To complete our Exposure Triangle Unit we now examine ISO. Just as shutter speed relates to the camera’s shutter curtain and aperture relates to the size of the opening in your lens, ISO relates to the image sensor. The image sensor is arguably the most expensive part of a camera and it is responsible for gathering light and transforming it into an image. ISO determines the sensitivity of image sensor to light. If there is a lot of light your sensor can easily capture the image. If there is a low level of light you can boost the sensitivity to capture the 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 “noise” to the pictures. So a lower ISO speed is almost always desirable. As a result, ISO speed is usually only increased from its minimum value if the desired aperture and shutter speed aren’t otherwise obtainable.
Note: actual amount of noise varies with camera sensor size, camera model, and noise reduction settings. Always try to use the lowest ISO speed possible as it is difficult to reduce noise in existing images.
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. 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.
Instances where you may need to go with a higher ISO speed are in low light locations where you subject is moving so you cannot decrease your shutter speed or your aperture to compensate for the amount of light the camera needs to properly expose your image. Likewise if you don’t have a tripod at your disposal making it impossible to hold the camera still for the length of the exposure and your aperture is opened all the way, cranking up your ISO would be your final choice.
Typically, ISO numbers start from 100 (Base ISO) and increment in value: 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!
Activity: ISO Exercise (10 points)
This activity requires a camera, tripod and subject. Work with a partner and share a camera. Each person should take a series of 4 different subjects (8 total images each).
- Portrait (Framing: Close up) on the cove using available light.
- Portrait (Framing: Close up) on the green background in the outdoor hall.
- Photo object #1 (Framing: Close up/Fill the Frame) of an object.
- Photo object #2 (Framing: Close up/Fill the Frame) of an object.
Taking your photos:
- Set your camera to P (Program mode), take the first image with your ISO set to 100.
- Keep the same framing on your subject, take a second photo with your ISO 6400.
The goal is to photograph the same subject using ISO speeds on the opposite end of the spectrum (100 and 6400). Before importing to Photoshop review the exposure data in Lightroom, record the ISO, shutter speed & aperture for both images. Rename the files:
- “Portrait 1_ISO 100” and “Portrait 1_ISO 6400”
- “Portrait 2_ISO 100” and “Portrait 2_ISO 6400”
- “Object 1_ISO 100” and “Object 1_ISO 6400”
- “Object 2_ISO 100” and “Object 2_ISO 6400”
Creating your side by side ISO comparison photo in Photoshop:
- Open Portrait 1_ISO 100 in Photoshop
- Place Portrait 1_ISO 6400 into Photoshop as well: FILE > PLACE EMBEDDED
- Use the marquee tool
to select half your image. - Click on the add layer mask button
in your layer panel. - Using the text tool
add text with the exposure data to both sides. - Save the image as a JPG, rename the file ISO Test 1, ISO Test 2, 3 and 4.
- Post to your images to your blog. Write a short description describing the effects of ISO on your image. Use your photos to support your findings.
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