Understanding how cameras handle ISO and low light

- When you change ISO on your camera, you gain the ability to use faster shutter speeds and/or smaller apertures. When ISO is set to a higher level, your sensor seems to become more light sensitive, meaning it doesn't need to be exposed to light for as long to register an image. In truth though your sensor doesn't become more light sensitive when you crank up the ISO setting. Rather the camera is altering one component deep in its guts. While we call them digital cameras, an image sensor actually outputs an analog signal. That signal is passed through an analog-to-digital converter after you shoot, and that's where the digital part of digital photography begins.

Before that conversion process though, that analog signals that comes off a sensor has to be amplified. The actual electrical impulses that come off the sensor are very faint, and without amplification they can't be easily measured and then converted into numbers. When you increase ISO, you're doing nothing more than increasing that level of amplification. As you do, fainter light levels captured by the sensor become visible, and as you should already know, any noise captured by the sensor also becomes visible.

The ability to do this, to change ISO on a shot-by-shot basis, is a boon to photography and it gives us tremendous flexibility in crafting exposure settings. All cameras have an ISO range and most cameras these days start at ISO 100 or 200 and go up to around 6400, possibly with options to turn on higher, much noisier ISOs. This is all achieved by increasing amplification. I say most cameras because we're now starting to see something new in the way that some cameras handle ISO.

Cameras such as this Fuji X-T1, the Sony a7R II, the Nikon D810, the Nikon D750, D5500, the Pentax K-5 and a few others, these cameras are known as ISO-invariant or ISO-less cameras. I'm gonna use this Fuji as my example, but what I'm describing here is true for the other cameras that I just mentioned. On the X-T1 when you increase ISO, the amplification of the signal coming from the sensor is increased until you get to ISO 1600.

Once you pass 1600, there's no more amplification of the sensor's data. Instead when the image is processed, it is digitally brightened during RAW conversion. What that means is that there's no difference in image quality if you shoot at ISO 3200 or if you shoot at ISO 1600, and then brighten the image by one stop in your RAW converter. Both images will appear identical. What's interesting about an ISO-less camera is that this is true across the entire ISO range.

Here's what I mean. I set up this little still life in fairly low light and shot it with the Fuji at ISO 1600. This is what I got. Then keeping the exact same exposure settings, same shutter speed, same aperture, I switched to ISO 200, the lowest ISO on the camera and shot. I got this, which is pretty much what you'd expect, an image that's way too dark. But then I took this image into Lightroom and brightened it up by three stops, and I got this.

This image is identical to the ISO 1600 image. If I bring them up side-by-side here. I know the screen is small, but when I go in and look at them up close, I find that as I move back and forth, I'm not seeing any difference in noise. There's a change going on over here. That's me moving around, that's just a reflection in the lens. But if you look at this area here, you'll see that it's staying remarkably clean in both images. So then I went and did the same test with my Canon 5D Mark III.

I shot this at ISO 1600. Then using the exact same exposure settings, I switched down to ISO 100, the lowest ISO on the camera. The image went a little bit darker, so I had to brighten it up by four stops this time. When I did, I got this, and this image looks different than what my original ISO 1600 image did. From back here you can see that first of all there's been a color shift for some reason. I can't find an explanation for that. I tried locking down white balance and it didn't make any difference. I don't know if you're gonna be able to see this on your compressed smaller version, but I've got a lot of noise in here on the image that I brightened.

It is both luminance noise, meaning I'm getting more speckly patterns, but there's also some chrominance noise. I can see bad magenta, green, and even some secondary colors here, some orange splotches spread throughout this. So this trick does not work on a non-ISO-invariant camera like the 5D. All right this is interesting and all. It's plainly a very different approach to the way that the camera is handling data, but other than being curious, so what? Well, there's one situation where this ISO-invariance capability is something that you can use to your advantage and that's shooting in low light.

Here's an image that I shot at ISO 1600 with the Fuji, and this is behaving pretty much the way any camera would. It's done a good exposure. Like most cameras these days at ISO 1600, it's very clean. But it blew out the lights here. These are LED lights on some kind of rope that are, or it's actually inside some kind of plastic rope that are wrapped around this tree, and they're totally overexposed. As I was standing here, I could actually see a lot of detail in here. I could see individual lights, I could see more flare around them, I could see shape to them.

So keeping the same exposure settings, I went down to ISO 200, and here's what I'm talking about. This is what I was seeing with my eye. I was seeing a lot more detail in here, but the rest of the image is underexposed. But as we've seen already, I can brighten this image back up and get it back to how it looked at 1600. But I've still got all of that great detail in there from the ISO 200 image, so with a little select and masking, I can put that detail back in. There's been some other brightenings and things that I've done in here so that's why you saw a little bit of change throughout the rest of the image.

But this is an image that I could not shoot with my 5D because when I tried to underexpose these areas in here, there's no detail there. Now that I've got this mask in place, something that's kind of cool, is I can change the amount of brightness on those restored areas. I did this with the selection brush here. I just brushed in a negative exposure compensation here, and you can see that I can change that. Let me zoom in here, and you can see that.

I've got a lot of latitude in here because the camera has captured so much data. So if you do a lot of low light shooting and you've been frustrated by the fact that bright lights in your scenes lose detail when you properly expose for the rest of the scene, this is a way to get it back in a single shot. I didn't have to do an HDR technique, I didn't have to shoot multiple shots at different exposures and combine them. This is just one shot at low ISO that I was able to correct into this final image. Now the disadvantage of shooting this way is that at ISO 200, the resulting image is very, very dark.

So I shot and framed, I was working on a tripod, I shot, or I framed and shot the ISO 1600 image first. I was able to work with my camera normally. I could see the viewfinder just fine, I could review the image, everything was very easy. When I was done, I just dialed down the ISO control to 200, took a second shot, and that's the one that I used to get my final image. So ISO-invariance is a very interesting technique. I think it's gonna be a really useful tool for me to have in my toolbox. If you go digging through your camera manual, you're not gonna see something in the table of contents or the index that says ISO-invariance or ISO-less or anything like that.

It's a sneaky feature that's in here. But if you Google around on your camera model and those terms, ISO-invariance or ISO-less, you'll probably find some documentation, not official documentation but some commentary about whether your camera is like this or not.