What the [bleep] is ISO?

If you’ve been using a digital camera, or have been around people who do, chances are you have heard of this thing called ISO. Everyone seems to be using it. It’s like the hottest buzzword amongst anyone with a camera. But what the [bleep] is it?

You must have thought that if you figured out what the letters stand for, your understanding would be better, right? So you found out that ISO stands for International Organization for Standardization. Big help isn’t it?

I thought so.

The world famous ISO organisation also lends its name to a format of optical disc archive files, also called (hold your breath) ISO!

The Names

Shall we discuss the history of names for a while? You can skip it, but if you are interested in stuff like this, read on.

There was a time when there were two popular denominations of film speed. What’s film speed? In a while, Watson.

These two systems were called ASA (American Standards Association) and DIN (Deutsches Institut für Normung). Yes, both of them are again ‘associations for standardization’. They were as related to each other as fairies and scientific calculators are. When ASA said a film is 100, DIN insisted it’s just 21. ASA said 3200, DIN would still be lingering around 36.

So the big daddy of Standardization Associations, ISO, steps in and says let’s make it rational guys! How do they do that? By selecting one and sticking to it. And we all got together and renamed ASA to ISO (American becomes international: typical huh?). Simple!

Now with that out of the way, let’s get to what these bodies were trying to standardize.

What is ISO?

Would you like some more history? Try it, it’s not that bad!

With digital cameras you do all these fancy ISO things. But back in the days of film, there was something we called film speed. It was a measure of how sensitive the film is. In the olden days of film, you used to get film which was ASA 25 & 50, as compared to the recent standard ISO (or ASA) 100 & 200 & 400 films. What this meant is that the film back then used to be half or a quarter as sensitive as the ones we saw last when film was still being made.

Sensitivity of the film simply is a measure of the light the film would need to get to a certain amount of exposure. The lower the ISO number, the less sensitive the film, the more the amount of light the film needs. The higher this number, the more sensitive it is, and the less amount of light you need.

This ties in directly with those other two devils you always worry about: shutter speed, and aperture. Higher the ISO, higher the shutter speed you can shoot on, given that the light, aperture and required exposure remain the same. Or, with higher ISO, you can shoot with a smaller aperture, given same shutter speed, light and required exposure. Or, keeping everything else the same, a higher ISO would give you more exposure in the picture.

Technicalities

How is it done? In film, there was something called grain. Slower film (lower ASA/DIN) film had finer grain.  Which meant more grain density. Which meant more light needed to ‘activate’ each of these grains to get them to your desired exposure value. In faster film (higher ASA/DIN), the grain would be coarser, which meant lower density, and less required light for desired exposure levels.

So why would you use slower film at all, you’d ask? Well, these grains, they can be seen! Which is why in photographs shot on really fast film (say ASA 800), you’d see spots all over, even where there should be solid patches of colour. If the same picture was taken using a slower film like ASA 50, these patches would be extremely smooth and homogeneous. In addition, slower film would capture more detail than faster film, because the grain was finer. Thus, if you were shooting things like studio portraits and landscapes, where you had the luxury of using a tripod, and didn’t have to capture a ‘moment’, or were blessed with expensive fast primes, you would prefer to shoot slower film. For journalists who were capturing events like natural calamities, or sports photographers, where speed and the moment mattered, faster film was a necessity. Even street photographers used faster film, to use faster shutters to capture the moment, and/or because they didn’t have access to faster lenses.

Enter Digital

In the days of film, once you bought a roll, you were stuck with it for 12 (or 36) frames. Buying the next roll would be the moment when you’d get a chance to rethink your choice. So you had to be wise while buying film, preempting the type of photography you needed to do. For professionals it’s not very difficult, since a. they are aware of the kind of pictures they are going to shoot, and b. they shoot faster through a roll than amateurs and the general public.

Enter digital, and with it, digital sensors. These sensors can simulate the sensitivity of various speeds of film. Most commercial level DSLR sensors today can be sensitive from the levels of ISO 100 to ISO 3200 and beyond.

The principles of light sensitivity here stay the same. But how do the grains become coarse and fine on demand? The answer is, that it’s not “grain” here per se. Digital sensors are made up of pixels, which do not change size. Instead, each of these pixel sensors can be made more sensitive or less sensitive as and when you need with help from the camera’s electronics. Neat eh?

The Difference

So do we get the “grain” in the picture? And do we lose details at higher speeds? Yes. But the mechanism is different here.

You see, these pixels and the electronics work as “light amplifiers”. They pick up light signals and amplify them by a factor you specify (the gain, in electronics engineering parlance) so that the picture becomes “visible”. This factor that you specify is nothing but the ISO. At base levels, the gain is low, while at faster ISO levels, this gain is high. This gain is the reason you get the noise when you shoot at high ISOs. It’s the same thing as the background noise that you get when you play a home-recorded tape at high volumes. And this is the thing that sets good mics apart from bad ones, and good camera sensors apart from bad ones. For the good mics, and good sensors, the signal-to-noise ratio is high, which means that they record signals at higher level than background noise, or random data used to fill up the space where there’s no signal. At low ISOs this ratio isn’t that critical, since the noise isn’t amplified enough to cause any concern. At high ISOs however, the noise gets amplified with the signal, and for older or smaller sensors (and cheaper mics), it becomes visible (or audible) enough to bother you. You see this noise as hot pixels (bright pixels at random places where the surrounding pixels are comparatively darker) or variation in pixel colours where there should be solid colours. Digital noise can be broken down into two components: luminance and chromatic. Luminance noise is variation of brightness values, while in the case of chromatic noise you see pixels showing colour variation.

Digital bad?

So digital gives you the benefit of varying the ISO levels, while in film, you’re stuck with the ISO rating of the film you’re shooting. But still digital’s not all good. Remember the pictures you saw in sports magazines back when you were a kid? The ones which had this moody feel, and lots of dithering? That dithering was film grain. It added an aesthetic quality to the picture. A certain mood. It gave me a feel of rainy overcast days, of desolate corner cafes in a big city with people reading books while sipping coffee, of a journalistic picture of a high-adrenaline event.

Digital noise, on the other hand, is ugly. The cheaper the sensor, the uglier it is. However, luminance noise is not as aesthetically jarring as the chromatic type, but still is undesirable in most situations. Photoshop and other specialized noise reduction software can help you control this noise as well, albeit the more you reduce noise, the more details you lose. An overdose of noise reduction can render your image looking like it’s been over smoothed, just like pictures from a small phone camera.

What does recaptured do

I keep the ISO as low as possible to get the smoothest & cleanest pictures. But when I have to capture motion, or events emotion and the shutter speed is too low for the surroundings, I bump up the ISO a bit. When I am shooting using a tripod or when I’m using my f/1.4 lenses or when I’m shooting a long exposure, I don’t often go above the base ISO (which is 200 for my camera).