Understanding Digital Exposure – Step 1. – ISO

Before diving into your camera’s meter, I want to break down the components of exposure.

I like to start off with ISO. When I walk into a shooting situation, I ask myself is these questions:

  • What is the LOWEST ISO I can use to achieve the cleanest file and best colors?
  • Do I want to use HIGH ISO for creative purposes? (ie, B&W Conversion and would like a little grain)
  • If I use the LOWEST ISO possible, will I be able to use a fast enough shutter speed to avoid camera shake?
  • If I use HIGHER ISO am I compromising the clarity and color of the file?

The more experienced you become (and more refined your style becomes), the less you will even think about the answers. It will be second nature and you just do it.

ISO dictates how sensitive your Digital Sensor is to light. The LOWER the number the LESS sensitive to light. The HIGHER the number, the MORE sensitive it is too light.

Digital Cameras have a large range of ISO values to choose from. We are going to stick with 1 stop increments.
ISO 100 – 200 – 400 – 800 – 1600 – 3200 – 6400

There is 1 stop between each of these values. So 100 to 200 is one stop. 100 to 400 is 2 stops. 100 to 1600 is 4 stops. So: ISO 200 is 1 stop more sensitive to light than ISO 100.

ISO and Shutter Speed
While shooting ISO 100 with an f/2.8 Aperture and a 1/250 Shutter Speed, I could switch to ISO 200, f/2.8 Aperture and a shutter speed of 1/500 and achieve a correct exposure. Because I increased my ISO setting by 1 stop, I was able to increase my shutter speed by 1 stop as well (changed from: 1/250 to 1/500).

ISO and Aperture
While shooting at ISO 100 with an f/2.8 Aperture and a 1/250 Shutter Speed, I could switch to ISO 200, f/4.0 Aperture and a shutter speed of 1/250 and achieve correct exposure. Because I increased my ISO setting by 1 stop, I was able to decrease my aperture size by 1 stop as well (f/2.8 to f/4.0).

What does stop mean? You will hear this term when discussing exposure. It can be used for ISO, Aperture (aka f/stop) and Shutter Speed. It is 1 full increment of Light Change. The light change can be related to ISO (the sensitivity), Aperture (How much light gets in), Shutter Speed (How long the light gets in) The combination of these 3 values (ISO, Aperture and Shutter Speed) determine Correct Exposure.

This may seem a little confusing/overwhelming right now. That is OK, we will continue to build on this and you will be rocking in no time. I do not want to bombard you with everything in one post. In short I can tell you that no matter how many bells and whistles your camera has, it still comes down to Manual Mode and ISO, Aperture, Shutter speed to make an exposure. I also throw in White Balance because I love to get it ALL right in the camera, to ease the post-production phase of photography.

Here are 3 scenarios to get you thinking:

SCENARIO 1: It is a Bright Sunny Day and you are headed to the park to photograph your children. Chances are you will use the lowest ISO setting on your camera. This is most likely 100 or 200. Bright and Sunny = lots of light, which enables you to use smaller apertures and faster shutter speeds. Lower ISO will give you better colors, saturation and lower grain performance.

SCENARIO 2: It is an overcast day and your son just started riding his bike with no training wheels. You want to Freeze the action of him riding his bike. In order to Freeze the action you would like to use a shutter speed of 1/500 of a second or faster. Due to lower light conditions you will want to choose an ISO value of 400 or 800.

SCENARIO 3: You are second shooting on your first wedding assignment. You walk into a beautifully designed catholic church, however there is very little available light. You think, hmmm it is time to use flash, right? Nope. The restrictions of this church do not enable you to use flash during the ceremony. What to do? You will want to use an ISO value of 1600, 3200 or 6400 (depending on your camera/lens combination) so that you can use fast enough shutter speeds to avoid camera shake.

Hopefully this is the beginning of the great Digital Exposure Journey for you! Please leave any comments or questions in the comments section.

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Comments 6

  1. David –

    Great explanation of understanding exposure and use of examples. It really is confusing to people at first but if you keep pounding it into their head (my wife :)) they will understand.

    Something you might want to add is the metering modes for cameras and how that changes what your camera thinks is the correct exposure (spot vs. center-weighted vs. evaluative… etc…).

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  2. Yes please….metering is something that I just can’t get through my head! I’m glad to have found your blog….and I’m now a subscriber, looking forward to the lessons to come!

  3. David,
    I think a great tool for anyone really struggling with Exposure is Understanding Exposure by Bryan Peterson. Not only does he go through exposure step by step, but he always makes you think creatively, to see all the different areas of correct exposure, not just the couple that do make it perfect.
    LOVE this blog by the way, but what photographer wouldn’t? How stale would our craft be without always thinking, always learning! Great job!

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      I agree Ginger. I have not read the book but back in 2002 I took an online course with Bryan Peterson via betterphoto.com. That course set the foundation for my journey of Understanding Exposure (on film) which I then took with me to the digital world. Thanks for the tip!

  4. Thank you so much for this — great explanation! I’m new to your blog and pretty new to DSLR photography as well. I understood Aperture & Shutter Speed and how they worked together, but had new clue how ISO fit in there. I appreciate the clear explanation and especially the examples/scenarios.

    I’ll be subscribing to your blog and looking forward to learning more!

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