ISO sensitivity and dual ISO on the smartphone: is it important?

As we already recommend in our ultimate guide to buying the best possible camera phone: Forget the ISO sensitivity of your smartphone. Value doesn’t matter outside of marketing communications.

Contents:

1. What is ISO sensitivity anyway?
2. And what is ISO sensitivity in mobile phones?
   1. So what does ISO 409,600 mean?
   2. ISO values ​​and computational photography
3. What is dual ISO sensitivity?

However, because the value keeps popping up, whether in Pro mode or on various data sheets, we’d like to put it together once and for all for ISO sensitivity, from a smartphone perspective.

Story Time: ISO sensitivity comes from the days of analog photography. A layer of silver halide crystals was bonded to the base of the film. The incident light converts the sensitive silver compounds to elemental silver and thus captures the corresponding image areas on the film.

The larger these crystals are on the film, the more sensitive they are to light and therefore better suited for low light conditions. However, larger crystals also mean lower resolution. Images become grainy. You’ve probably noticed this before with old analog photos.

Before international standardization came to photography as well, there were all sorts of names and units for film speed, for example ASA, DIN or GOST.

Before we get to the ISO sensitivity of smartphones, one more note: We have set out to explain all the details about your smartphone camera. Here you will find an overview:

The age of digital cameras has traded in the grain of analog film for pixels made of silicon. The light-sensitive material converts incoming light particles into voltage. The more light, the higher the voltage. In the case of CMOS sensors, this analog signal first passes through an analog amplifier.

The ISO sensitivity of the sensor now depends on the pixels themselves and the gain of this analog hardware amplifier. And just like analog film grain on the newly inserted substrate, in today’s digital photography there is no way to swap “digital film grain” and change the ISO sensitivity of the sensor. We will come to an exception later.

After the first analog amplifier, our image signal now goes to an analog to digital converter which, as the name gives away, digitizes the signal. From here the magic of computational photography kicks in and combines image information with other photos, reduces noise, bakes white balance, lightens dark areas and darkens light areas, etc. The result then ends up as a JPEG file in your smartphone’s memory.

“But I can set the ISO sensitivity in Pro mode!” I hear you say. That is true, but setting the ISO value, as you know it from the manual mode of your smartphone or your SLR camera, does not change the sensitivity of the sensor. In a nutshell, the ISO values ​​set here only change the gain, that is, the brightness setting in the image processing behind your image sensor’s analog-to-digital converter. This happens after the photo has already been taken.

So what does ISO 409,600 mean?

Incredibly high sensitivity like ISO 409,600 now just means the manufacturer allows a steep increase in brightness via software, which is of dubious use at this scale, even with full-frame cameras. Half a million ISOs on smartphones? Little sense.

And this is exactly where the answer is, why the Canon EOS 5D Mark IV can “only” ISO 102,400 and the Huawei P40 Pro ISO 409,600. Firstly, Canon’s marketing department is less bold and secondly, it caters to a target group that is more camera-savvy and, pardon my French, you can’t sell every piece of shit to them.

ISO values ​​and computational photography

With the Huawei P40 Pro+, you can actually select ISO 409,600 manually in Pro mode. But RAW images are only available up to ISO 6400. That smacks of software trickery (which is basically completely legit, by the way) that somehow could offer an equivalent to ISO 409,600. But can you still call it “ISO” and advertise with it?

Modern smartphones usually don’t just take a photo when you press the shutter button anymore. Especially with high-contrast subjects or poor lighting conditions, there are sometimes more than a dozen messy photos in one shot. In the end, it doesn’t matter to the user which gain was set somewhere between the analog to digital converter and the JPEG compressor. The main thing is bright, the main thing is noise-free, ISO does not matter.

And above all: ISO sensitivity is a measure of a given light output. It says nothing about the quality of the image; at best, it says something about the manufacturer’s self-confidence that the results at the maximum ISO equivalent value are somehow good enough for customers. The connection between the highest ISO value in the data sheet and low-light image quality is precisely: zero.

Like I said: whether it’s analog or digital film: there’s always a “hard” sensibility. With one exception: Because some so-called dual ISO sensors actually have two different hardware sensitivities. Manufacturers accomplish this with an additional analog gain loop in the sensor before the analog to digital converter.

As a result, developments in terms of image quality at different ISO values ​​are less intuitive, even for camera professionals. However, with smartphones, there is great potential to do the thinking for the user and cleverly exploit changes in dynamic range in highlights and shadows for higher dynamic range.

Blackmagic is very transparent about the dynamic range of its Pocket Cinema 4K. Above ISO 1000, the camera switches from low to high. The graph provided by the manufacturer shows that adjusting the ISO values ​​within the low or high settings does not change the dynamic range, only the gray point (the transition between pale yellow and dark yellow on the graph). Ultimately, this just means a redistribution of image information, while the dynamic range itself remains constant.

This dual ISO option is now exciting and contradictory. ISO 1000 in Blackmagic Pocket Cinema 4K is more suitable for capturing subjects with lots of bright detail. You can tell by the fact that there’s simply a lot more information in the lighter spectrum above the 18 percent neutral gray point. As a result, detail reproduction for bright details at ISO 1000 with 6.8 compared to 3.5 light values ​​is significantly better than at ISO 100.

At the same time, we can also see from the diagram that ISO 100 is actually much more suitable for night scenes, or low-key shots where the scene is mostly made up of dark areas. After ISO 100 (film sets excluded) isn’t usually practical for low-key shots, the higher sensitivity of a dual ISO sensor is an interesting option here – the gray dot in dynamic range suddenly jumps back.

What we commonly understand as ISO sensitivity comes from a heavily limping comparison with analogue photography, which is, however, losing more and more substance. In the age of computational photography, ISO sensitivity is about as relevant as a throttle on the Tesla Model 3.

conclusion

Bottom line: While it’s always been indicative of a camera’s quality, you can really forget about smartphone ISO sensitivity in the days of computational photography. It doesn’t really say anything about how well a particular camera performs in low light.

On the other hand, it might look different with dual ISO sensors, which are rightly used in high-end professional cameras. With the IMX689, the Oppo Find X2 Pro has the first sensor that I know of with this property. However, in the test I couldn’t bring about the behavior described above, which is characteristic of dual ISO sensors, with the Find X2 Pro.