ISO in Digital SLR Photography

Many digital SLR cameras also have an auto setting for ISO. Canon and Nikon cameras have this feature; however, many Nikon cameras will not allow you to use the auto feature when shooting in aperture priority, shutter priority or manual. I do not recommend the auto setting for anyone who wants to learn photography as it will impede the learning process.

The exposure triangle is an important fundamental in SLR photography. The exposure triangle consists of an aperture (or f/stop), a shutter speed and the ISO. The aperture is the size of the lens opening that allows light to reach the camera’s image sensor. The shutter speed is the amount of time the shutter is held open that allows light to reach the sensor. A change is one of the items will impact the other two. Generally you will set the ISO first based on the available light. Next you will set either the aperture if a certain depth of field is desired, or you will set the shutter speed if you want to stop or blur action. If you see blinking in the viewfinder then you need to make an adjustment. If a Nikon camera blinks “subject too dark” you must boost up the ISO. With a Canon, if you see the aperture blinking then you have to boost up the ISO. Keep increasing the ISO until the blinking stops. You can also change the aperture and/or shutter speed.

According to Cambridge in Colour from the Exposure Triangle: Aperture, ISO and Shutter Speed, “There are many combinations of f/stop, shutter speed and ISO that will result in the same exposure. Each setting however, can impact the final result of the image, so it is important to know that aperture controls the depth of field, that shutter speed affects how you want to show motion (stop it or blur it) and how ISO controls the sensitivity of the sensor to light and that a high ISO could result in noise.” For this reason, it is sometimes better to change the ISO instead of the aperture or shutter speed so that the final outcome of the image will be as expected.

Below are examples of the three settings which result in the same exposure but the results would be very different:

• f/22 at 1/15 ISO 100
• f/16 at 1/30 ISO 100
• f/11 at 1/60 ISO 100
• f/8 at 1/125 ISO 100
• f/5.6 at 1/250 ISO 100
• f/4 at 1/500 ISO 100
• f/2.8 at 1/1000 ISO 100

There are some trade-offs using high an ISO (800 or higher). Shooting at a high ISO might allow you to photograph indoors in an auditorium or an outdoor night scene; however, digital noise results from using a high ISO. Noise is degradation in image quality which results in discoloration of the image. This greatly depends on the camera type and size of the sensor. Full-frame sensors found in more expensive digital SLR cameras tend to have significantly less noise than lower priced digital SLR cameras that do not have a full frame sensor.

Like everything else, learning how to select an ISO will come with practice. Experiment shooting at night and in different lighting conditions so you can test a different ISO with the available light. Soon you will be looking at a scene and dialing in an ISO without hesitation. Once you become an experienced photography you will be able to look outside and say “it’s a 400 ISO kind of day.”