Introduction - ISO Basics

ISO or ASA in the most basic terms is the speed with which your film or digital camera responds to light, so the higher the ISO/ASA rating the more sensitive the film or CCD/CMOS sensor is to light.

In terms of film those with with lower sensitivity (lower ISO speed rating like 50 or 100) requires a longer exposure and is thus called a slow film, while stock with higher sensitivity (higher ISO speed rating such as 400 or 800) can shoot the same scene with a shorter exposure and is called a fast film.

The same holds true for digital camera, but you are adjusting the sensitivity of the CCD or CMOS not actually using different film, this is one of the beauties of digital cameras, you can change ISO on every shot if you wish, you don’t need different physical films!

The basic rule would be a higher ISO gives a higher shutter speed with the same Aperture settings, so less blur. The trade-off is that higher ISO also gives more noise or grain to your images, which can be a bad thing if it’s not a look you appreciate.

Slow shutter speed will give you pics like this:

Because you just can’t hold it steady enough!

or

Because your shutter speed isn’t fast enough to capture someone in motion!

ISO or ASA

ISO is the term generally used on Digital Cameras, the standard was ASA and in the later years ISO.

To get a bit more technical it was known as the ISO linear scale, which corresponds to the older ASA scale, doubling the speed of a film (that is, halving the amount of light that is necessary to expose the film) implies doubling the numeric value that designates the film speed so 50, 100, 200, 400, 800 and 1600.

Nikon digital ISO ratings tend to be exactly the same as the real film counterparts where as some others like Sony Alpha A-100 are slightly off, I think the Canon 350D was out too.

As for other manufacturers I’m not really sure.

For example with the Alpha:

The first column is the indicated sensitivity the second is the actual.

ISO 100 ISO 125
ISO 200 ISO 250
ISO 400 ISO 500
ISO 800 ISO 1000
ISO 1600 ISO 2000

And for the Canon 350D:

ISO 200 ISO 250
ISO 400 ISO 500
ISO 800 ISO 1000
ISO 1600 ISO 2000

Noise or Grain

Noise is what happens when you crank the ISO up, it’s because you are making the sensor more sensitive to light so you are also making it more sensitive to it’s flaws. The contrast, ISO and grain/noise are all linked.

Visual Noise is what occurs on digital cameras where as Grain is what occurs on film, Grain tends to be a lot more pleasing than noise, especially in Black & White photography.

In terms of film, the ASA or file speed is roughly related to granularity, this is the size of the grains of silver halide in the emulsion, since larger grains give film a greater sensitivity to light. Fine-grain stock, such as portrait film or those used for the intermediate stages of copying original camera negatives, is “slow”, meaning that the amount of light used to expose it must be high or the shutter must be open longer. Fast films, used for shooting in poor light or for shooting fast motion, produce a grainy image.

In digital pictures the image noise is a random, usually unwanted, fluctuation of pixel values in an image. Image noise can originate in film grain or in electronic noise in the input device (scanner or digital camera) sensor and circuitry.

Noise in digital images looks like this:

Now this looks ok in a small size, but if you blow it up the noise is very visible, here is a crop from the image:

Noise is also directly related to sensor size, so camera phones generally give terrible noise even under quite reasonable lighting conditions. Digital SLRs have much better noise performance than compact cameras and even better than that is full frame sensor Digital SLRs like the Canon 5D.

Also note noise tends to be more obvious in shadows or underexposed pictures, so do be careful when shooting.

We will discuss noise more in detail later, what causes it, what are the types of noise and how to combat it. There a few options for software based noise reduction, which again we will discuss later.

Interestingly the most sensitive sensor common in commercial photography may be the Silicon Intensified Target Vidicon, at ASA 200,000, used in TV cameras.

Rule of Thumb for ISO

The most basic rule is keep the ISO as low as possible at all times, especially on cameras like the Panasonic Lumix range and other super compacts with small sensors, keep the ISO at 50 or 100 even if you are shooting at night because the noise these cameras generate can be quite terrible.

When shooting in low light or darkness with a camera with decent light sensitivity (the best compacts on the market now for high ISO shooting are the Fuji cameras after that would come Canon), adjust the ISO up one setting until you get a clear shot.

You can check the sharpness of your shot by zooming in on the LCD.

You can check your camera manual or have a look around online to find out how you can change the ISO, most compact cameras have it inside a menu somewhere and are often using auto-ISO.

For example here is the Canon S3 IS ISO button and it’s usage.

Every digital SLR from medium range and above has a dedicated ISO button.

Be careful when you start shooting something that your ISO is set correctly as well, as the night before you may have been shooting at ISO800, if you keep shooting like that in daylight you will have a lot of overexposed pictures and unnecessary noise in all your shadows.

I’ve made this mistake before! I blame it on an oversight in the D70s user interface..

If it’s really dark and you are taking still scenery it’s always best to use a tripod and the lowest ISO possible.

Summary

Just think a little bit before you take your shot, don’t just keep shooting and hope for a lucky shot Understand your camera and it’s limitations and learn to get the most out of it.

ISO is an important tool in getting us clear and sharp shots by getting the optimum shutter speed out of our camera for any given situation, but it’s also something that can cause a lot of noise and terrible artifacts.

These can ruin your picture if you aren’t careful.

So do learn to use ISO, just be wary of it’s effects as like most things it’s a double edged sword.

You can read more here:

Film Speed

If you need and clarifications please leave a comment below, do check out the previous article in the series on What is Aperture or f-stops/f number & Depth of Field and next to come is Shutter Speed!