What ISO Really Means on a Digital Camera

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If you remember the days of film or not, the link between ISO and old school film speed always comes up. It’s an easy way to explain what ISO or how the US standard equivalent ASA works in the digital realm or a simple way to explain digital ISO. However, a direct comparison between the two is far too simplistic. ISO in the film and digital world don’t work exactly the same.

At first it seems like a contradiction. Turn up the ISO on a DSLR usually due to working in low light conditions, the other settings change so you can use faster shutter speeds and smaller apertures, etc. The higher the ISO number the noisier/grainier the images become. Isn’t that a bit like how old school film worked?? Again, not exactly in the digital world.

Digital photography school.com which is a great resource, shows the meaning of digital ISO, like many do, ‘In Digital Photography ISO measures the sensitivity of the image sensor.’

Nikon USA relate digital ISO to film ISO in a basic way, but say there are also caveats. They say upping the ISO number in the film days produced more grain. In the digital world this is now noise. Larger sensor pixels result in less noise. Noise reduction is put to use on every image as all pixels produce some noise, which is getting better all the time, meaning less noise at similar high ISOs. So there take is, it’s kind of similar to film ISO, but not quite.

The topic of digital ISO reminds me of asking the question of if planets go round the sun. Yes, but ask a physicist and they will say that its not entirely true with a two-hour lecture on the effects of gravity, subatomic particles and so on. Nothing is ever that straightforward when you want a simple answer.

What? No Light Sensitivity…?

Because we’re thinking of modern ISO in film terms, turning up the ISO number does not increase sensitivity. The sensor itself doesn’t become more sensitive to light per se, the image is just made lighter. Many camera sensors can use amplification when ISO increases, but it’s not across the board. It may seem this way with an increasing ISO as amplification creates more noise, but this itself is a lot to do with your aperture and shutter speed settings. Most of the time the image sensor data goes through various amplifiers which have their own gain value and noise level so when the gain increases so does the noise. So the ISO figure relates to the amount of amplification used on the image data.

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The ISO standard itself only links the lightness of the final file to initial exposure, which can be used via amplification or not. This means as a general rule, increasing ISO only reduces the time of exposure, thus producing more noise from each color and less light hits the sensor.

Knowing the differences between how a digital sensor and film capture and recover light is helpful. Digital hard clips highlights at a certain threshold, while film has a more linear curve, which means things like highlights can be more easily recovered with film. With this in mind, you exposed differently for the two mediums. This we know already, with ‘exposing to the right’ in the digital world where you get as much exposure as you can on an image without clipping the highlights (this is where the histogram and blinking highlights are your best friends). This gives the best signal-to-noise ratio and in turn shows a lower ISO number. It’s not a set in stone rule as there are always times where you need to expose more for the shadows or midtones, leaving the highlights to their own devices.

Highlighting the Brightness

So upping the ISO number doesn’t increase sensitivity per se like film, but the image brightness for each make of camera may handle this differently. Therefore, you may have to dig into how your own camera brand implements ISO so you know how to expose properly for your model of camera. Histograms and clipping work to an extent, but they are based on the output of JPEG’s and midtones as a start point. If the same tools were available for RAW files, this will give us a more accurate output, but how well is still debatable. For now we can only use what we are given to provide the correct exposure and not clip the highlight as a rule.

In reality, how does this affect our day-to-day shooting? In many ways, you have to know how your individual camera model responds in the most common situations you shoot. Learning as much as you can about the sensor and how it works is always ideal, but not everybody’s cup of Darjeeling digging into the stats and data this deeply.

I dug through a lot of the comments and feedback of articles on this matter and through all the discourse most are more interested in practical applications for ISO. Overly simplifying, if the sensor gets too much light, it can oversaturate and blow out the highlights. ‘Exposing to the right’ is a general rule of thumb, but it really depends on the scene and subjects you are capturing. The histogram gives you a general awareness of the image, but it’s not a completely precision tool. As my grandad used to say, ‘owt’s better than nowt’ (perfectly understandable to people from the North of England!).

We know the lowest ISO settings give us the most noise free images, even if your camera is rated up to 5 billion ISO. In changing the ISO parameters it changes all your other exposure settings. Include some of the other guidelines above and think digitally from now on.

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