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Photography Lesson 4 – What does ISO mean?

ISO followed by a number (ex. ISO 100) is a measurement of how sensitive to light a film or digital sensor is. In the older days of film it was called ASA. Your camera probably has a setting that will allow you to change the ISO that you are using. In the next few minutes I will explain the uses of ISO.

Generally speaking, without getting super technical, the lower the ISO the more light is needed to expose a photograph. In turn, the greater the ISO, the less light is needed to make the picture. In the old film days, this was called the ASA or speed of the film. For instance, if one was going to be shooting in bright sunlight, he or she would buy Kodak ASA 100. If one was going to shoot sports or be in lower light, one might choose Kodak ASA 400. For really dark photography one may choose ASA 800 or ASA 1600.

I bring up these numbers because it will help you understand the relationship between ASA and ISO. It will also demonstrate the similarities of “grain” and “noise”. Generally speaking, the lower the ASA in film, the smoother the photograph looked. So, for a portrait the photographer would generally use a ASA 50 or 100 for that dreamy smooth look. Back in the day if one used an ASA 800, the photograph would have “specks” in it and not be that smooth, dreamy look.

ISO works the same way, but, instead of “grain” you get digital “noise”. Digital noise is is the random variation of brightness or color information in images produced by the sensor and circuitry of a scanner or digital camera. I will show in momentarily what I am talking about, but it shows up in little colored specks on your digital photograph. Since I believe in the power of pictures, lets look at these two examples.

Crossed Swords of Victory

at 100%

The above photo was taken in Iraq with an ISO 200. Notice how smooth the image looks.
The crop underneath the main photo is a portion of the photo at 100%

Tsavo Lions

at 100%

These are the actual Tsavo Lions that were the real-life man-eaters that the movie “Ghost in the Darkness” depicted. They are on display at the Chicago Field Museum. This photo was shot behind glass in a very dark environment. The only way I could get this photograph was to shoot at an ISO 3200. Notice all of the noise in the picture. Not the grandest photo but I was able to capture that little moment in time.

Ok, now that you kinda see the results of different ISO lets talk about how to apply them in your picture taking experience. Doubling the ISO number of the film doubles its sensitivity to light.  ISO 100 needs twice the light to take the same picture as ISO 200. ISO 400 film needs a quarter of the light that ISO 100 needs. This means, you could capture a low-light scene with a shutter speed of 1/15 second with ISO 100 film, or 1/60 second with ISO 400. That’s the difference between getting a blurry mess and a sharp photo.

While turning the knob up on ISO may sound like the savior to all bad pictures, there is a price to pay for jacking it way up. The price is noise like you saw in the lion pictures above. That’s the decision you have to make….get a noisy picture or no picture. While I will not be making millions selling the above lions, I have captured a moment in time that I will be able to tell stories about for years to come. Coffee and storytime anyone?

Here are some basic rules of thumb for ISO adjustment…

  • Use the lowest setting possible that allows you to get a non-blurry picture.
  • In daylight with bright sunlight, use ISO 100 or 200
  • On an overcast day, use ISO 400
  • Inside or no flash ISO 800, 1600

These are general guidelines and each camera varies greatly on the quality of each ISO setting.

Homework Assignment

  1. Pull out that camera manual and figure out how to adjust your ISO
  2. Take at least 5 photos at each setting for comparison
  3. Compare the noise levels in each photo
  4. Take note of how fast your shutter clicks (shutterspeed) at each ISO

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