Continuing the blog serialisation of my popular X-Pro1 lust/hate/love story:
Part Seventeen: Fujifilm X Cameras: Understanding the ‘ISOless’ Sensor
The Fuji X cameras (and indeed most digital cameras) use what’s affectionately known as a ‘ISOless’ sensor.
But what does this mean?
And can understanding how it works help us in any way?
Firstly I think it would help for us to go back to the absolute basics!
The exposure triangle:
In principle, the exposure triangle is not dissimilar to other famous triangles!!
Get the exposure triangle wrong, and much like the obtuse triangle; things will come out looking rather funny and weird!!
Get the exposure triangle wrong, and much like the Bermuda Triangle; things will disappear!!
Get the exposure triangle wrong, and much like a love triangle; you might well come home empty handed.
I wont labour the point on the exposure triangle, a quick google search will yield you a lot of well written information about it, whereas I’ll probably just continue to make triangle jokes!
However:
The exposure triangle is basically the 3 parameters of exposure. In order to get the correct exposure in a shot, you needed to correctly balance these 3 points. All of these 3 points effectively control sensitivity to light.
Back in the days of film, the ISO was a fixed value determined by the film you were using. So if you put in a roll of ISO 100 film, then if half way through the roll you wanted to swap to ISO 800, it was tough luck, unless you physically removed the roll of film and substituted it for the 800 one.
These days, it’s the sensor in your digital camera that does the job of film. I don’t just mean that it’s the part that captures the image, but it’s also the part that controls the ISO
You can change ISO as frequently as you like, of course your sensor has a fixed set of ISO values, for example in the Fuji X-Pro1 (and most of their other cameras, bar the X-Pro2) the native range of the ISO is 200-6400.
You can even set the camera to pick the best ISO for you (in fact you can set the camera to control the entire exposure triangle for you!)
So on the one hand, digital cameras have a sensor that can provide you with multiple ISO values; but on the other hand, digital camera sensors are often referred to as ‘ISOless’ namely having no actual ISO.
That’s not at all confusing is it?!!
Basically your camera is set to it’s lowest native sensitivity. Which on Fuji cameras is ISO 200.
When you go up on ISO, you’re basically trading fine detail resolution for brightness and noise.
In what will I can only describe as a hideous over simplification:
An ISO-less sensor, (when set to a value other than the base ISO), is a mechanism of telling the camera to underexpose (make the image darker), and note that underexposure in the metadata of the RAW file, which then boosts the brightness to get the luminance level back. This causes image degradation in the shape of noise (which is a grain like textured pattern) and a loss of details.
But it’s better than a shot that’s so dark you can’t see anything!
So what’s ISOless really mean in straight forward terms?
The rhetoric goes:
Well if you shoot RAW, then it shouldn’t make any difference if you artificially brighten your image in the camera, or if you do it yourself later on the computer.
But does this statement hold true?
As you know, I’m not one for making statements that I’ve not tested and taken the photographs to demonstrate!
So let’s take a look.
Here’s an overview of the test.
(Excuse the file names on the right. If you’re doing something like this, it pays to note stuff like that!)
These RAW files were then imported into LightRoom.
The higher ISO shots were left alone, just exported as a Jpeg.
The 200 ISO shots were treated to brightening, as per the number of stops they needed to match the exposure that the camera recommended. This was done via the exposure slider. No other changes were made. These files were then exported as a Jpeg
So, that’s what I did; but how did it turn out?
ISO6400 Vs 200 + 5EV
6400
200 + 5EV
ISO3200 Vs 200 + 4EV
3200
200 + 4EV
ISO1600 Vs 200 + 3EV
1600
200 + 3EV
ISO800 Vs 200 + 2EV
800
200 + 2EV
ISO400 Vs 200 + 1EV
400
200 + 1EV
So what do we think?
Is the Fuji X-Trans sensor in my X-Pro1 ISOless?
Do you see a big drop in quality, between shooting in camera at ISO 6400 and doing nothing on the PC Vs shooting at base ISO (200) and then adding the missing stops of light back in later on the computer?
In these screen shots… give or take, more or less. The shots look APPROXIMATELY the same the brightness isn’t 100% consistent, but I think it’s safe to say:
This sensor is ISOless. IN THAT WE CAN PUSH A FILE BY AS MANY STOPS AS MISSED DURING EXPOSURE AND STILL GET AN IMAGE
But so what? What’s the advantage? In one case we get the camera to brighten the image, in another we can do it ourselves later.
Is there even any difference, are we gaining or losing any quality by brightening it ourselves?
And is there anyway way that we can use the ISOless sensor to our advantage.
I think it’s advisable that we take a closer look at those images!
So we’ll do that and take a look at those questions next time!
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Adam Bonn Fotografia 
There is no such thing as an exposure triangle. ISO isn’t part of exposure; rather ISO is used to work out what the exposure should be to give an image with a predefined brightness. If anything, it would be a ‘brightness triangle’, or – to play it safe – one would use the term that Bryan Peterson in his book “Understanding Exposure” actually used: “Photographic Triangle”.
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“Exposure Triangle” is a perfectly valid term used throughout the photographic world and has been in circulation since long before digital. But yes – perhaps one day an author will coin a new term for “brightness” and that’ll gain traction.
In digital terms it’s actually ISO that’s a anachronism, not exposure – in photographical terms, exposure is a synonym for brightness, and refers to sensitivity to light. Of course the reason I’ve just referred to ISO as a anachronism, it because a digital sensor doesn’t physically change it’s sensitivity to light, it under exposes, then performs pre-ADC &/or post capture amplification to brighten the image.
Modern digital camera makers tend to use one of two value sets for ISO (REI & SOS), which makes the term “pre-defined brightness” vaguely misleading (because the attribute data of each system refers to a different numeric variable)
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“Exposure Triangle” may be a valid term, but only because it’s been parroted all over. The term that described the concept was already coined (Photographic Triangle), but somebody, somewhere, somehow mistakenly wrote “Exposure Triangle”, and the term unfortunately stuck. If you have a reference where the term was formally proposed or introduced, I’d be more than happy to consider it.
Also, if you decide to perpetuate that term, what do you propose to use for what is considered “exposure” within science, technology, and medicine? BTW, it is only a subgroup of photographers that uses the term in that way, by no means all of them.
Getting to ISO: ISO relates to how initially captured intensities get mapped to output intensities (JPEG). It doesn’t exist outside the context of a JPEG-processing engine, and it doesn’t exist for sensors or when it comes to raw data. Therein lies the anachronism: namely that a raw shooter must use the ISO setting when all s/he wants is to adjust the gain. For a raw shooter, or when considering raw data, it does not make sense to speak of ISO. I other words, there is a big difference between saying “the sensor is at ISO200” (which doesn’t make sense) vs “The sensor uses the gain of ISO200” (which gets at the hart of it).
When I wrote “pre-defined brightness”, I was referring to the fact that the photographic triangle as a tool to determine the ‘proper exposure’ works with the stipulation of a certain target output brightness, say, that of 12% gray. If one parameter changes, one or both of the other parameters must change to compensate (maintain the output brightness). Of course, it’s not really a triangle after all, but a quadrangle. Scene luminance is the missing variable and it is often controllable too.
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If I ever decide to write an article about photographic nomenclature then you’ve certainly giving me some interesting pointers. Thanks very much
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