UNDERSTANDING EXPOSURE

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Home    Lesson Plans    #1: Understanding Exposure

What is ISO?

The last element that contributes to a proper exposure is the ISO setting. ISO is the acronym for "International Standards Organization". In photography, ISO is an indicator of how sensitive a film or digital sensor is to light. The ISO setting determines how much light is needed for a correct exposure of your photograph.

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How do we measure ISO?

Similar to aperture and shutter speed, ISO can be measured in full stop increments and are represented by numbers.

100
200
400
800
1600
3200
6400

The lower the ISO number, the lower the sensitivity of the digital sensor or film is to light which means that more light needs to hit the digital sensor or film to get proper exposure. So, ISO 400 is twice as sensitive to light as ISO 200, for example.

One of the main advantages of modern digital cameras over film camera is how easy it is to change ISO settings on a digital camera. On film cameras, you would need to use an entire roll of film before you can change your ISO because the ISO rating is built into the film roll. With a digital camera, you can change your ISO after every photo you take (if you need to).

The example images below shows illustrations of the effect of ISO on the exposure of the photo. In this example, the aperture and shutter speed remain constant to isolate the effect of ISO on the exposure of the photograph.

ISO Effects
Figure 1: Effects of ISO on Exposure

In the photos above, the photo taken with ISO 400 has the correct exposure. Do you notice anything diferent about the photos taken when the ISO was increased or decreased? Increasing the sensor's sensitivity to light while maintaining the aperture and shutter speed tends of overexpose the photo. On the other hand, if the ISO sensitivity is lowered, the photo will be underexposed.

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General Rule of Thumb for ISO

Here's a quick overview of how ISO rating is generally used:

  • ISO 100-200: Outdoors (Sunny)
  • ISO 400: Outdoors (in the shade or overcast (cloudy)); Indoors (when there's lots of light)
  • ISO 800 and above: Indoors (with minimal light); Low light conditions, sports or action shots, night shots

ISO and Digital Noise

Just like aperture and shutter speed which has a unique side effect to the photograph, ISO also has its own unique side effect. In digital photography, the side effect of higher ISO setting is called Digital Noise. Digital noise lowers the overall clarity of the photograph. It makes the photograph more "grainy". Sometimes, this grainy look can be used by photographers for creative purposes so in some circumstances, it is desirable to have a photo with a lot of digital noise.

The sample images below shows photos with different ISO settings.

ISO and Digital Noise
FIGURE 2: ISO and Digital Noise

Notice that the photo taken with ISO 6400 has a lot more digital noise or "grain". The photo taken with ISO 100 looks a lot smoother and has minimal grain and is not even noticeable. As a general rule, you should choose the lowest ISO setting possible to get the smoothest and grain-free photos.

Once again the aperture and shutter speed varies between the two photos in the above example. This is how the 3 elements (aperture, shutter speed and ISO) work together to achieve the correct exposure. You cannot change one and not affect the others.

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ISO and Digital Noise

Your DSLR camera usually will have the option to allow you to change the ISO setting. Each camera manufacturer is different. On my Nikon camera, I go to a menu and set the ISO or there are button combinations on my camera that allows me to change the ISO setting. Consult your camera's product manual to figure out this feature.

  • It's time to tinker with your camera! This time, take a few minutes to figure out how to change your camera's ISO setting.
    Consult your manual if you do not know how.

When to use higher ISO settings

Your DSLR camera usually will have the option to allow you to change the ISO setting. Each camera manufacturer is different. On my Nikon camera, I go to a menu and set the ISO or there are button combinations on my camera that allows me to change the ISO setting. Consult your camera's product manual to figure out this feature.

There are situations where the amount of lighting you are dealt with is not quite enough for your image. This is an indicator that you need to increase your ISO setting. Typical examples include:

  1. Low light conditions (indoors). In this case, the amount of natural light is not enough for your camera to get the correct exposure even with using the longest shutter speed and widest aperture without using a flash.
  2. Your subject is too far away for your camera's flash to have any effect. To properly expose your photograph, you need to increase your ISO.
  3. Low light conditions (night). If you want to take photos at night, you will need to increase your ISO.

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Assignment 1.3: Get familiar with ISO

For this week, I would like you to become familiar with the ISO setting and how it affects the exposure of a photo. Experiment... discover... this is the only way to learn.

Assignment Instructions

  1. Set your camera to Program Mode. Consult your user's manual if you do not know how. Setting your camera to Program mode takes away your responsibility for setting the aperture and shutter speed - the camera will do this for you. All you have to change is the ISO setting.
  2. Follow the ISO guidelines discussed in General Rule of Thumb for ISO. Set your ISO depending on how much amount of light you have.
  3. Take pictures outdoors when it is sunny. Set ISO to 50, 100 or 200.
  4. Take pictures outdoors in the shade or if it is cloudy. Set your ISO to 400.
  5. Take pictures outdoors in the early evening or indoors with limited lighting. Set your ISO between 800-2400.
  6. Lastly, download your photo and view it on your computer. Study your photos and try to understand what the differences are between the photos taken at different ISO ratings. Pay particular attention to how much digital noise is created on the photograph based on the ISO setting used.

Project 365 or 52 Week Project for the serious hobbyist

If you are really serious about learning photography then I highly recommend that you try Project 365 or 52 Week Project. Both of these projects are great ways to practice and learn photography. For more detailed information about these projects and how to start, read my Project 365 or 52 Week Project blog. I have carefully organized a Weekly Theme Suggestions for this project to coincide perfectly with our step by step Lesson Plans.

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