, will allow you to understand how to get the best image quality and manage digital noise in low light conditions.
Spoiler alert – surprisingly, on a lot of ISO variant cameras the digital noise actually reduces when increasing ISO in low light situations! Be sure to check out the video following this article to see the difference for yourself.
So let’s look at the topic and try to simplify things into understandable language. Let’s get started…
THE SOURCES OF NOISE IN A PHOTO
To more fully understand ISO invariance, we need to understand a little more about why noise occurs in a photo and the types of noise you might see in your image. Noise occurs both due to the scene in front of your camera and because of problems in your camera’s ability to read said scene. Let’s look at the types of noise in a little more detail:
1. PHOTON NOISE
– Photons are the basic unit of all light and the scene in front of your camera will produce noise purely because the photons from your subject don’t all arrive equally. This type of noise is present in the atmosphere so will always be present in your images.
2. FRONT-END READ NOISE
– Front-end read noise (also known as upstream read noise) originates in your camera. There are lots of reasons for front-end noise, including imperfections in resetting your camera’s pixels after a photo.
3. BACK-END READ NOISE
– Back-end read noise (also known as downstream read noise) also comes from your camera and is often caused by factors such as the physical electronic components in your camera.
WHAT IS ISO INVARIANCE AND VARIANCE?
ISO is part of the Exposure Triangle and describes the sensitivity of your camera is to light. The higher the ISO number, the more sensitive the sensor of your digital camera is to light.
It is well known that different ISO settings can potentially affect the amount of digital noise in an image but the key thing to remember is that the as you increase ISO you decrease dynamic range, which is the difference between the darkest and lightest areas of the scene.
Also, as you increase the ISO, you risk blowing out the highlights and will lose detail and color in those areas of the image.
It is for this reason that ISO invariant cameras have become a hot topic of discussion within the astrophotography community.
So, what is ISO invariance? In very simple terms it is the fact that the electric signals coming from a camera’s sensor are more or less the same no matter what ISO you’re using.
ISO INVARIANT CAMERAS
What actually makes a camera ISO invariant? Well, certain cameras have very low levels of back-end read noise and this makes them perform consistently with low levels of digital noise no matter the ISO setting. Typically, most cameras that have Sony sensors are ISO invariant.
ISO invariant cameras can have images taken at any ISO value and there is no real effect on the digital noise within the images. For example, let’s imagine with an ISO invariant camera, you took two images in low light conditions with the same focal length and aperture setting, but one at ISO 100 and the other at ISO 3200. If you then boosted the exposure of the ISO 100 image by five stops in postproduction so as to match ISO 3200, it would result in images that are essentially the same in terms of quality (i.e. have the same amount of noise). The amount of noise is invariant of the ISO setting that you use.
What most people don’t realise that you can take a really underexposed image in low light conditions, shot at a very low ISO, & correct the exposure in post-production to produce a clean image that looks like an image that was shot at the correct exposure. This is a viable method of shooting low light scenes.
So, the idea is that if your camera is perfectly ISO invariant there would be no noise (digital grain) penalty for brightening a photo in post-production instead of increasing your ISO in-camera.
ISO VARIANT CAMERAS
ISO variant cameras, however, have varying levels of back-end noise dependent on the ISO value selected. For example, again, imagine you took images in low light conditions with the same focal length and aperture setting but at ISO 100 and at ISO 3200. If you then boosted the exposure of the ISO 100 image by five stops in post-production to match ISO 3200, when comparing the two images, surprisingly, the ISO 100 image would have more digital noise.
And the higher you increase the value the less digital noise is introduced.
In short, for ISO variant cameras, the amount of noise in the image depends on the ISO setting you use and generally image quality improves with a higher ISO which is against some of the misguiding generalizations out there!
With ISO variant cameras you have to be careful what ISO setting you use in the field because you don’t have the freedom to boost or drop the exposure in postproduction. Most Canon cameras are ISO variant and have to use the right ISO in camera.
It is also possible to get ISO variant cameras where at some point the camera begins to show some form of invariant behavior from a specific high ISO value upwards.
As I’m sure you’ll agree, with all this disparity it’s a good idea to understand exactly how your camera performs.
TESTING YOUR CAMERA FOR INVARIANCE AND VARIANCE
The good news is that it is pretty easy to test your camera to determine if it is ISO invariant or variant and understand the best ISO setting to use for optimal image quality.
Here’s howyou can test your camera for this:
• Go into a low light scene where there is little light pollution. You could do this in a dimly lit room in your house or out in the field at night.
• Set the camera to be in manual mode with long exposure noise reduction and high exposure noise reduction turned off
• Set a constant shutter speed (e.g. 15 seconds) and aperture (f/2.8)
• Then take exposures with different ISO settings – 100, 200, 400, 800, 1600, 3200, 6400 etc. Just use full stops of light for the increments.
• Then import the images into your photo editing software and match the total exposure values of the images so that you can check the different levels of noise for each image versus the others. You can do this manually on each image or have the program automatically match the exposure.
o ISO 100 = +5EV o ISO 200 = +4EV
o ISO 400 = +3EV
o ISO 800 = +2EV
o ISO 1600 = +1EV
o ISO 3200 = no change – leave at 0 o ISO 6400 = -1EV
o ISO 12,800 = -2EV
o Etc.
You can then confidently identify the lowest ISO value that produces the least amount of digital noise.
SUMMARY AND CONCLUSION
So, to capture the best quality low light image, it would be best to shoot a potentially underexposed image at the lowest ISO that you’ve tested and determined to be the happy medium between noise and dynamic range and correct the exposure in post-production.
For an ISO invariant camera, this ISO value will be low thereby allowing you to preserve dynamic range and avoid blowing out highlights better than a variant camera, hence invariant cameras being desirable for low light photography. You also have flexibility in your ISO selection being able to use ranges that will still be lower than a variant camera’s with little impact to image quality.
For ISO variant cameras you however have to be aware of what your lowest ISO value is (out of relatively high values) that produces the least amount of digital noise, loss of dynamic range and blown out highlights. All is not lost with variant cameras though as, by testing, you can find the sweet spot which will produce very acceptable results.
As you increase ISO, you decrease dynamic range
As you increase ISO, you risk blowing out the highlights and will lose detail and color in those areas of the image
ISO Invariant cameras allow you to shoot at a very low ISO, & correct the exposure in post-production to produce a relatively noiseless image without loss of dynamic range or blown out highlights
ISO Variant cameras generally have less noise at higher ISO's so you have to determine the happy medium between noise and dynamic range
I hope this article manages to provide a simple explanation of ISO invariance although it should be remembered that this is a hugely technical topic. If you’re so minded you can explore it further but the tips above should allow you to start trying it out on a suitable camera.