Understanding Exposure

Before venturing out of your camera’s Auto Mode, opening a whole new world of photographic possibilities, we need to develop a basic understanding of exposure.  Don’t worry, this is not going to hurt.  There are really only 3 things that you need to follow:

1.  ISO: the measure of a digital camera sensor’s sensitivity to light
2.  Aperture: the size of the opening in the lens when an image is created
3.  Shutter Speed:  the duration of time that the shutter is open

If you’re trying to decide whether or not this is worth your time to learn (come on…it’s only 3 things), skip down to the bottom of this post and read the last 2 paragraphs.  If any of it rings true in your photographic life, come back up here and dig in.  The whole thing is going to take about 5 minutes to turn you into an exposure pro!

I’ve heard a number of metaphors to help explain these exposure elements, but the one that has always made the most sense to me is to think of it as a window in your house.  I like this one best because a window actually has shutters on it.

In this example, your camera is like the window and your eyes are like the camera’s sensor.  The amount of light that comes in the window can be controlled by two things; how wide the shutters are opened (this is your aperture) and how fast they are opened and closed (this is your shutter speed).  You might have to use your imagination a little bit for the shutter speed example, because light travels so quickly.  But, it makes sense that if you open the shutters for 1/2 a second, less light will enter the room than if you opened them for 3 seconds.

If you need more light in the room, you can either open the shutters wider (a larger aperture), or you can leave them open longer (a slower shutter speed).  ISO would be like a pair of sunglasses that you wear inside the room.  They don’t really effect/change the amount of light that comes in the room, but they do change how much of that light your eyes actually see.  Another way to increase the light in the room would be to leave the shutters open the same amount, for the same time duration, but raise your sunglasses off your head (raising the ISO).

Now, with the window metaphor in mind, here’s what actually happens in the camera:


The aperture is in the lens.  The wider it opens, the more light it lets in to the sensor.  It’s opening is determined by what we call f stops.  They work, at least in my mind, backwards.  The larger the f stop number, the less it opens, allowing less light to come in.  It’s very important to remember that last sentence.  So, an f stop of 2.8 lets in twice as much light as 4.

Aperture note: The lower your aperture, the softer/more blurry the background will be behind your subject.  So if you’re focused on your subject, trees/buildings/traffic in the background will be blurry if you shoot at f 2.8 and will be very clear if you shoot at f 11 or f 16.

Shutter Speed

You can think of the shutter speed as how long that aperture stays open.  It’s measured in fractions of a second (250 is 1/250th of a second).  The bigger the number, the less light that gets to the sensor.  Exceptions would be for long exposures like 1 (meaning 1 full second).  An example would be a shutter speed of 250 letting in twice as much light as 500.

Shutter Speed note: Faster shutter speeds will stop/freeze action while slower shutter speeds will cause action to blur. Most normal body movement will be clearly captured at a shutter speed of 250 or above.  Sports, like soccer, or a quickly moving toddler, might need 500 to 1000.  1000 or above will clearly capture almost everything moving.  Shutter speeds below 60 will require a still subject, a very steady hand and probably a tripod.


The ISO determines how sensitive your sensor is to the light let in by the shutter speed and aperture settings.  The higher the ISO, the more light that your sensor receives.  So an ISO of 200 lets in twice as much light as 100.

ISO note: ISO 100 is considered normal.  The higher your ISO goes, the more grain/noise/fuzzy edges you may notice.  Some cameras handle this better than others.  For the most part, anything under ISO 800 will provide great results, with some cameras doing well at 3200 and above.

What’s it all mean?

Light is what a camera always wants more of.  The 3 ways you can give it more light are (print this out and carry it in your camera bag if needed):

1. A bigger aperture, meaning a lower f stop number
2. A slower shutter speed (keeping in mind that you need to stay above 60)
3. A higher ISO.

In Auto Mode, your camera handles all this for you, but sometimes at the cost of capturing a good image.  An example would be a shot of your children in the shade or at the end of the day when the amount of available light is a little low.  We don’t want to turn on our flash, because flash ruins everything (more on this in another topic)!  So, in Auto Mode, our camera decides to drop the shutter speed down to 60 or 120 to have enough light to get the shot.  The problem is that your children are playing (meaning moving) and it’s that interaction that you want to capture.  You know that, but your camera is so dumb when it’s in Auto Mode, that it doesn’t understand what you want.  By lowering the shutter speed to let in more light, your camera gives you a picture of 2 blurry objects that sort of resemble your kids.

The purpose of understanding exposure is to learn different ways to give our camera the light it begs us for, while maintaining the control we need to get the shot we’re begging for.  We’re going to take this amazing new knowledge and use it get what we want…better pictures!  We’re saying NO to Auto Mode and YES to Aperture Priority Mode and YES to Shutter Priority Mode!  Depending on what your photographing, one of these modes will produce a WAY better result than Auto, and I’m going to show you how simple they are to use.


51 Responses to Understanding Exposure

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