Back to the Basics: ISO

Back to the Basics: Exposure, Part III: Last week, we discussed Shutter Speed. Two weeks ago, we discussed Aperture / F-stop. Today, we’re going to discuss ISO. Next week, we’ll tie it all together into the Exposure Triangle. If you haven’t already read the posts regarding Aperture and Shutter Speed, consider checking those out before reading about ISO.

Exposure is the capturing of an image on film or digital sensor through the use of light. Three factors go into making an exposure: Aperture, Shutter Speed and ISO. Changing one of those factors will effect your exposure AND one or both of the other two factors.

ISO

Film is manufactured in a variety of different ISOs.

ISO is a measurement of how sensitive the film or digital sensor is to light. It’s not just for your film cameras. Although you’re not loading a digital camera with a specific speed of film, setting the ISO is just as important.

In traditional photography, photographers typically choose the ISO they’d like to shoot, then choose a film to match. In digital photography, photographer still chooses the ISO they’d like to shoot, even though film is not used.

Manual Film camera with ISO set to 100

ISO is measured in numbers. Typical ISO numbers are: 100, 200, 400, 800, 1600, and 3200. The lower the number, the less sensitive the film / digital sensor is to light. Therefore, lower ISOs require more light to make an exposure.

Lower ISO = Less Sensitivity = Needs More Light

LCD screen of a digital SLR shows the ISO setting

High ISO’s have a greater sensitivity to light. They can capture much dimmer beams of light. As a result, higher ISO’s require less light to make an exposure.

Higher ISO = Greater Sensitivity = Needs Less Light

As with both Aperture and Shutter Speed, the difference from one ISO to the next is half or double. In aperture, it’s half or double the amount of light. In shutter speed, it’s half or double the amount of time. In ISO, it’s half or double the light sensitivity.

ISO 100 is half as sensitive to light as ISO 200; and it’s twice as sensitive to light as ISO 50.

ISO 1600 is twice as sensitive to light as ISO 800; and it’s half as sensitive to light as ISO 3200.

Also, as with aperture and shutter speed, there are half and third stops. Although it’s less common, film is (or perhaps, was, by the time you read this) available in 50, 64, 100, 125, 160, 200 etc. Some digital cameras have half and third stops available, from 100 through 6400 (even higher, in some cases!).

Third stops:
100, 125, 160, 200, 250, 320, 400, 500, 640, 800, 1000, 1250, 1600, 2000, 2500, 3200 4000, 5000, 6400

This provides photographers with much greater flexibility. On a digital camera, the photographer can change the ISO from one shot to the next simply by turning a dial or two. In traditional photography, the photographer generally wants to shoot the entire roll with the same ISO setting.

Terminology: Film Speed
Slow Film: Lower ISOs are called Slower Films (or Slower ISOs) because they are less sensitive to light and therefore the film / sensor needs to be exposed to light for a longer period of time (or exposed to greater amounts of light) in order to capture enough light to make a good exposure.

Lower ISO = Less Sensitivity = Needs More Light = Longer Exposure ∴ Slower Film

Fast Film: Higher ISOs are called Fast Films (or Fast ISOs) because with the greater sensitivity to light, they need shorter exposure times (or a smaller amount of light) before they capture the amount of light needed to create a good exposure.

Higher ISO = Greater Sensitivity = Needs Less Light = Shorter Exposure ∴ Faster Film

The Nitty Gritty Details

There is a visual by-product of using a higher ISO: increased grain or noise. The higher the ISO, the more grain or noise there will be in the image. This is true in both film and digital photography. Without getting too technical, in traditional photography, the grain is caused by the silver halide crystals that make up the light sensitive film. In digital photography, the noise is caused by color discrepancies recorded by the photo diodes of the digital sensor. There are several different types of noise in digital photography. Noise caused by a high ISO is random noise.

Lower ISOs produce images with less noise and grain (sharper images). The photographs below demonstrate this point. Both photos were shot with a tripod, under the same light, using a digital camera. The photo on the top was shot at 100 ISO. The image on the top right is a cropped section of the same photograph, to show a detail of the noise. The photo on the bottom was shot with a 6400 ISO. Again, the image on the right is a cropped section of the same photo, to show a detail of the noise. As you can see, there is a drastic increase of noise in the image shot at 6400 ISO.

100 ISO

100 ISO Cropped

6400 ISO

6400 ISO Cropped

Does this mean that you should never use a high ISO? Absolutely, not! Decide which ISO to use on a photo-by-photo basis.

Why might you choose a high ISO?

- low light situation, rules prohibiting the use of flash photography (such as indoor basketball or museums)

- no tripod to hold the camera steady in a low light situation

- flash would create a undesirable lighting effect

- grain / noise may create a neat artistic effect

We’ll get into this even more next week when I’ll talk about adjusting your Aperture and Shutter Speed in order to choose the right ISO for the shot.

Additional Photos:

Traditional photograph with 3200 ISO Film

Same image, cropped to show detail of grain

Additional Reading:

Back to the Basics: Aperture / F-stop

Back to the Basics: Aperture / F-stop part 2


Back to the Basics: Shutter Speed


Back to the Basics: Exposure Triangle