I will put to the test the claims that Fujifilm cameras are ISO invariant at some iso settings.

What this means in broad terms is that an image shot at iso 200 – f7.1 – 1/4s should have the same image quality as the image shot 3 stops under exposed at iso 200 – f7.1 – 1/30s and then pushed +3 stops in the raw processor.
The noise in the image should be equal or at least very close to the correctly exposed image.

If this is the case it would mean that you could under expose to get a higher shutter speed and still get the image back to propper exposure without compromise.
For situations where the light is low and you need a higher shutter speed this could be very helpful.

So I have taken a set of images with the Fujifilm X-T3 and XF 100-400 at iso 200-12800.
Each iso value have been shot 5 times:

1. normal exposure
2. under exposed 1 stop
3. under exposed 2 stops
4. under exposed 3 stops
5. under exposed 4 stops

I have then added back the exposure in Capture One Pro to level the exposure to match the normal exposure.

First we have the normally exposed images:

ISO 200

The next set are at iso 200.
Normal exposure followed by the under exposed images:

Now lets level up the exposures.

Now lets look at the 100% crops from the bottom left corner as that was the darker part of the image.
The normal exposure is the left half and the corrected exposre on the right half.

Click on the gallery below and use the option in the exif data box to open each image at full size in a seperate window:

 

Conclusion at iso 200

As we can see the noise increases very slightly, but it is as good as iso invariant at iso 200.
At iso 200 it is realively safe to under expose by 3-4 stops without sacrifising image quality.
The benefits of a faster shutter speed or more depth-of-field probably outweigths the very slight increase in noise.

 

ISO 400

Following the same procedure as above lets now look at iso 400.
I will not provide the gallery for the un-corrected images as it follows the same pattern as for iso 200.

Here are the corrected images:

Now lets look at the 100% crops from the bottom left corner as that was the darker part of the image.
The normal exposure is the left half and the corrected exposre on the right half.

Click on the gallery below and use the option in the exif data box to open each image at full size in a seperate window:

 

Now lets compare +4 at iso 200 with +3 at iso 400:

ISO 200 vs 400 +4_+3

There is very little difference to note here. I looks like the sensor behaves in a linear way at these iso settings.

Conclusion at iso 400

The noise is now increasing by about 1 stop as you would expect. Without knowing what the sensor and camera processor is doing I will conclude that it makes little difference to shoot at iso 200 or 400 as long as long as the overall exposure remains constant.

 

ISO 800

Following the same procedure as above lets now look at iso 800.
I will not provide the gallery for the un-corrected images as it follows the same pattern as for iso 200.

Here are the corrected images:

Now lets look at the 100% crops from the bottom left corner as that was the darker part of the image.
The normal exposure is the left half and the corrected exposre on the right half.

Click on the gallery below and use the option in the exif data box to open each image at full size in a seperate window:

 

Now lets compare iso 200 @ +2 vs iso 800 normal exposure:

ISO 200 +2 vs 800 Base

They are as good as identical.

Conclusion at iso 800

The noise is now increasing by about 2 stops as you would expect.
I will conclude that it makes little difference to shoot at iso 200 or 800 as long as long as the overall exposure remains constant.

 

ISO 1600

Following the same procedure as above lets now look at iso 1600.
I will not provide the gallery for the un-corrected images as it follows the same pattern as for iso 200.

Here are the corrected images:

Now lets look at the 100% crops from the bottom left corner as that was the darker part of the image.
The normal exposure is the left half and the corrected exposre on the right half.

Click on the gallery below and use the option in the exif data box to open each image at full size in a seperate window:

 

Now lets compare iso 200 @ +3 vs iso 1600 normal exposure:

ISO 200 +3 vs 1600 Base

Here the native iso 1600 image looks a little cleaner than the iso 200 pushed 3 stops.

 

Conclusion at iso 1600

The noise is now increasing by about 3 stops as you would expect.
It looks like iso 1600 at its normal exposure will perform better than iso 200 pushed in post.

 

ISO 3200

Following the same procedure as above lets now look at iso 3200.
I will not provide the gallery for the un-corrected images as it follows the same pattern as for iso 200.

Here are the corrected images:

Now lets look at the 100% crops from the bottom left corner as that was the darker part of the image.
The normal exposure is the left half and the corrected exposre on the right half.

Click on the gallery below and use the option in the exif data box to open each image at full size in a seperate window:

 

Now lets compare iso 200 @ +4 vs iso 3200 normal exposure:

ISO 200 +4 vs 3200 Base

Here the iso 3200 shot is a little cleaner, but perhaps slightly less detailed.
It is possible that Capture One Pro is adding some noise reduction behind the scenes that I cannot change.

 

Conclusion at iso 3200

The noise is now increasing by about 4 stops as you would expect.
It looks like iso 3200 at its normal exposure will perform better than iso 200 pushed in post.

 

ISO 6400

Following the same procedure as above lets now look at iso 6400.
I will not provide the gallery for the un-corrected images as it follows the same pattern as for iso 200.

Here are the corrected images:

Now lets look at the 100% crops from the bottom left corner as that was the darker part of the image.
The normal exposure is the left half and the corrected exposre on the right half.

Click on the gallery below and use the option in the exif data box to open each image at full size in a seperate window:

 

Now lets compare iso 400 @ +4 vs iso 6400 normal exposure:

ISO 400 +4 vs 6400 Base

 

Conclusion at iso 6400

The noise is now increasing by about 5 stops as you would expect.
It looks like iso 6400 at its normal exposure will perform better than iso 400 pushed in post.
We are now quickly seeing a severe degredation in image quality as we are approaching the normal limits for this sensor.
It is best not to push the sensor much beyond this point.

 

ISO 12800

Following the same procedure as above lets now look at iso 12800.
I will not provide the gallery for the un-corrected images as it follows the same pattern as for iso 200.

Here are the corrected images:

 

Now lets look at the 100% crops from the bottom left corner as that was the darker part of the image.
The normal exposure is the left half and the corrected exposre on the right half.

Click on the gallery below and use the option in the exif data box to open each image at full size in a seperate window:

At iso 12800 the base exposure is still very usable, but beyond that it is a rapid fall off in quality.

Now lets compare iso 800 @ +4 vs iso 12800 normal exposure:

ISO 800 +4 vs 12800 Base

Again we see a very comparable result as we are within the normal range of exposure settings for this camera.

 

Conclusion at iso 12800

The base exposure at 12800 is still very usable, but beyond that it is best to avoid.

 

Final conclusion

It is better to shoot at the correctly exposed higher iso settings than underexposing at a lower iso value and then correct in post.

However, the impact on image quality, if you underexpose, is very slight and I wouldn’t worry about it with this camera.

I would avoid any exposure values that pushes the overall exposure beyond 12800.

 

 

 

 

2 thoughts on “Fujifilm X-T3 ISO Invariance

  1. Wow, what a body of work on this project and overall in your portfolio. If you look at Tony Northrup’s recent video (April 2019) on ISO invariance and the website he referenced (photonstophotos.net – on the ‘read noise’ page) you can see how many, many cameras perform. I use the XT3 (Fuji) and there is a drop-off point at ISO 800 where there is less noise, which is really helpful to know. I just discovered your page and am enjoying your images and blog. Thanks!

    Like

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