I could have started this blog post with “When father was young”, film speeds were described in terms of low, medium and fast. We referred to DIN, ASA, ISO, and ISO Latitudes settings. The term ISO still applies to today’s photography. It still indicates light sensitivity for modern digital cameras. There is no ISO rating that suits every light condition, it is essential to know when to use a slow, medium or fast ISO to optimise and provide the best results. This blog post is here to outline advantages and disadvantages of the different ISO settings, and discuss such terms as Base ISO and Auto ISO.

 

Exposure Triangle

The exposure triangle in the middle of this diagram illustrates the relationship between ISO, shutter speed and aperture. If you change one parameter, you will have to compensate one or more other parameters in order to keep balance and maintain correct exposure. Increasing the f-number decreases the size of the lens’ diaphragm, resulting in a reduction in the amount of light hitting the image sensor, but also increasing the depth of field in the final image. Reducing the shutter speed affects the capture of motion, but it also increases the amount of light hitting the image sensor, so everything is brighter. Increasing the ISO lets you shoot in lower light situations, but you increase the amount of digital noise due to amplification in the photo.

 

Beginner’s ISO Cheat sheet

*Note this is an approximation as different cameras have different levels of sensitivity to light.

 

LOW

Low refers to the ISO range 50-200. Using a low ISO setting allows you to get the very best quality from the sensor. Colours will be at their best, as will tonal distribution. Noise will be at minimum too, as amplification is at its minimum. A low setting is generally desirable when you want to blur moving subjects such as beautiful running water flowing through the landscape. I can recommend a LOW ISO when shooting backlit to gain the shortest exposure times. The lack of amplification comes at a price, because the camera often needs more time to get the right exposure. So when shooting handheld with low ISO, camera shake is easily introduced. To avoid that we can increase aperture, which in turn reduces depth of field.

MEDIUM

Medium refers to the ISO range 250-400. Image quality and noise are still within reasonable limits. Image quality is very similar to lower ISO ratings, but you have the benefit of being able to use faster shutter speeds or smaller apertures (or a combination of the two). Colours tend to be more saturated.

FAST

Fast refers to the ISO range 500 and beyond. I rarely go higher than 6400 as noise quickly builds up. A High ISO allows you to handhold your digital camera in low light. Noise is not always your enemy – for instance when shooting black and white with high contrast. In these types of pictures, grain is desirable. It often adds the mood that I am looking for. What you need to be aware of when shooting at fast ISO is increased noise, loss of accuracy in colours and loss of overall sharpness.

Shot at 1/1000 f/2.8 ISO 500 with a Sony Alpha A99V equipped with a Minolta 80-200 HS APO G @ 160mm

 

Base ISO

The term “Base ISO” or “native ISO” refers to the unamplified sensitivity of the camera. In other words, the base ISO is the single ISO setting at which your sensor/processing pipeline produces its best signal-to-noise ratio and where the sensor can achieve its full dynamic range. The ISO settings below the base ISO setting are usually marked “LO” to avoid confusion. For my Sony Alpha A99V, the base ISO value is 100.

Practical use of ISO and Sony Alpha A99V

The combination of the A99’s great full-frame sensor and the Minolta 80-200 HS APO f/2.8, and not least Sony’s Steady Shot technology works wonders. I try to stay under a strict maximum limit of ISO 800 but it is my experience that it is possible to produced great-looking photos up to 1600. When shooting at night, I often go even higher. Especially when shooting black and white with high contrast, when I use ISO settings as high as 6400.

Auto ISO

Fortunately most of the time you do not need to worry about selecting the correct ISO. Most cameras have an “Auto ISO” setting. With Auto ISO, the camera will look at the amount of light in the scene and change the ISO appropriately so that the shutter speed does not get too slow. Remember Auto ISO has its limits; the camera sets the ISO without regards to noise. On the A99V is it easy to change the automatic adjustment range in [AUTO]. When [AUTO] is selected, press B, select [ISO AUTO Maximum] or [ISO AUTO Minimum], then select the desired range.