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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