ISO in digital cameras defines sensitivity as well, but in a different way. The digital sensor has only one base sensitivity. Cameras use circuitry to boost that sensitivity in steps that relate to traditional ISO values. The sacrifice for higher sensitivity in digital cameras is not grain, but noise introduced by electronic amplification, heat and other factors. On the first digital cameras increasing sensitivity from the base of 100 ISO to even 800 ISO introduced visible noise in the form of stray colors and clumpy pixels. This wasn’t that long ago; many of those “early” digital cameras are still making photographs today.
There’s isn’t one generation of digital camera where we can say digital high ISO problems were solved, but through 5 or 6 generations of sensor technology it is radically improved. If you have a camera built in the last 3-4 years your higher ISO settings are probably similar to the base ISO setting. Each new innovation brings further improvement. Anything I write about this today is only valid until the next sensor technology comes along.
Digital photographers have a fairly simple task as far as learning to set ISO. We can use ISO as the variable that allows us to choose the settings we want for aperture and shutter speed while still keeping our overall exposure correct. To successfully manipulate ISO without negative effects we need to know our cameras. We need to know what happens at each ISO value for each camera we use. ISO choice is very much a question of taste, or noise tolerance. My threshold will be different than yours. Yours may change over time.The following examples show how ISO effects image detail. I made these samples with the oldest camera I own, a Canon 5DII, and the newest, an Olympus EM-5. The 5DII is a generation old, but by no stretch an outdated camera. The EM-5 is currently the best Micro43 camera, but the sensor of the EM5 is almost 4 times smaller than the sensor in the 5DII, which is a major factor in terms of noise.
These images are 100% crops, a tiny portion of the whole image to show you individual pixels. When you look at an actual photograph you are not looking at individual pixels. A typical photographic print has 300 pixels across every inch, and even the worst monitor shows you a photo at 72 pixels to the inch. In short this is detail you will not regularly see. Take some time to compare the subtle differences in these samples.
(Disclaimer: This is a beginner’s guide, so I am using examples of Jpeg files coming right out of the camera. That is how most beginners will cut their teeth, and I recommend it. The step from shooting Jpeg to RAW is an important step that will come in any photographer’s evolution, but only after you get a better sense of the basics.)
The EM5 has a base ISO of 200. To my eyes, on the EM5 the difference from ISO 200 to ISO 800 is slight. At ISO 800 the solid color starts to get just the tiniest bit pebbly and the edges lose some clarity. I am comfortable using ISO 800 for all but the most detail demanding applications.
The EM-5 still holds up at ISO 1600 and 3200. The solid colors now have texture introduced by electronics. Having stray pixels in with the correct pixels softens the color. These setting would probably not be suitable for a finely detailed photograph like a landscape image with leaves, grass or sand. On the other hand I would not hesitate to use ISO 1600 or 3200 for people, except for formal portraits.
ISO 6400 is the last useable ISO on the EM5, and then, only in desperation. The color clumps are bigger and there are now stray bits of contrasting noise in the solid colors. The edges are gone, turning jagged. It would have to be a one-time shot, like Aliens landing at night, to get me to even try ISO 12800.
I would never use ISO 25600 on the EM5. I’m not sure why they even bother to include it. Starting at ISO 6400 the overall color starts to shift as well. By 25600 the subtle colors are gone. In the image above you can see how in the 25,600 side the “Espresso” leather on my chair looks like grey mud.
The Canon 5DII has a base ISO of 100. The camera limits ISO to 6400 unless you go into the custom functions of the camera and activate “ISO Expansion.”
From ISO 100- through ISO 800 there is almost no difference in the 5D2. If you look closely at the black edge on the ISO 800 sample you will see it’s just starting to lose its crispness. At ISO 800 the solid colors have just the tiniest bit of sandiness, but is still smoother than the Olympus at its base of ISO 200. ISO 400 and even ISO 800 are still suitable for detailed landscape or architecture photos.
At ISO 1600 the 5DII still is not a compromise. There is only the slightest loss of detail. I wouldn’t use ISO 1600 for a landscape photo because you usually don’t need that level of sensitivity in good light. It is completely usable in all portrait applications.
So why does ISO Matter?
It would be easy to assume from these images that ISO just isn’t that important. But it is. In a way I have oversimplified the situation in the above images. These were carefully shot with an exact manual white balance. They were photographed in tungsten light, which is more challenging to a sensor than daylight, but not nearly the problem that fluorescent, sodium vapor, or mixed light present. In short, the examples present good light with careful technique. Under or over exposure of one f-stop is enough to make ISO 800 look more like 1600. At ISO 1600 and above a white balance mismatch can completely distort colors. Combine incorrect white balance with under exposure and high ISO and you can get a capture that will never make a satisfactory photograph.
Set up your own ISO test for each camera you own. You don’t need a color chart, but use something with solid areas of tone, and sharp edges. Learn where you don’t like the results. Until then try these guidelines. If you are in bright light there is no reason not to use a lower ISO. If you are taking photos where you want the highest level of clean detail by all means use ISO under 800. When you need a higher ISO it’s almost always better (now) to choose ISO 1600 over ISO 3200, but you shouldn’t avoid ISO 3200 or even 6400 when it means being able to use a fast enough shutter speed to stop action. As ISO goes up, when the light is poor, your exposure technique becomes more critical. When the chips are down err on the side of being too conservative with ISO.