What is Exposure?

Imagine yourself getting a sun-tan.  In the morning when the sun power is weak, it will probably take a couple hours before you get a sunburn.  At noon, it will probably take less than half an hour for you to get the same burn (ok, tan) because the sun is much more powerful.  Now imagine sun power as aperture, and the time it takes to burn yourself as shutter speed.   Get it?  Then… a third variable is skin sensitivity.  People who are usually under the sun (like a surfer) are probably less sensitive to sun power; therefore, it will take more time to get a burn.  So skin sensitivity is similar to ISO.  A surfer would be ISO 100; while Ginny is probably ISO 800 (more sensitive).

No a little more technical.  How does the camera know how much exposure is needed?  Some how, some genious researches concluded that 18% gray (imagine mixing 82% white paint and 18% black paint) is the optimal exposure for all pictures.  So the camera sensor will try to calculate the correct aperture, shutter speed, and ISO based on average 18% gray brightness.  This is also called “metering”.  It does a very good job most of the time.  It’s when you go outside of 18% that you start to get into interesting pictures.  Try it.

Camera Scene Modes

Typically, point-and-shoot cameras have a few “scene” modes which allow users to take better pictures in special occasions.  Today, we will try to figure out what the camera does in each of these scenes.

Portrait – in portrait mode, camera will try to make the background as blurry as possible (so viewer will pay attention to the person).  To make background blurry, it will open up the aperture (hole) as much as possible to decrease depth of field (DOF).  Typically, the camera will use the maximum aperture value of the camera.  Here is an example of a picture with blurry background equivalent to portrait mode (f/1.8).

Landscape – in landscape mode, the camera will do the exact opposite of portrait mode:  it will try to make everything as focused as possible.  To do this, it will close up the aperture.  Typically, it will use the smallest aperture (biggest F number) while keeping shutter speed above 1/30 (cause less than 1/30 will capture blurry picture from hand shake, remember?).  Here is a sample picture with equivalent landscape mode where aperture is closed up (f/8 ) to make everything focused.

Sports – in sports mode, the camera needs to “freeze” something that is moving fast.  To do this, it needs to make shutter speed as fast as possible (for example 1/125 of a second).  Therefore, it will try to set the shutter speed to a fast value while keeping aperture to a reasonable setting (such as f/8 ) for easier focus.  Here is an example where the motorcycle is “frozen” (f/22, 1/200).

Night Landscape – for example, city lights only (no foreground person).  Camera will increase ISO sensitivity to highest value (for example, 800), then balance both aperture and shutter speed.  Typically, you will need a tripod.

Night Portrait – for example, portrait in a dimly lit restaurant.  The goal is to use flash to light the person, yet capture the background in it’s original lighting.  To do this, camera will keep shutter speed about 1/30 of a second, open up the aperture (small F number), and fire the flash to lit the person. This is an example with shutter speed of 1/60 and flash on.

See, it’s really not all that complicated once you understand how to expose the sensor. Science rocks!

What to do when it’s dark?

Ginny asked me what to do with her camera when the environment is dark.  Point-and-shoot cameras nowadays are very smart; so most likely, it will turn on the flash and try to light up the subject.  This works just fine for an indoor dinner.  But, it will not work well for outdoor night scene cause the background will not be lit by the tiny flash.

First of all, a little camera basics.  The camera takes a picture by opening a hole, and let the light “expose” the sensor (or film).  There are 3 things that affect a picture’s exposure:

Aperture –  Usually called the “A” or “F” number in the lens.  This determines how big the “hole” opens.  The bigger the hole, the more light hits the sensor for the same amount of time.    The smaller the “F” number, the bigger the “hole”.

Here is a lens set to 2 different apertures.

F/16

F/1.8

Aperture affects background sharpness.  The bigger the hole, the more blurry background becomes.  This is known as Depth Of Field (DOF).  We will talk about it some other day.

Shutter speed – This is the amount of time the “hole” is opened.  For example, 1/250 means the sensor is exposed for 1/250 seconds.  The more time it is open, the more light hits the sensor.

ISO Sensitivity – the 3rd variable is how sensitive the sensor is.  The higher the ISO, the more sensitive the sensor (film) is.  For example, when sensor is programmed to ISO 400, it will take 1/4 less time to get the same exposure as ISO 100.  i.e., at ISO 400, it will take 1/4 of one second, while it will need 1 full second to get the same exposure at ISO 100.  There is a catch, higher ISO usually increase noise; so your picture will start to look grainy, or even spotty.  Yuk!

So… what to do when it’s dark?  The easiest thing to do is increase ISO.  If that’s not good enough, then open up your aperture (say from F/8 to F2.8).  If that does not work still, then increase exposure time (slow down shutter speed).  Say from 1/60 to 1/15.  There is a catch on shutter speed.  Usually the hand movement (shake) will become noticeable from 1/30.  So if you are shooting 1/15 or slower, then you will need a tri-pod.

Case Study A – portrait in a fancy dimly-lit restaurant.  The ambient is not bright, so you are going to increase the ISO high enough to capture some ambient light at 1/30 exposure (why? cause slower than that will cause camera shake, remember?)  Then, turn on the flash so it will light up the person.  Now, you will have a properly lit portrait with some ambient light in the background.

Case Study B – city lights in Monte Carlo.   There are actually two sub-cases.  1) if you want a person in the picture, you can do the same as in case A.  However, if you just want to capture the scenery, then you need to increase ISO even higher.  If you have tripod, use it so you can increase shutter speed to more than 1/15 of a second.  If you don’t, then try opening the aperture to maximum (smallest number).

Assignment – Yes, you’ve got homework.  Take a self-portrait next to a candle.  The room should have no other light except candle.  Here is the recipe – Shutter speed 1/30.  Aperture widest allowed by the camera.  Flash on.  Experiment with different ISO setting until you get a good “moody” picture where only you and the candle are lit.

Tomorrow, we will use camera’s special modes “sports, portrait, night scene, etc” to explain how the camera use these 3 variables to get better pictures,