Understanding Exposure

Proper Exposure

I’ve been putting off writing this post for a while since it’s a complicated enough topic to write a book about..  Simplifying it had me delaying the inevitable.  So let’s roll up our sleeves but I’ll try to keep it out of super-geek zone.

When we talk about exposure, we’re simply talking about how much light is allowed into the camera to make your picture.  If we under-expose, the pictures turn out too dark.  Therefore if we over-expose they are too white or “blown out.”  Cameras have the following three tools to balance exposure:

  • Shutter Speed
  • Aperture (f-stop)
  • ISO

Shutter speed is simply how long the camera is “open”.    Keep it open too long and too much light gets in and vice versa

Aperture is the size of the hole that you are letting light through.  Bigger the hole, the more light gets in.


In days of old, ISO referred to the sensitivity of the film.  If you were shooting indoors with less light available, you’d want very sensitive film – typically 1600 (the trade-off was grainy prints).  Today, we can adjust ISO without having to change the film.  Essentially it’s the sensitivity of the image senor.

I’m going to devote a separate post to each of these.  I want to discuss how they are measured, how you can adjust them, and when to adjust them.  For now however, I just want to drive home the delicate balancing act that is creating a proper exposure.

You’ve probably heard the term “stops” in reference to camera adjustments.   A long time ago, people figured out that if there existed a common language between exposure adjustments; it would be much easier make better photos.  Remember, up until a few years ago, you had no idea if your camera settings were correct until a few days later when you got your prints back from the lab.  Given the super complicated nature of light, lots of math was involved.   The outcome produced a way for photographers to increase and decrease the light allowed into the camera by the exact same amount across all adjustments.  One “stop” of light is the same amount regardless if you’re adjusting the Aperture, Shutter Speed or ISO.  In fact, increasing exposure by one stop is exactly twice as much light.  Decreasing is exactly half as much.  Additionally it gave all cameras and lenses a level playing ground.  f8 on one camera allows exactly the same amount of light in as f8 on another camera.

Given the goal of an overall “proper” exposure, there are many combinations that could get us there.  If you increase the Shutter Speed by two stops and decrease the Aperture by two stops; you’re at exactly the same exposure!  I’ll discuss more in the coming posts about why you might make such an adjustment.

A photograph that has been properly exposed will allow the viewer to see details in the darker areas and in the lighter areas.  We try to avoid big dark shadows that look like black blobs on prints while at the same time, try to avoid “pure white” areas where there should be details.  Below is an example of an under exposed, properly exposed, and over exposed photo.  Notice how the highlights on the top image are blown out (specifically the waves).  Similarly, notice how little detail is available in the shadows of the under exposed photo.

Over Exposed

Properly Exposed

Under Exposed

Your camera likely has settings that allow you to take control of one input, while calculating the remaining options for you thus giving you a good exposure.  “S” or “Tv” mode allow you to dictate the shutter speed while  “A” or “Av” mode give you control over the aperture.  Only when in the M or Manual mode does the camera give you full control of both and put the ball entirely in your court.  ISO isn’t typically adjusted via the main dial, but more often has a separate “hard” button on the body.  Some cameras may require you to dig into the menu screen to adjust however.

Over the next few days, get out your camera and figure out where those adjustment dials are.  We’ll go in-depth on controlling each one of them in the weeks to come.   Also, try to identify photos you’ve taken in the past that are underexposed or overexposed.

Please, feel free to leave questions in the comments field.


One response to “Understanding Exposure”

  1. […] discussed in the prior post, we’re lifting up the hood of each of the three variables that can contribute to the exposure […]

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