ISO Invariance: The End of the Exposure Triangle?

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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…

ISO Invariance: The End of the Exposure Triangle?

This squirrel was astonished that I could take an underexposed picture of him at ISO 100 and turn it into a completely usable photograph in Lightroom.

Two constants – aperture and shutter speed

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.

ISO is different

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.

ISO Invariance: The End of the Exposure Triangle?

Nikon D7100, 50mm, f/1.8, 1/90 second, ISO 100 (pushed 3 stops in Lightroom)

Raise the ISO

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?

ISO Invariance: The End of the Exposure Triangle?

Nikon D750, 35mm, f/4, 1/1000 second, ISO 100. No need to adjust exposure in post-processing.

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.

Explaining ISO Invariance

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.

ISO Invariance: The End of the Exposure Triangle?

Nikon D750, 50mm, f/2.8, 1/60th of a second, ISO 3200

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?

ISO Invariance: The End of the Exposure Triangle?

Nikon D750, 50mm, f/2.8, 1/60th, ISO 100 (pushed 5-stops in Lightroom – to effectively ISO 3200)

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.

ISO Invariance: The End of the Exposure Triangle?

Nikon D750, 50mm, f/2.8, 1/60th, ISO 100 (no editing in Lightroom)

Whoa – what?!

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.

Is it the end of the ISO as we know it?

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.

ISO Invariance: The End of the Exposure Triangle?

Nikon D750, 200mm, f/5.6, 1/350 second, ISO 100. Using a low ISO allowed me to pull out lots of color detail in post-production on this image.

Down on the upside

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.

A demonstration

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.

ISO Invariance: The End of the Exposure Triangle?

Nikon D7100, 50mm, f/4, 1/60 second, ISO 3200.

Now take a look at a similar photo taken at much lower ISO:

ISO Invariance: The End of the Exposure Triangle?

Nikon D7100, 50mm, f/4, 1/60 second, ISO 100.

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.

Nikon D7100, 50mm, f/4, 1/60 second, ISO 100 (pushed 5-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.

Limitations

There are also some significant limitations to take into account when looking at ISO invariance:

  • It only matters if you shoot in RAW, which allows you to have as much data from your camera’s image sensor as possible. RAW file sizes are huge and must be processed by a program like Lightroom before they can be shared or printed.
  • Processing takes time. Many photographers, myself included, like to get the exposure right in camera if at all possible.
  • In almost every situation you won’t get better results from shooting at low ISO values and then raising the exposure afterward. Most of the time you will get results that are roughly equal to what you would have achieved with just raising the ISO, to begin with.
ISO Invariance: The End of the Exposure Triangle?

Nikon D750, 50mm, f/8, 1/200 second, ISO 1250.

Conclusion

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.

Read more from our Tips & Tutorials category

Simon Ringsmuth

is an educational technology specialist at Oklahoma State University and enjoys sharing his enthusiasm for photography on his website and podcast at Weekly Fifty. He and his brother host a monthly podcast called Camera Dads where they discuss photography and fatherhood, and Simon also posts regularly to Instagram where you can follow him as sringsmuth.

  • Ricardo Castro Nunes

    Hi Simon
    First of all Thank you for a well written and enlightening article on one of the subjects that is still somehow misunderstood , the ISO and mainly ISO invariance that is a term I hear for the first time .
    I work with a Nikon D810 and some time ago I needed to shoot a Jazz performance for a friend , the light conditions were just terrible and I don’t use flashlight because it ruins the results on live music (and many other things) so I shot at ISO 12800 and was amazed that the slightly overexposed shots came out almost without grain whilst the underexposed had lots of ugly grain and banding and were almost useless even after I processed them in ACR , Photoshop and Topaz Denoise so my advice would be , when shooting in poor light conditions you always gain if you overexpose the photos by 1/2 to 2/3 stops .
    I hope this somehow contributes for the reader’s enlightenment .
    ric

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  • Alan D Granger

    When shooting at higher ISO”s it is very important to get the exposure right. Pushing an underexposed high ISO picture will not yield good results. I try to overexpose high ISO pictures a little.

  • Charles G. Haacker

    Thanks, Simon, for a well written and intriguing piece. I have for years now relied on letting my ISO “float,” i.e., putting it in auto. I use compacts by choice, my current mainstay being an original issue Sony RX10. My favorite keep-it-simple technique is to leave it in Aperture Priority with ISO on auto but “capped” at 3200 to avoid really excessive noise. I mostly process in Lightroom and control noise with luminance. I’d rather a little noise than too little sharpness. I think I will try underexposure (exposing to the left) and raising the exposure in Lightroom just to see what happens, but I have an idea that the 1″ sensor in the Sony isn’t going to lend itself to the technique but it will be interesting to experiment. I incidentally am perfectly capable of using the camera fully manually, I just tend to think, well Why Should I? ?

  • Tim Lowe

    A tripod is essential to keeping ISO in the optimum range. Yes, you can manipulate a raw file but getting the exposure right in the first place is still the better way to go. I would see this technique as useful for images you couldn’t otherwise get with a longer exposure. Sports. Street. That kind of thing.

  • You’re exactly right Alan. Shooting at 3200 or 6400 gives you a lot less color data overall and a lot less wiggle room in post. Best to get the exposure right when you take the shot.

  • Ed Hannon

    When raising ISO is better than pushing in post processing depends on when the camera does analog gain vs digital. Most cameras do analog ISO gain up to some ISO level and then switch to digitally multiplying the captured levels in camera. Analog ISO gain is always better than post processing (when shooting in Raw). When the camera does digital ISO gain, you can get the same or better results with post processing (better when the software uses advanced algorithms to push). You can determine the point where the camera switches to digital ISO by displaying images at various ISO settings using 16-bit histograms (e.g. rawdigger). If you expand the histogram so you can see each discrete digital level you will see smooth histograms with analog gain and gaps with digital.

  • walwit

    I read somewhere that shooting with high ISO makes low light areas darker but there is no mention of this in your article, could you comment on this?

  • Thanks for sharing all that, Ricardo. I appreciated hearing about your experiences, and I’m sure other readers will too.

  • Hmm…I’ve never heard of that before but it’s certainly possible.

  • Good explanation, Ed. Thank you!

  • Mark Miller

    Reminds me of the not so ‘good old days’ when, if you used a good lab, you could take your ASA 100 Kodacolor film and push it to 400.

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  • Michael

    Thank you Simon! I love technical articles like yours that make us passionate hobbyists more professional photographers. I always strive to shoot with the lowest ISO as possible but still without taking a chance by shooting with longer shutter speed than my lens focal length at that moment. You were right saying that underexposing 3 or even 5 stops and later recovering in LR was not always a good idea because you would not be able to recover all the details in shadows. As far as I know the exposing to the right is the normal way to do. However, sometimes I end up with some underexposed images and always successfully recover them in LR making these images looking great. I’ve never underexposed any given image for more than 1 to 1.5 stops. Another reason is to shoot exclusively in RAW so you always have a lot of data to post-process your images.

  • I think you hit on the big takeaway here, Michael: always shoot in RAW! 🙂

  • kurt squire

    Thank you for taking that damned video game book out of the darkness. I feel like it’s one of those $1 bill projects where you sign them and see where they end up.

    Otherwise, this is really cool. Have a barebones knowledge of photography, but this is unreal. The intro was well written for a newb btw, and the gain metaphor was especially helpful for me.

  • Hahaha! I haven’t cracked open that video game book in years! I didn’t even realize it was on my shelf and in the picture until you mentioned it 🙂 I’m glad you found the article to be useful, and if you have a DSLR you should try shooting in RAW and see what you can recover. It’s fun.

  • sly

    Hello I invite anybody to have a look at the read noise (expressed in electron) of your favorite camera there : http://www.photonstophotos.net/Charts/RN_e.htm
    The moment the read noise becomes flat or almost is when your camera becomes ISO invariant. And it can happen before digital amplification. Also anyway un camera with 2 or less electron of noise can be considered as ISO invariant because anyway other source of noise (photon noise) will dominate in most cases.
    The d750 : http://www.photonstophotos.net/Charts/RN_e.htm#Nikon%20D750_14 have a read noise bellow 2 electron quickly after ISO 200.

    Something not mentioned here, is that shooting with low ISO and fixing exposure in post allows you to get a bigger dynamic range which can be handly.

    I could not refrain to notice some miss conception here :
    -“shooting with a low ISO of 100 or 200 means your camera sensor is not very sensitive to light”
    – “I had to shoot at ISO 3200 in order to let in enough light”

    The sensitivity to light of your detector does not change with ISO, a higher ISO is only a higher amplification after that the light was resgistered in the pixel (at least on CCD or CMOS detectors).

    The D750 have big pixel so is more sensitive to light is a flooded concept. They are differences between, for instance, 4 small pixel and 1 of the size of 4 (notably the total efficient area) but in modern camera this differences are getting smaller and smaller.

    Cheers.

  • sly

    Actually that not concretely the case. The read noise to ISO relation becomes often flat before the digital amplification : http://www.photonstophotos.net/Charts/RN_e.htm#Nikon%20D750_14

    The CMOS have now very low read noise and as soon as you have light in your picture, the (natural) photon noise dominate. However, the black (no light) part of the picture is still dependent of the read noise.
    The ‘noise’ seen on the D7100 is more a pattern than a noise coming from the imperfect electronic and some ‘dark current’. Most of this pattern can be remove by taking dark images (same iso, same exptime) and subtracting it to the original.

  • walwit

    Comment by Alan D Granger right here below seems to validate this.

  • bongiss

    in all of my testing for iso invariance. it is better to compare photos in 800 or 1600 iso vs 3200 iso.. the 200 iso vs 1600 iso..invariant iso is from 200 – 1600 while 3200 iso upwards are just digital or applied gain.

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