Understanding ISO

It is extremely hard to take good pictures without a good understanding of how ISO works and what it does. Camera ISO is one of the three pillars of photography (the other two being Aperture and Shutter Speed) and every photographer should thoroughly understand it, to get the most out of their equipment. Since this article is for beginners in photography, I will try to explain ISO as simple as I can.

The Sensor Chip

A typical digital camera sensor and mount.

The sensor inside your digital camera actually works on the same principle as a solar cell. When light hits the chip, a tiny electric current is generated: the brighter the light, the stronger the current. Instead of there being one giant solar panel generating lots of electricity, there are millions of extremely tiny solar cells collecting very small and precisely measured amounts of light. These photosites will eventually create the pixels in the image you capture.

It should be noted that a photosite is not the same as a pixel. Several photosites will add their captured light, which is filtered and processed and eventually combined to make a pixel. That’s a complex topic for discussion in another lesson.

All of this carefully measured electrical current, which reflects light intensity, is measured and stored by the camera’s circuitry. This data is called the signal. The signal, however, must compete with the noise inherent in all electrical equipment.

Signal vs Noise

Noise is at a constant background level to the signal.

An unavoidable fact of electronics is noise. While the sensor is measuring the tiny electronic currents generated by the light, there is also a tiny electric current that comes from other places. This unwanted current is called noise, and it mostly comes from the ambient heat of the sensor. The difference between the value of the signal and the value of the noise is called the Signal to Noise Ratio. When the ratio becomes smaller, the noise is more apparent, and the signal may get lost in it.

There are two ways that the signal to noise ratio can become low: by a drop in the signal, or an increase in the noise. In modern cameras, the noise is mostly a constant value, so we only need to worry when the signal drops – that is, when we’re photographing a dark subject. The graph to the right may help to visualize the situation.

What is ISO?

ISO (I.S.O.) is the abbreviation for the International Organization of Standardization, a governing body based in Europe that provides the standards for a wide variety of subjects. 

For photographers the key standard is Film Speed ratings. In the past this was known as ASA or the American Standards Association (Now discontinued and replaced by the American National Standards Institute or ANSI), and you could buy your films in ASA 50, 100, 200, 400, 800 and 1600. There were specialist films that would go higher or use infra-red although these were generally known as the standard speeds. Since digital cameras do not use film but use image sensors instead, the ISO equivalent is usually given.

Most decent cameras now have interchangeable ISO settings which are especially useful for digital photography.

In very basic terms, ISO is the level of sensitivity of your camera to available light. The lower the ISO number, the less sensitive it is to the light, while a higher ISO number increases the sensitivity of your camera. The component within your camera that can change sensitivity is called “image sensor” or simply “sensor”. It is the most important (and most expensive) part of a camera and it is responsible for gathering light and transforming it into an image. With increased sensitivity, your camera sensor can capture images in low-light environments without having to use a flash. But higher sensitivity comes at an expense – it adds grain or “noise” to the pictures.

Take a look at the following picture (click to open a larger version):-

ISO 200 and ISO 3200 Comparison

The difference is clear – the image on the right hand side at ISO 3200 has a lot more noise in it, than the one on the left at ISO 200.

Every camera has something called “Base ISO“, which is typically the lowest ISO number of the sensor that can produce the highest image quality, without adding noise to the picture. On most of the new Nikon cameras such as Nikon D5000, the base ISO is typically 200, while most Canon digital cameras have the base ISO of 100. So, optimally, you should always try to stick to the base ISO to get the highest image quality. However, it is not always possible to do so, especially when working in low-light conditions.

Typically, ISO numbers start from 100-200 (Base ISO) and increment in value in geometric progression (power of two). So, the ISO sequence is: 100, 200, 400, 800, 1600, 3200, 6400 and etc. The important thing to understand is that each step between the numbers effectively doubles the sensitivity of the sensor. So, ISO 200 is twice more sensitive than ISO 100, while ISO 400 is twice more sensitive than ISO 200. This makes ISO 400 four times more sensitive to light than ISO 100, and ISO 1600 sixteen times more sensitive to light than ISO 100, so on and so forth. What does it mean when a sensor is sixteen times more sensitive to light? It means that it needs sixteen times less time to capture an image!

ISO Speed Example:

ISO 100 – 1 second
ISO 200 – 1/2 of a second
ISO 400 – 1/4 of a second
ISO 800 – 1/8 of a second
ISO 1600 – 1/16 of a second
ISO 3200 – 1/32 of a second

In the above ISO Speed Example, if your camera sensor needed exactly 1 second to capture a scene at ISO 100, simply by switching to ISO 800, you can capture the same scene at 1/8th of a second or at 125 milliseconds! That can mean a world of difference in photography, since it can help to freeze motion.

ISO Examples

A series of images taken with the Canon 5D MkII, changing the ISO and other setting to keep a constant exposure. Noise increases dramatically at higher ISO settings.

The lower the ISO number, the slower the speed. The higher the ISO number, for example 1600 the faster the speed. 

Take a look at this picture:-

Black Skimmers, 1/2000th of a second at ISO 80

What would have happened if I had ISO 100 on my camera instead? My sensor would have needed 8 times more time to capture the same scene, which is 1/250th of a second. At that speed, I would have introduced motion blur into my picture, because the birds were moving faster than that. In short, I would have ruined the picture.

When to use low ISO?

The standard ISO that most people use everyday, giving accurate color rendition and "clean" noise-free images is ISO 100.

You should always try to stick to the lowest ISO (base ISO) of your camera, which is typically ISO 100 or 200, whenever possible. When there is plenty of light, you should always use the lowest ISO, to retain the most detail and to have the highest image quality.

There are some cases where you might want to use low ISO in dim or dark environments – for example, if you have your camera mounted on a tripod or sitting on a flat surface. In that case, bear in mind that your camera will most likely need more time to capture the scene and anything that is moving is probably going to look like a ghost.

Oh No! It's a GHOST ATTACK!

Camera is set to the lowest ISO to retain the detail, which also resulted in a long exposure of 5 seconds. The person sat still, while other person stepped in for a brief moment to introduce the ghost :)

When to increase ISO?

You should increase the ISO when there is no enough light for the camera to be able to quickly capture an image. If you want to shoot indoors without a flash, set the ISO to a higher number to be able to freeze motion. Other cases where you might want to increase ISO are when you need to get ultra-fast shots, like the bird picture. But before increasing the ISO, you should think if it is OK for you to introduce noise to the image.

The biggest problem when using high ISO's is the quality deterioration. If you have ever used high I.S.O. film such as 800, the emulsion used on the film contained larger, more sensitive "grains" which were perfectly visible in your images thereby reducing quality.

On many of the newer DSLRs, there is a setting for “Auto ISO”, which works great in low-light environments. The beauty of this setting is that you can set the maximum ISO to a certain number, so when the ISO is automatically increased based on the amount of light; it does not cross the set barrier. So, if I want to limit the amount of grain in my pictures, I typically set the maximum ISO to 800.

How to change your digital cameras ISO setting?

To change the ISO setting on your SLR camera, press the ISO button then use the 'up down left and right buttons' to change the speed. Again, if you don't have this option refer to your cameras manual as it may be slightly different between brands. 

 

 

 

 

 

 

General Rules and tips for ISO Settings:

• Use an ISO of 100 or 200 when taking photographs outside in sunny conditions.
• If the sky is overcast or it is evening time, then use an ISO within the range of 400 to 800. 
• Night time or in cases of low light you might need to set your digital camera ISO to 1600. If you don't then your photo will appear too dark if at all.