ISO – how does it affect the image?
How ISO Shapes Your Photography
ISO in photography refers to the sensitivity of your camera’s sensor to light. It’s one of the three key elements of exposure, along with aperture and shutter speed.
ISO seem like something we just adjust to the need of light, getting enough sensitivity to have a fast enough shutter speed so images aren’t motion blurred due to camera shake, or making people running being motion blurred because the shutter speed is too slow.
The aperture opening is also a factor. If the aperture is wide open (like 1.4-2.8, which is lens dependent), the result is a narrow space of focal depth, and demands a lower ISO, whereas an aperture from 5,6-11, needed for having a deeper area kept inside the focal range, lets less light into the sensor, and therefore needs a higher ISO setting.
On film cameras, this is maybe even easier to understand, because each film is chosen not only for colour or BW, but especially with focus on the ISO.
Before digital sensors, ISO was tied to film speed—the sensitivity of the film emulsion to light.
Low ISO film (e.g., ISO 50 or 100): Fine grain, high detail, perfect for bright daylight or studio setups.
High ISO film (e.g., ISO 800 or 1600): More sensitive, allowing shooting in low light, but introducing noticeable grain.
What Does ISO Do?
- Low ISO (e.g., 64, 100, 200)
The sensor is less sensitive to light.
Produces clean images with minimal noise and maximum detail.
Best for bright conditions or when using a tripod.
- High ISO (e.g., 3200, 6400, 50,000)
The sensor becomes more sensitive to light.
Allows you to shoot in low-light situations without a flash.
Adds noise/grain, reduces dynamic range, and can affect colour accuracy.
The M10 to M11
The transition from the Leica M10 to the Leica M11 isn’t just about a new sensor or added megapixels; it’s about how these changes influence the way we handle light, especially through ISO sensitivity. ISO is one of the three pillars of exposure, alongside aperture and shutter speed, and understanding its impact is essential for any photographer aiming to master their craft.
Remember that the Leica M system relies on manual focus lenses, and there is no built-in stabilisation, which demands faster shutter speed and therefore also needs higher ISO settings in low light, to keep an image without motion blur.
Whereas the Leica Q (Typ 116) – the original Q introduced in 2015 has optical image stabilisation built into its fixed 28mm f/1.7 Summilux lens. The newer Leica Q2 and Q2 Monochrom retain the stabilised 28mm f/1.7 lens from the original Q, and the Leica Q3 features optical image stabilisation (OIS) built into its fixed lens, not sensor-shift (IBIS), which helps with slower shutter speeds, allowing handholding to some degree.
Leica M10 vs. M11: What’s The ISO News?
The Leica M10 introduced a 24 MP CMOS sensor with a native ISO range of 100–50,000. It was celebrated for its classic rendering and low-light performance. The M11, however, steps up with a 60 MP BSI CMOS sensor, offering improved dynamic range and an ISO range of 64–50,000.
This lower base ISO of 64 allows for slightly better highlight retention and cleaner files in bright conditions, while the backside-illuminated design improves noise control at higher ISOs. On older cameras, the very low ISO often spoiled the dark areas a bit, not having the same depth and nuances in the shadows.
If you know the M9 and like how it performs, this is not a real issue, because this camera doesn’t allow much alteration to the dark areas of the image.
What ISO Really Does
ISO determines the sensor’s sensitivity to light:
- Low ISO (64–200): Produces the cleanest images with maximum detail and dynamic range. Ideal for daylight or controlled lighting.
- Mid ISO (400–1600): A sweet spot for handheld shooting in mixed light. Slight grain begins to appear, but modern sensors handle this gracefully.
- High ISO (3200–50,000): Essential for low-light environments. Expect visible noise, reduced dynamic range, and colour shifts—but also the ability to capture moments that would otherwise be lost.
From One End of the Scale to the Other
- At ISO 64 on the M11, you’ll see crisp detail, rich colour, and smooth tonal transitions. Perfect for landscapes or studio work.
- At ISO 50,000, the image tells a different story: grain becomes part of the aesthetic, shadows lose depth, and colours desaturate. Yet, this is where the Leica philosophy shines—embracing imperfection for the sake of capturing the decisive moment overrules the technically perfect image.
Practical Takeaways
- Use base ISO whenever possible for maximum quality.
- Don’t fear high ISO—modern Leica sensors handle noise beautifully, and grain can add character.
- The M11’s improved dynamic range means you can push ISO further without sacrificing as much detail compared to the M10.
ISO isn’t just a technical setting—it’s a creative choice. From the silky smoothness of ISO 64 to the gritty charm of ISO 50,000, each point on the scale offers a different mood. The Leica M11 gives photographers more flexibility and confidence to explore that spectrum.
ISO in Film Photography: The Historical Context
Unlike digital cameras, where ISO can be changed shot by shot, film ISO is fixed for the entire roll. If you load ISO 400 film, you shoot everything at ISO 400. This limitation forced photographers to plan and embrace grain as part of the aesthetic. Grain in film is physical (silver halide crystals), while noise in digital is electronic, but both influence the mood and texture of an image.
Understanding this history helps us appreciate why Leica cameras still celebrate the character of high ISO images. Grain and noise aren’t flaws—they’re part of the storytelling.
Practical Takeaways
- Use base ISO whenever possible for maximum quality.
- Don’t fear high ISO—modern Leica sensors handle noise beautifully, and grain can add character.
- The M11’s improved dynamic range means you can push ISO further without sacrificing as much detail compared to the M10.
Although described so, it is maybe a bit misleading that changing the ISO setting on your camera alters the sensitivity of the sensor. In reality, that’s not how it works. Let’s break down what ISO truly does and why it matters even when shooting in RAW.
Image from a previous workshop in Paris. Challenging light inside a café in the Latin Quarter.
Leica M9, Summilux 50mm f1.5 ASPH
Does ISO Change Sensor Sensitivity?
No. Unlike film, where ISO represented the actual sensitivity of the emulsion, a digital sensor has a fixed sensitivity. It always converts incoming photons into electrical charges at the same rate, given by the quantum efficiency of the sensor, and the transmission of the optics and filters over the sensor.
Increasing ISO does not make the sensor more sensitive to light, but in a historical context, it’s often told like it does because it’s easier to understand.
Why Is ISO Often Described as “Sensor Sensitivity”?
This terminology comes from the film era, where ISO (or ASA) was a direct measure of the film’s light sensitivity. When digital cameras arrived, manufacturers kept the term because it was familiar and easy to understand for photographers transitioning from film.
From a practical standpoint, raising ISO does make the image brighter, which feels like “increased sensitivity,” even though technically it’s just signal amplification. So, while it’s not correct scientifically, it’s a convenient shorthand for describing the effect.
On a bright day, the ISO can be lowered, and a detailed rendering of tones is achieved with low noise.
A shot from a local village, Kerteminde, in Denmark, this summer.
Leica M9, Summilux 50mm f1.4 ASPH
What ISO Actually Does
ISO in a digital camera is essentially a gain control.
This is known better from video, because in specifically professional video cameras, “gain” earlier has been the normal expression when shooting in low light, and needing more light to make the correct exposure.
When you raise ISO, the camera amplifies the electrical signal coming from the sensor before converting it into digital data. This makes the image appear brighter, but it also amplifies noise.
ISO and RAW Files
Here’s the subtle but important point:
- If you shoot RAW, the ISO setting still matters because the amplification happens before the RAW data is recorded.
- The RAW file contains the amplified signal, not the original unamplified one.
- If you keep ISO low and brighten later in post-processing, you’re doing a similar amplification digitally—but usually with slightly worse results because the camera’s analogue amplification is cleaner than digital brightening done afterwards.
In the low light conditions like in the Metro in Copenhagen, higher ISO (gain) adds noise, most visible in the dark areas, but often it’s necessary to zoom in to really see it, and it may not disturb the image unless this is shot on very high ISO. Using a fast lens going down to f1.4 in this example helps keep the gain/ISO down.
Leica M9, Summilux 50mm f1.4 ASPH
Should You Always Keep ISO Low?
Getting as much light as possible and avoiding high ISO is generally good because:
- More light means a stronger signal relative to noise.
- Lower ISO preserves dynamic range and avoids unnecessary amplification.
However, in very dark situations, raising ISO can help because:
- At very low ISO, shadows may be severely underexposed and require heavy digital brightening later, which can introduce more noise than if you had raised ISO in-camera.
- Many modern sensors are “ISO invariant” in certain ranges, meaning brightening in post vs. raising ISO gives similar results—but not always perfectly.
Think of ISO as a tool to balance exposure and noise—not as a magic sensitivity booster.
In short, ISO doesn’t change sensor sensitivity; it changes signal amplification. RAW files reflect that amplification. Understanding this helps you make better decisions in low-light situations and get the most out of your camera.
Low light conditions in the Metro in Copenhagen.
Leica M9, Summilux 50mm f1.4 ASPH