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Since the beginning of photography, there have been three basic elements that need to be taken into consideration when making an image; the size of the aperture on your lens, the speed of the shutter, and how sensitive your film (or, in modern times, the digital image sensor) is to light. Indeed almost all of photography boils down to an understanding of how those three variables affect the overall exposure of your image. Like death, taxes, and the morning sunrise – they are immutable and must be considered no matter the type of camera you have or the subject you are shooting.
Some photographers prefer to shoot in manual and control each of these elements individually, while others prefer the full Auto route. Still others find themselves somewhere in between by letting the camera make some decisions while they control one or two of the parameters themselves.
But at the end of the day the aperture, shutter, and ISO always work together to help you get the photos you want. Until now. Sort of…
Two of the three elements of the Exposure Triangle are, and will always be, limited by physics. Changing the size of the aperture on your camera lens determines not only how much light will be let in, but other parameters as well such as the depth of field. Using a fast shutter speed will always be necessary to freeze motion, particularly with fast-moving subjects. Conversely, a slow shutter speed is an unchangeable requirement for capturing light trails and other types of long exposure photography.
But ISO is not nearly the limiting factor it once was and on some newer cameras is almost irrelevant. Almost!
There are many analogies to help you understand ISO, but one of my favorites is that of a microphone. If you have the gain turned down low you will need to speak very loudly for your voice to be heard. But this gives you the benefit of cutting out low hums and other noises that might otherwise be picked up, such as a desk fan or overhead ventilation unit. Turn the gain up high and you won’t have to speak as loudly, but along with your voice, a myriad of other background sounds will be picked up as well. Once those other sounds are recorded, it’s almost impossible to get rid of them!
In similar fashion, shooting with a low ISO of 100 or 200 means your camera sensor is not very sensitive to light. So you will need a lot of incoming light in order to take an image. This is accomplished through either a large aperture, slower shutter speed, or combination of the two. The benefit is that ISO 100 results in a picture that is free of digital noise that often looks like colored static – the kind you might see on an old tube-style television set.
If you don’t have much incoming light, there has always been a simple solution – raise the ISO. (In the days of film the solution was similar but not as easy. It required the photographer to physically remove the film in the camera and put in another roll of 400 or 800 ASA film that was more sensitive to light.) Like I mentioned earlier, though, photography is all about tradeoffs. Shooting at a high ISO of 3200 or 6400 can help you get the shot you want, but will often result in a picture that is far more noisy and grainy than you might prefer.
What’s a photographer to do?
Thanks to incredible advances in modern sensor technology, shooting at ISO 3200 or 6400 on a new camera like a Canon 5D Mark IV or a Nikon D7200 results in images that are far more usable than their counterparts from five or 10 years ago. But there’s another solution that’s gaining prominence as well.
Some camera sensors today are so good they are essentially what’s known as ISO Invariant, which is a fancy way of saying the ISO doesn’t really matter. Not in a practical sense, anyway. Shooting with a camera that is ISO Invariant means that you will get roughly the same results if you shoot at a low ISO of 100 or 200, and then change the exposure in post-production, as you would from shooting a higher ISO (like 1600) in the first place.
Allow me to illustrate what this means with the help of some visual aids. The following two images look fairly similar, but a closer look reveals something a bit deeper under the surface. They were both taken with the same camera using the same exposure settings, with one key difference – the ISO.
The above photo looks decent, but the sun was almost down and I had to shoot at ISO 3200 in order to let in enough light…or did I?
Here is the same image except this one was shot at ISO 100, then in Lightroom, I pushed the exposure up by five whole stops. Five stops! That’s a lot of adjustment, yet the final image looks almost identical to the one shot at ISO 3200. Just for the sake of comparison, here is the original ISO 100 image before any editing in Lightroom.
I’m not kidding here – the original image was almost entirely black. If you look closely you can just barely make out the roof of the bird house and a bit of color in the sky. Yet so much data was captured by the sensor at ISO 100 that I was able to create a file that was not only usable but in my opinion, superior to its ISO 3200 counterpart. Even doing a bit of pixel-peeping reveals little difference in terms of the noise levels between the two images.
Not only are the two images quite similar, I would go so far as to argue that the one shot at ISO 100 and boosted 5-stops has richer colors and better noise levels than its high-ISO counterpart. This is, in a nutshell, what ISO invariance is all about. It gives you the ability to shoot at virtually any ISO value and still get a usable image, assuming you shoot in RAW and have the ability to adjust the photo in post-processing software. ISO, then, no longer becomes much of a determining factor in the overall exposure.
Modern camera sensors are getting so good at picking up luminance and color data from incoming light. So the idea of a photographer needing to manually adjust the sensitivity of the image sensor is becoming almost moot. Note that I’m hedging my bets here, as I have throughout this whole article. I will stop short of saying that ISO is not a factor to be considered anymore. Far from it!
The fact is that camera sensors are getting so good that they are significantly outperforming their counterparts from a few years ago in all areas, not just ISO invariance. ISO 3200 or 6400 on many cameras today, in general, results in images that are perfectly usable. Whereas the same settings on a digital camera from yesteryear would yield an image so muddy it might have been well-nigh unusable. This has also resulted in the same sensors being able to capture significantly more details at low ISO that can, if desired, be used to edit an image ex post facto.
Of course, not all is sunshine and roses in the land of ISO invariance. It’s important to note that not all cameras are even capable of such a feat. The Nikon D750 is a 24-megapixel full-frame camera. So its individual pixels are physically larger, and therefore much more sensitive to light than a 24-megapixel crop-sensor camera like a Canon Rebel T6 or Nikon D3300.
Only a handful cameras today can actually be described as ISO Invariant. Whereas most cameras still follow the same rules about using ISO in tandem with aperture and shutter speed that have been the foundations of photography since its inception.
To illustrate, here’s another series of images taken with a Nikon D7100. It’s a few years old but quite similar to many cameras today in terms of its imaging prowess.
Now take a look at a similar photo taken at much lower ISO:
Whoops! That’s the unedited version in which you can barely tell that there’s a picture at all. Here’s the same image with the exposure bumped up by five stops in Lightroom.
Even on these smaller web-sized photos, you can see some key differences. The colors are not as natural, the darker portions are muddier, and there is significant banding in the shadows. “Wait a sec,” you might be saying right about now. “What’s banding in the shadows?” Follow the advice of Rafiki from The Lion King and look harder…
Here you can clearly see that the ISO 3200 image is superior. All throughout the ISO 100 picture, there are horizontal lines crossing through the image, which is a phenomenon known as banding. It often happens when you try to recover details from darker portions of an image. Add this to the fact that the colors are harsh and the whole thing required such a significant amount of processing and you start to see why ISO still matters. It matters quite a great deal.
There are also some significant limitations to take into account when looking at ISO invariance:
I like to think of ISO invariance as a backup tool to use when I really need it, not something on which I can rely for everyday shooting. Who among us hasn’t come back from a photo session only to find out that some key pictures were horribly underexposed by accident? (Raise your hand if this is you. Go on, I’ll wait.) I know I have, and it’s nice knowing I can still get a usable photo in these situations as long as I have not blown out the highlights.
We’re still years, perhaps decades, away from a point at which ISO is no longer a practical consideration, and it’s possible we will never get there entirely. However, if you look at where we have been and where we are now in terms of camera sensor technology, it’s easy to draw a line to a point in the future when ISO might not matter nearly as much as it does now.
In the meantime, my suggestion for most photographers remains the same; use all three elements of the exposure triangle to get the shot you want. And if you’re worried that using a fast shutter speed will result in a photo that is underexposed, just raise your ISO as you are shooting instead of fiddling with sliders back at your computer. You’ll learn more about photography by doing so, and you will probably have more fun since you’ll be taking pictures instead of being hunched over your laptop.