From a Nikon D700 at ISO 6,400
If you’ve shopped for a DSLR… well, pretty much ever, you might notice reviewers, blogs, and spec sheets often list two sets of ISO ranges the camera has. One will be smaller, and often called “native,” and the other range will be bigger and use words like “push” or “extended.” And, you might be wondering what exactly a “native” ISO is, and how it differs from those “extended” ISOs, and maybe why most people tend to ignore the extended ones when discussing how well a camera does. And that’s what we hope to clear up today.
Like a stereo, your camera’s sensor moves between ISO settings by increasing the “gain,” or how much the values received at the pixels are amplified. The higher the ISO, the more gain. The native ISO range of a sensor is what settings you can access just by changing the gain. See, the problem is, sensors don’t just get noise from the environment (“shot” noise), a lot of noise is actually caused by the sensor picking up on thermal variations, or getting electron interference from its own circuitry. The more you amplify these readings, the more two things happen. One, you amplify the noise with the signal. Two, you send more juice through the sensor, possibly introducing even more noise from that. So, sensors are designed as best as they can be to perform with a high signal-to-noise at any of their gain settings. These form the physical, real ISO settings for your camera.
But, in today’s ISO wars, that’s not always enough. Much like optical zooms on point-and-shoots are supported by digital zoom when you need to get even closer, ISO ranges on DSLRs are supported by extended ranges. These extended ISOs don’t change the gain on the sensor. This is done with either push ISOs (ISOs higher than the highest native ISO), or pull ISOs (ISOs lower than the lowest native setting). In the case of a push ISO, what happens is the camera increases the sensor gain as high as it can go. Then, it’ll take a picture at it, but under expose it by a stop. This way, the exposure time matches a higher ISO. Then, the processor adds one stop of value to all the values recorded. Or two stops. Or three. however many push ISO settings your model may have. This is why push ISO settings fall apart faster than native ISO ones. They’re no longer altering sensor gain, but taking an under-exposed, high-ISO image and pushing the exposure up. Much like you’d do in Lightroom for an underexposed image. Actually, pretty much exactly like that, except at the data level.
A pull ISO works the same way. For example, many Nikon cameras had a lowest native ISO of 200. But, for a lot of reasons, many people like shooting at ISO 100. Enter a pull ISO for 100. What happens is the camera exposes an image at ISO 200 (the lowest gain setting) for the same time as an ISO 100 exposure would be. Then it pulls all the values down one stop. If pulling up all the noise is the bane of push ISOs, highlight clipping is the bane of pull ISOs. Again, for the same reasons as you’d have in post. if you accidentally expose an image too long, you can pull a lot of it down, but, you’ll find sometimes bright areas have already saturated to pure white, and there’s nothing to recover. On a model like a D700, there’s a lot of headroom for that, and it’s rare to clip with just 1 stop of overexposure. So, it’s rare to see clipping in the pull ISO 100. But, every model differs, as does every scene. At the end, you’re still taking an overexposed picture at a higher ISO and then pulling it back.
So, why use extended ISOs at all? Well, they’re admittedly handy if you’re a JPG shooter. or a journalist where getting the shot at all matters more than clipping and or noise. But, if you’re a raw shooter, you can mimic the settings yourself by just dialing in a stop or two of exposure compensation on either end of your native ISO range, and then fixing it in post. It’s the same process, except you’ll have a bit more control over it.
By the way, I’m sure this has all been horrendously familiar for you film shooters, since push and pull developing has been used for ages to do exactly this same trick, but in an analog format. But, there it is. Your native ISO range is what your camera’s sensor can actually do through the much better manipulation of its gain. The extended ranges are just the processor doing some push or pull developing for you at the data level, but there’s no more adjusting the actual sensor. And now you know, and as 80s cartoons taught me, that’s half the battle.