Setting Aperture, Shutter Speed, ISO, and Exposure Modes

This is a big, important lesson in my “How it’s Done Series”.  In this lesson I am going to finally explain the aperture, shutter speed, and ISO relationship.  In the past several lessons we covered aperture to death (here and here).  We also talked about your image sensor and ISO.  In this installment you’ll learn how aperture, shutter speed, and ISO interact to properly expose your camera’s digital sensor.

First,  we need to review some things already covered in past lessons.  If you want, you can reread lesson three on understanding exposure.  If you are too busy to read that lesson over again, here is a quick and dirty review…  Think of your camera’s image sensor as 16 million tiny empty measuring cups (one cup for each pixel).  Now, imagine the light that enters your camera as water.  When you push the shutter button, light pours into the sensor and fills up these tiny measuring cups.  If a cup fills halfway it measures grey.  A full or overflowing cup measures white, and an empty cup measures black.  Your camera can measure 255 different levels of brightness for each of these 16 million cups (pixels).  From this information a photo is constructed.

Your camera’s light metering system is designed to let an exact amount of light into your sensor.  This exact measurement is called an exposure.  If you let in too much light you get a washed out over exposed picture.  Not enough light equates to underexposure.  Lucky for you, modern digital cameras have wonderful light meters and exposure software working in the background that helps your camera achieve the correct exposure in most situations.  However, all this engineering does nothing to help you maintain creative control over your photos.  Nor does it help you set up your camera for maximum performance in challenging conditions.  The world is simply too random for automatic mode.  In order to be a good photographer you have to understand how aperture and shutter speed work together to create an exposure.

Proper exposure is a three way relationship between aperture, shutter speed, and ISO.   Shutter speed and aperture are similar in that they both can be used to restrict light or increase light.  Leaving the shutter open longer or using a wider aperture are two methods of letting more light into the camera.  Using a shorter, faster shutter speed or a smaller aperture will both decrease the amount of light entering the camera.  ISO is the other half of the equation.  Your ISO setting determines how much light is required to make a correct exposure.  At a low ISO setting your image sensor requires more light during exposure than it does at a high ISO setting.

We are kind of getting ahead of ourselves.  I’ll break it own piece by piece and use a couple animations to show you exactly how this three way relationship works.

What you are looking below at is a camera.  The bluish sideways cup is the image sensor.  The aperture and shutter are represented by the opening and closing divider between the lens and the image sensor.  The yellow is supposed to be light.  If you can’t see that, I am a failure as an animator.  Of course everything is slowed way down in this animation.  For now, we are going to ignore ISO.

Discussing the above diagram — Notice I marked the aperture as f/4.0.  That’s a pretty wide aperture for most lenses.  It’s a “fast” aperture.  By fast I mean it lets a lot of light into the sensor.  A wide aperture requires the shutter to be open for a shorter period of time than a small aperture.

A wide aperture is useful when you want to freeze a moving object.  Here is a picture of my two wild dogs playing in the snow.  Without a wide aperture this photo would be a blurry mess.

A wide aperture is also helpful if you don’t have a tripod and you are shooting in low light.  This image was photographed handheld at f/2.0 and 1/200 sec.  The light in this photo was window light.  It was pretty dim.  I had to open the aperture wide and turn up the ISO a little in order to get a workable shutter speed.

A helpful rule of thumb states, “to get a sharp photo while hand holding your camera you must have a shutter speed equal to or greater than the focal length of the lens.”  For example, if you are shooting with  your zoom lens set to 30 mm, you will need at least 1/30th of a second shutter speed.  If you are hand holding your 200mm lens, you should use at least 1/200th of a second.  The longer the lens, the faster the required shutter speed.

Now, take a look at the above animation.  What do you see that’s different from the first animation?  The aperture is smaller.  Also, notice it takes the sensor a lot longer to fill than the first animation.  I am trying to show that with a narrow aperture you’ll get a slower shutter speed than with a wide aperture.  And, just in case you  didn’t pay attention to the last few lessons, let me remind you that f/16 is a smaller aperture than f/4.  Remember,  the bigger the aperture value, the smaller the aperture.  It’s backwards!

Here is one more animation.  What I am trying to illustrate here is the effect that available light has on shutter speed.  In this example I kept the aperture at f/16 just like in the previous example.  Except this time I made the light source brighter.

Notice the shutter doesn’t have to remain open as long because of the brighter light.  With a really bright light source you can get away with a smaller aperture and still retain a shutter speed that is fast enough to hand hold your camera.  For example, a lot of my aerial photos are shot at f/11.  That’s a pretty small aperture considering I am taking photos while flying at mach .77.  But working in my favor is the bright sun at high altitudes.  I can get shutter speeds of 1/300th even with the small aperture because the light source is so bright.

We must now return to the sometimes hard to understand subject of ISO.  As discussed in a previous lesson, ISO is the amount of amplification applied to the signal coming out of your image sensor.  That definition of ISO is kind of hard to grasp and is a difficult concept to work with when trying to make pictures out in the real world.  For that reason, I like to use a a slightly incorrect, but easier to understand, definition of ISO.  Think of ISO as how sensitive your image sensor is to light.  A higher ISO means more sensitivity.   Imagine you are shooting in low light and you don’t have a tripod.  If you want a fast enough shutter speed to hand hold the camera you are going to have to increase ISO.  There is a trade off.  Remember, as you increase ISO you are also increaing the amount of digital noise in your photos.  I always try to keep my ISO at the lowest possible setting for any given situation.  You don’t want to ruin your photo by having too slow of a shutter speed, but you also don’t want to add any unneeded digital noise.

Below is another exposure animation.  This is the way I like to think of ISO.  In terms of our cup filling with water analogy ISO is like using a smaller measuring cup.  Because the cup is smaller you don’t need as much light or aperture to fill the cup rapidly.

High ISO settings are useful in less than ideal lighting situations.  You can use a high ISO setting to help maintain a fast shutter speed.  This is useful if you are hand holding the camera (no tripod) or, if you’re trying to photograph action without any motion blur.

Exposure Modes

I think it is important now to pause and talk about camera shooting modes.  Throughout this series I realize I’ve been saying things like, “with a wider aperture you’ll get a faster shutter speed ”.  Up until now I haven’t really explained how to set aperture and through what magic of technology your shutter speed is changed.  I think we have finally reached the point where I need to break down the basic shooting modes.

Auto Mode:  In auto mode the camera decides both the shutter speed and aperture.  How this decision is made is completely up to the camera manufacturer.  Mostly the camera takes a look at how much light there is, how far away the photographer is focusing, and whether or not flash is being used.  From this information the camera’s computer makes a guess as to what is being photographed.  For example, if the light is dim, and the photographer focuses about five feet away, the camera’s software might guess that the photographer is taking a hand held indoor portrait.  The camera’s number one worry  for a hand held indoor portrait is camera shake.  In this case the camera would pick the widest possible aperture to ensure a decent shutter speed.  That’s great if your camera guessed right and you are indeed shooting a handheld indoor portrait.  However, it’s not so great if your camera guessed  wrong and you actually shooting a dimly lit landscape and your camera is securely mounted on a tripod.  Your camera has no idea what it is shooting.  Camera technology is great these days, but it’s not that great, yet.

Because I have absolutely no idea what decisions my camera will make in auto mode I never use it.  My three year old Canon 5D has only shot in auto mode twice, and that’s when I gave it to my mom to take a portrait of my family with me in it.

Shutter Priority Mode:  In this mode the photographer sets the desired shutter speed and the camera then decides the aperture.  This mode is useful if you are shooting action and you want to maintain a certain shutter speed to stop the action.  It’s also useful if you are using flash and you want your camera to always shoot at its maximum sync speed.

Here is an example of how shutter priority mode works in real life.  Image you set your camera to hold 1/200th of a second shutter speed.  Now imagine you point your camera at a brightly lit subject.  Your camera meter looks at the light, does a quick calculation, and then decides that f/9.0 is the right aperture to properly expose your sensor with a shutter speed of 1/200th.  Now, imagine you point your camera at a dimly lit subject with this same 1/200th second setting.  In this situation the meter calculates the camera needs a wider aperture to support the 1/200th of a second shutter speed.  Depending how dark the scene is your camera might open the aperture to its widest possible setting.  If your camera calculates that you need an aperture wider than your lenses’ maximum aperture, then your camera’s display will flash some sort of warning indicating that you have to select a slower shutter speed to maintain a correct exposure.

Do I ever shoot in shutter priority mode?  Rarely, almost never.  The only time I ever use this mode is when I am using fill flash and I am shooting in dappled, mixed light.  When I am trying to get a picture of my daughter running through a sunlit forest and I am using fill flash is about the only time you’ll ever see me in this mode.  I hate not being in control of the aperture.  Aperture has a huge affect on how the photo will look and I really hate to give up that control unless conditions absolutely demand it.

Aperture Priority Mode:  In aperture priority mode you, the photographer, set the aperture and the camera calculates the correct shutter speed for that aperture.  I might be biased, but I think aperture priority is the best mode to shoot in.  You have control over depth of field in aperture priority mode, and with a tiny bit of practice you can make very quick and intuitive decisions based on aperture mode.  Here is a quick example of my thought process while shooting in aperture mode.

Let’s say I am walking down the street with my camera doing some general street photography.  It’s sunny late afternoon.  The shadows are long but the sunlit areas are still bright.  I have my camera set in aperture priority mode and set to f/8.0.  I am ready for anything.  Out of nowhere a dump truck careens through an intersection and smashes into a Toyota Corolla.  The accident happens in a bright sunny intersection.  With my camera at f/8.0 I begin snapping away.  After a few frames I notice my shutter speed is a whopping 1/600th.  I don’t need that much shutter speed.  This scene isn’t moving anymore.  However, I could use more depth of field so I can keep the smashed Toyota and the dump truck in sharp focus.  I reset the aperture to f/11.  I still have a good shutter speed and now I have the depth of field I was looking for.  As I work my way around the wreckage I notice some blood draining out of the drivers side door of the Toyota.  The pool of blood is in a shaded area.  When I aim my camera at the blood soaked pavement I notice that my meter is giving me 1/20th second shutter speed with the aperture set at f/11.  I really want to get a sharp image of the blood because I think it will add a nice detail to this bit of photo reportage.  I’m going to need a faster shutter speed.  Since I am in aperture priority mode I know that I need a wider aperture to get a faster shutter speed.  I dial the aperture to f/5.6.  Now, I see that the camera has calculated a shutter speed of 1/100th.  Hey, that’s all I need as long as I hold the camera really steady.  Keeping in mind that f/5.6 gives a fairly limited depth of field I recompose the picture and make sure to focus on the most emotionally important part of the scene, the blood flowing out the drivers side door.

Manual Mode:  In manual mode the camera calculates nothing.  The photographer sets the shutter speed and the aperture value.  It’s all up to you in this mode.  It’s a common misconception that pro photographers always shoot in manual mode because they have the ability to out think their camera’s built in meter.  It is also a common misconception that manual mode is more difficult to navigate than third semester calculus.

I use manual mode once in awhile.  Manual is useful in scenes with tricky lighting — when you find your light meter isn’t doing the right thing.  Scenes with lots of contrast, bright highlights and dark shadows, sometimes call for manual mode.  If you are shooting a scene where there is too much dynamic range you have a choice to make.  Do I overexpose the highlights or underexpose the shadows?  Your camera’s metering technology is great at finding an exposure that puts everything in the center of the histogram.  If you purposely want to have your shadows turn to ink black, it’s time to switch to manual.  If you want your highlights to blow out to pure white, it’s time to switch to manual mode.

Unless you carry a light meter, the best way to get started with manual mode is to first use one of the automatic modes to find an in the ballpark exposure.  Set the settings auto mode selected into manual mode, and then tweak your settings as you see fit.  If you want your picture to look darker restrict the light by either increasing shutter speed or dialing in higher aperture setting.  To lighten do the opposite.

Ok, that’s enough for this lengthy lesson.  Stay tuned for more to come.  By now you should be getting a handle on how you can set your camera’s aperture and shutter speed.

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