Last time out we looked at the ISOless sensor and demonstrated how it worked.
To recap on that; basically if you turn the ISO up in the camera, it makes the resultant image brighter.
Or APPARENTLY, if you prefer; you can shoot at a lower ISO and turn the brightness up yourself when you work on your RAW file
Possibly you’re thinking, ‘well all roads lead to Rome’
Indeed.
The thing is… it’s not quite that simple.
Excessive noise reduction is (in my opinion) well controlled on the X-Trans sensor, but owners of cameras with a X-Trans II sensors have to put up with details being smeared by excessive noise reduction.
Admittedly this is mainly a SOOC jpeg problem, but although RAW data is just that (RAW data), it does contain a great detail of subsidiary information… Obvious stuff like which lens, what camera etc but it also contains information about the exposure of the shot, whether or not you used any dynamic range settings, how much noise reduction may be required etc
Now in real terms, the only reason you’d want to raise the ISO when taking a shot is if the shutter speed was too low for the situation, perhaps you need to freeze movement, or perhaps the shutter speed is too low to be hand held.
It’s these situations that make us sacrifice the superior image quality and dynamic range of base ISO and select a higher ISO
The apparent trick to the ISOless sensor is that we could use a lower ISO, get the benefits of this, yet still use a higher shutter speed.
So for the next test, I did the following:
Here the ISO6400 shot has exposure parameters chosen by the camera, which I’ve then manually replicated whilst dropping the ISO, then compensating on the computer by raising the brightness via the exposure slider.
6400 0EV
3200 +1EV
1600 +2EV
800 +3EV
400 +4EV
200 +5EV
As we discovered last time (especially at these resolutions and with photobucket UPDATE July2017, photobucket decided to break all my website, so I’ve had to manually update the entire site – not happy with photobucket right now! my new site host) it’s all SEEMS pretty much a muchness!
So let’s get closer
Suddenly it’s not clear cut is it?!
It’s still quite close… and the ISO200 does PERHAPS have a tiny bit more detail, but it also has a lot more noise.
It’s also complicated by what which RAW processing software you use…
I did these in LR, because it offers 5 stops of EV adjustment, and I needed 5 stops of EV adjustment 🙂
I tried the same in a different (and I have to say a PERSONALLY preferred) piece of software and at +5EV the image completely fell apart!!
This complicates matters… because different RAW convertors handle things in different ways, they act upon (or completely ignore) the metadata from the camera that’s burnt into the .RAF file.
So, to my mind, it’s not as simple as set up the exposure triangle as if you were on the highest ISO, then set the lowest ISO, and sort it all out when you get home.
Plus, let’s be honest – pushing files 5 stops is nearly always going to lead to some quality problem or another…
So for what’s it’s worth, my PERSONAL OPINION only, here’s how I use my ISOless sensor…
In a word: “sparingly”
The above shots show EV push of 2 Stops in pairs.
800/200 + 1600/400 + 3200/800 + 6400/1600
I use the ISOless sensor in conjunction with the EV dial. (If I’m using aperture priority mode, if I’m in full manual, I just do the maths in my head, ISO/Shutter Speed is only division or multiplication by 2 after all!)
That offers the following advantages
I can just turn the EV wheel to a negative value to raise the shutter speed
If I’m using a RAW convertor that reads EV values (and I use more than one RAW convertor, so it pays for me to have a universal setting) then that convertor doesn’t mind me telling it to add +2EV in post. If I’m using one that doesn’t read EV values, then it’ll still let me push +2EV in post
That’s why all the shots above are only pushed 2EV. But also another reason, that I’ll come back to later in this page.
Why -2EV? well that’s as low as the dial goes on the X-Pro1 (On the X-T1 and other Fuji X cameras, it goes to +/-3)
Why select a minus value, when I want a brighter image?
Well I do (want a brighter picture) but at the time I want a higher shutter speed, which means turning the wheel down! (Not up)
And there’s another thing as well…
Remember my article about Automatic Dynamic Range, and why I don’t use it?
If you look closely at the images pushed 2EV above. You’ll notice that it’s only ISO1600 that’ll take +2EV without some loss of quality…
That’s because the X-Trans and X-Trans II sensors use a mixture of ANALOGUE and DIGITAL gain (gain = increase in the brightness) not arriving to a full digital gain solution until after ISO1600
It’s the analogue data that’s set into the RAW file. This amplified data is placed before the Analog-to-Digital Conversion (ADC) process.
The full ‘nuts and bolts’ of digital image creation are too complex (and a little off topic) to go into here…
As previously noted, when we raise ISO, we drop quality (namely dynamic range and resolution).
Basically, the ISO amplification (on X-Trans and X-Trans II) between 200 and 1600 is inserted BEFORE the RAW file is created. This means that if you clip data, by shooting at 200, but with exposure settings for 6400, then this data is lost. If you attempt to push this file in post, you’re just amplifying clipped data.
At ISO1600 and beyond, the camera is DIGITALLY amplifying the POST ADC data (the same as you dragging the exposure slider on your computer)
It’s worth mentioning that you MIGHT very well get a nicer file, shooting at 1600 and pushing 2 stops, than if you select ISO6400. This is because the camera will include noise reduction data for 6400 in with the .RAF file, and you might want to handle noise reduction yourself in post. (For example, this why on X-Trans II cameras, skin gets unnaturally smooth after 1600. If you shoot SOOC X-Trans II jpegs, I highly recommend using ISO1600, with -2EV on the EV dial, then using the in-camera RAW convertor to push the file +2stops. Obviously, you’ll need to be shooting RAW + jpeg to do this)
So is the sensor truly ISOless?
Yes! But with the caveat that the first 3 stops of digital amplification are burnt into the metadata before the RAW file is physically created, and pushing clipped data might very well not exceed the quality of letting the camera do it.
Of course, (as we saw) you can get a bit more resolution from pushing ISO200, but at the expense of noise…
The ISOless sensor (in the incarnation within the X-Trans and X-Trans II cameras) is not some piece of unworldly magic, allowing you to run around at night shooting at settings that require sunshine.
You can’t 100% completely set the ‘ISO’ when you get home. At least not before ISO1600.
So for me?
On my X-Pro1 and X-T1, I leave auto ISO @ 200-1600, if the shutter speed is too low (and the ISO is at 1600), I bring it up, by using a negative EV dial setting, then brighten the file in post.
But before the ISO reaches 1600, I let the camera provide the signal amplification.
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Adam Bonn Fotografia 
Not sure if I’m missing something here. You discuss setting the EV dial but surely this is disabled when shooting in manual control. Good article by the way.
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Thanks Steve,
The final words of that article read:
“On my X-Pro1 and X-T1, I leave auto ISO @ 200-1600, if the shutter speed is too low (and the ISO is at 1600), I bring it up, by using a negative EV dial setting, then brighten the file in post.
But before the ISO reaches 1600, I let the camera provide the signal amplification.”
So if the camera selects ISO1600 AND a SS of 1/30 (ie in aperture priority mode) & I think ‘Hmmm I don’t fancy hand holding that at 1/30’ then I’ll dial -2 EV on the EV dial and the SS will rise to 1/125, then in post I’ll lift the shadows or apply a +2EV exposure boost (with the exposure slider) or sometimes a bit of both 🙂
Hopefully that explains.. If not, let me know
Adam
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The sensors are mostly ISOless (tested with Nikon D5500 and X-T20). You mentioned it’s the RAW converter that was causing the differences, and I think you’re spot on.
This can be seen easily with Darktable’s Profiled Denoise. Basically the Profiled Denoise module applies different denoise strengths at different ISOes, but when you shoot underexposed and bringing up the exposure in post, the module incorrectly applies the needed profile, e.g. using ISO800 profile to denoise an ISO800 image, while it should use ISO3200 profile but it doesn’t know that we bumped the image up +2EV. Luckily with Darktable, you can choose with profile to apply, so you can still fix it manually.
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All the sensors are ISOless in that they have a fixed native value, but as I it understand the truly “ISOless” chips are the ones that don’t reply on pre ADC data to provide brightening.
I suspect the D5500 and X-T20 might be the same Sony chip. The X-T20 is certainly ISOless.
I haven’t tried Darktable, but I’ve heard some good things, and what you’ve just told me sounds like a very useful feature.
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