Back to Basics Photography: In Depth Look at ISO

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Back to Basics Photography: In Depth Look at ISO

what is ISO


Back to Basics Photography: An In Depth Look at ISO

In the upcoming months John J. Pacetti, CPP, AFP, will be writing a series of basic photography lessons.  To find them all just search “Back to Basics” on our blog. This is the third article in this series. John is a frequent visitor to the MCP Facebook Community Group. Make sure to join – it’s free and has so much great information.


In our last article I gave you a look at the exposure triangle. This time we’ll go in depth with ISO.

ISO is the sensitivity of the sensor.  The sensor gathers light.  Light on the sensor is what creates your image.  The lower the ISO number the more light is needed to create an image, bright scenes.  The higher the ISO number the less light is needed to create an image, darker scenes.


Knowing what ISO is appears to be in, my opinion, the most miss understood of the three parts of the exposure triangle.  If you’re having trouble with this, you’re not alone.  Back in the film day, most people choose 100 or 400 film speed.  You were told to use 100 for outdoors and 400 indoors.  This is still true.  Today’s digital cameras, however give us a much greater ISO range than film ever did.  Most digital camera will give you a range of 100 to 3200 and higher.  Some of the newer cameras go as high as 102400.


The ISO is what I usually set first when determining my exposure settings.  Here are a few scenarios.

  • While I’m working outdoors, for example, a park with a bridal party or portrait session, engagement session or a family session, I do not need a high ISO.  I’ll use 100.  The only time I may choose 200 is if it’s very over cast or nearing dusk where I may need a bit more light sensitivity to get to my good exposure.
  • Now, if I’m working in a low light situation, for example, a church that does not allow flash photography, I’ll choose an ISO of 800, 1600, possible 2500.  I need the sensitivity of the sensor to be higher.  The higher sensitivity of the sensor will allow me to keep my F-Stop and SS where I want them to create my good exposure in that lighting situation.
  • Let’s say I want to work with available window light.  Window light is diffused (for the most part) sun light.   I’ll go with 400 possibly 800 if the light is not intense enough like a cloudy day.  Again, setting my F-Stop and SS once I have my ISO.


A little recap:  Use a lower ISO in bright light situations (100).  In low light situations, use a higher ISO (400, 800, 1600).  Once you decide on your ISO, you can than set your SS and F-Stop.

I hope this gives you a better idea as to how ISO works and how to use ISO to your advantage.  Education is the key.  Once you have that education, there is no stopping a rewarding photographic career.  Education never ends, no one person knows everything.

Next time we’ll look at F-Stop.


John J. Pacetti, CPP, AFP   –   South Street Studios

2013 Instructor at MARS School- Photography 101, The Basics of Photography

If you have question, feel free to contact me at  This email goes to my phone so am able to answer quickly.  I’ll be glad to help in any way I can.


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  1. Yza Reyes
    March 5, 2013 at 2:27 am —

    i’m learning free! thanks for this free gift of knowledge =)

  2. December 19, 2012 at 4:07 pm —

    ISO is also how sensitive film is to light.

    I haven’t gone digital in camera yet. Generally, I will have 400 speed film in my camera. I am ending a year of shooting exclusively in B&W, so Kodak BW400CN is my general purpose film. I will use 100 outdoors and I’ve used TMAX 3200 at a night baseball game and inside the Smithsonian Air & Space Museum. I’ve also pushed TMAX 3200 to 12800 for a rock concert.

    For 2013, I will resume using color film. I love the look of Ektar 100 when I used it in 2011 for the Space Shuttle launch. I haven’t tried Portra 400 yet, so I don’t know if that’ll be my primary film next year or not.

  3. December 19, 2012 at 2:23 am —

    I found this very clarifying. Thank you

  4. December 17, 2012 at 12:34 pm —

    This is very good article and helpful for anyone. I like it very much.

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Back to Basics Photography: In Depth Look at ISO