When I first heard about ISO Invariance it really threw me for a loop. I had to reconsider everything I'd learned about ISO and how it relates to Astrophotography. Normally I use ISO 6400 when photographing the Milky Way at night. I had always thought that using a low ISO, like ISO 400, and increasing the Exposure in Post-Processing would effectively destroy the photo. ISO Invariance would allow me to use any ISO and get the same results.
To understand how this actually works, you need to have a solid understanding of ISO first. New photographers are often told "Higher ISO's increase the camera's sensitivity to light". It doesn't work quite that way though. As the ISO is increased, the camera amplifies (brightens) whatever light was captured during the exposure. If you increase the ISO, you aren't capturing any more light. The only way to capture more light is a longer shutter speed or wider aperture. Raising the ISO in-camera only brightens the exposure. You could do the same thing in Adobe Lightroom or Camera RAW by increasing the Exposure slider. You will need to shoot in RAW to do this though. For astrophotographers, it's essential to capture as much light as possible. This will drastically reduce the grain in the image.
Imagine someone speaking softly a few feet back from a microphone. At the normal volume it will be very hard to hear them speaking. If you amplify the sound, or increase the volume, you can start to hear them more loudly. However, this amplification will also cause the background noise to become more apparent. You will hear lots of static. This random noise will obscure the voice and make it harder to hear them speaking. It would be better to have them either speak louder or get closer to the microphone. This is similar to how ISO works. If you capture more light, using a longer Shutter Speed or wider Aperture, the signal becomes more clear. If there's only a faint amount of light, it will become very grainy when amplified at a High ISO.
Some newer camera sensors are ISO Invariant. This means you can shoot at any ISO and brighten the Exposure in Post-Processing without any quality loss. Keep in mind, you can not recover that much data when shooting JPEG. You need to shoot in RAW. However, most cameras on the market are ISO Variant. On an ISO Variant Sensor, brightening the exposure in Post-Processing could ruin the image quality. That's why it's recommended to use the proper ISO in-camera.
If you skip down to the Further Reading section you can find different articles showing examples of ISO Variant and ISO Invariant sensors.
This morning I headed down to the local dark sky park, Scenic Vista, to capture my test images. Using a Nikon 14-24mm lens at f/2.8, with a 20 Second Shutter Speed, I took 7 photos. I started at ISO 6400 and decreased the ISO by 1 Stop each time, down to ISO 100.
First, let's take a look at the ISO 100 example. The Before image is ISO 100, clearly way underexposed. The After image is ISO 100 brightened by 6 Stops in Adobe Camera RAW. It's a night and day difference!
If the Nikon D750 is ISO invariant, then the brightened ISO 100 image should look nearly identical to the ISO 6400 photo. Let's do another comparison. The slider below shows the ISO 100 image brightened by 6 Stops on the left, and the ISO 6400 image on the right.
They look nearly the same, but we need to take a closer look. I cropped in heavily into the center of the foreground. You should notice a slight difference now. There is a weird color grain problem affecting the ISO 100 image. After brightening the photo 6 Stops in Post-Processing, it's expected that it would have some quality loss.
Finally, let's look at the stars. I don't see any real increase in noise between ISO 100 and 6400. However, I do see a slight color noise problem. It looks similar to what we saw in the Ground Crop photos above.
After directly comparing the brightened Low-ISO photos and the High ISO photos I am very surprised by the results. There is only a slight loss of quality when brightening the ISO 100 photo by 6 Stops!
Based on these results, it's clear that the Nikon D750 has an ISO Invariant Sensor. This is very important for night photography especially. No matter which ISO you choose, you can always brighten the Exposure in your Camera RAW processor without any major problems. In fact, you can intentionally underexpose the photo to save detail in the highlights, and then brighten the Exposure in post-processing. I will be using this method when I do light painting in my scenes, which tends to get overexposed at ISO 6400.
Another benefit of having an ISO-Invariant sensor is for wildlife photography. For example, the image below was taken at 1/320s, f/6.3, and ISO 200. It was getting dark, and there wasn't much light; plus, I needed a fast shutter speed to freeze any motion on the owl. Since I didn't want to jack my ISO up and lose fine-detail, I deliberately took an underexposed photo. Then I brightened the photo by 3 Stops in Adobe Camera RAW. The image looked incredible after, with no quality loss!
Many new photographers are told that increasing the ISO increases grain. As we've seen, this isn't exactly the case. There is an important distinction to make here. When shooting at night, the camera settings usually don't change. I use a Wide Open Aperture (f/1.4 - f/2.8), a long Shutter Speed usually around 20 seconds, and ISO 6400 most of the time. If the ISO is the only setting being changed on an ISO Invariant sensor, there should be no noticeable difference in grain between ISO 100 and ISO 6400 when the two photos have the same brightness. The examples above proved this.
If I am photographing during the day and I increase my ISO, there will be more grain though! Why is that?
The ISO 12800 image is clearly much more grainy than the ISO 100 image. This is because less light reached the sensor. When you increase the ISO in an automatic mode (Aperture Priority, Shutter Priority, Program, etc...), the camera will automatically compensate with a faster Shutter Speed or smaller Aperture.
The ISO 100 photo had a Shutter Speed of 1/8 sec. The ISO 12800 photo had a Shutter Speed of 1/160 sec. That's a loss of 4.3 Stops of light! That's why the higher-ISO image is more grainy; less light was able to reach the sensor. Remember, in the night photos the Shutter Speed and Aperture didn't change, only the ISO. Technically, both ISO 100 and ISO 6400 received the same amount of light. But in this example, the higher-ISO photo lost over 4 Stops of light because of the faster Shutter Speed.
This example clearly shows that it is the amount of light reaching the sensor that ultimately causes grain, not the ISO. Remember back to the microphone example. If you increase the volume listening to a faint voice, you will hear a lot of static. You need to get the person closer to the microphone, or make him speak louder to get better audio quality. If you want less grain in your photos either use a Longer Shutter Speed or a Wider Aperture.
Of course, this isn't always possible. When I photograph birds I need to have a fast Shutter Speed. My lens can only open up to f/5 - f/6.3. I'm stuck; my Aperture can't get any wider (unless I spend $10,000 on an f/4 lens) and my Shutter Speed can't get much shorter (or I risk motion blur). I have to accept that my images will have more grain in them simply because less light is reaching the sensor.
This article on Lonely Speck explains how to find the best ISO for Astrophotography. In fact, this is the article that first got me interested in ISO Invariance!
Rishi Sanyal has created a great post on how to effectively use an ISO Invariant Sensor. In this article he covers some real world uses for an ISO Invariant sensor. Read his article over on Digital Photography Review.
Daniel Laan also tested out the Nikon D750 for ISO Invariance in this Fstoppers article. Check out his article for even more comparison photos and information on the Nikon D750 as well as some ISO Invariant tips.
Spencer Cox recently published an extensive look at ISO Invariance for Photography Life. This article is the best resource on ISO Invariance that I've found so far.