Plesha’s Pick 2: Understanding Exposure

I remember a time when I was starting out in digital photography where I was trying to understand what photographers meant when they said the word “exposure.”  The today’s digital cameras make things a bit easier for photographers than the film cameras.  They allow us to experiment with photography without adding the substantial cost of developing film. 

Even though there have been many advances in today digital cameras, a camera still is basically a “box with a hole, a shutter mechanism,  and some sort of sensor that allows us to capture an image.” 

You will hear the term “stop” in referring to exposure.  I typically use this term as relative term.  Moving up or down a stop in a particular setting means that the setting will allow 2 times the amount of light or 1/2 amount of light for the exposure respectively.

Some people may call the following three parts of the camera the holy trinity of photography:

• Aperture(“hole”) -  This reflects the size of the hole of the lens that determines the amount of light allowed into the camera and affects the depth of field(“focus”) in the image.   Aperture is measured in F-stops.  The higher the F-stop number the smaller the hole and greater the depth of field.  For example,  F-5.6 aperture setting allows  2 times the amount of light into the camera than the one stop higher F-8.0 aperture setting  and has a lesser depth of field.  By the same token, F-8.0 aperture setting allows 1/2 the amount of  light into the camera than f-5.6 aperture setting but has a greater depth of field.  In essence, opening up the lens one stop of light(lower F-Stop number) allows twice as much light into the camera and decreases the depth of field.  Closing the lens one stop of light(higher F-Stop)  reduces the amount of light into the camera by  one-half and increase the depth of field.
  
• Shutter speed -   This determines how fast the shutter opens and closes to allow the light from the hole(Aperture) to hit the sensor that creates the image.  This setting can affect whether you freeze the action in an image  to allow some or all of the image to be blurry.  Doubling the shutter speed reduces the amount of light hitting the sensor by 1/2 the amount of light.  Slowing the shutter speed by one half increases the amount of light hitting the sensor  by 2 times.  For example, changing the shutter speed from 1/125 to 1/250 reduces the amount of light to the sensor by 1/2(1 stop).  Changing the shutter speed from 1/125 to 1/60 will allow twice the amount of the light to the sensor(1 stop).
  
• Sensor(ISO) setting  -  This determines how sensitive the sensor is to the light hitting it.  For example,   a  ISO 200 setting is 2 times(1 stop)  as sensitive to the incoming light as a ISO 100 setting.  a  ISO 400 setting is 2 times as sensitive to the incoming light as a ISO 200 setting and  4 times as sensitive(2 stops) as an ISO 100 setting.   Higher ISO settings allow us to work in lower light situation where minimal light is hitting the sensor. This can come with the price of high noise in the digital image.  

Exposure is combination of all the above.  Adjusting one of the settings does affect at least one of the other settings in a typical situation.  The following are just examples to show the relationships of the above three items.  They also assume that the camera’s built light meter is metering the scene correctly.

Fixed ISO example:

Let’s assume for example that we set the camera to the ISO 100(really could be any ISO setting).  Our plan is not to change this setting.  We frame a portrait of something and use  digital camera’s program exposure setting.  The camera’s  built-in light meter will help us determine the proper aperture and shutter speed settings.  In this case, the program mode set camera’s aperture to  F-8.0 with a shutter speed of  1/250.  We take the picture.

The overall image look good and it is not too dark(under exposed) or too light(over exposed).  We decide that we would like the background of the photo to be more blurry(out of the focus).  So we want a lesser depth of field for the image than the camera has suggested for us.  We also want to ensure that we keep the same over all exposure of the image(light hitting the sensor).   After switching the camera to manual exposure mode, we can increase the aperture by one stop by changing the F-stop from F-8.0 to F-5.6.   This change will allow twice as much light into the camera and reduce the depth of field to give us a more blurred background.  

Since we increased the amount of light hitting the sensor with this change, we run the risk of over exposing the image. We need to adjust the shutter speed to offset the increase in light.  So we would then increase the shutter speed from 1/250 to 1/500 which would allow 1/2 the amount of light hitting the sensor.  

So in this example,   the F-8.0/ 1/250 and 5.6/ 1/500 settings allow the same amount of light to the sensor which produces the final exposure.  If we wanted to have more depth of field than the original image, we could have also have chosen to change the F-stop to F-11..o (close down a stop from f-8.0- 1/2 the amount of light) and decreased the shutter speed to 1/125(twice the amount of light from 1/250)  .  The amount of light hitting the sensor would be the same in all three cases. There is a relationship between the shutter speed and the aperture as far the amount of light that hits the sensor.   We could continue to play with various aperture/shutter speed combinations like F16.c/ 1/160 and f-4.0/1/1000 etc.. 

Fixed Aperture example:

Let’s assume we had a similar scenario as above in a lower light situation.  We set the ISO to 100 and the program setting  set the camera’s  aperture to F-5.6 and a shutter speed of 1/30.  We are taking a family portrait in this case and are concerned that any movement of the family members will result in a blurred image.  Looking at the suggested shutter speed, this definitely could be the case. 

We take our first photo. We definitely like the depth of field and the overall exposure of the image but noticed some blurring around the children’s arm were they moved when the photo was taken.  Since we can not keep the subjects from moving, we need to have a faster shutter speed to “freeze” the action.  We like everything else about the image.  

So, let’s change the shutter speed to 1/125 in an attempt to freeze any slight movement in our portrait.  We could  go to manual mode and set the camera to the following: aperture f 5.6 , shutter 1/125, and ISO 100.  The change in shutter speed reduce the amount of light hitting the sensor by two stops( 1/30 to 1/60 – 1 stop and 1/60 to 1/125 additional stop) . 

Since we like the depth of field, we do not want to adjust the aperture in this case.  We could then change the ISO setting from  ISO 100 to ISO 400( ISO 100 to ISO 200 increase 1 stop of light sensitivity and ISO 200 to ISO 400 another stop of light sensitivity)  to offset the reduction in light caused by the increase in the shutter speed.   So  ISO 100/ aperture 5.6 / shutter 1/30  equals ISO 400/ aperture 5.6 / shutter 1/125 as far as the exposure goes.

 

Fixed Shutter speed example:

Assume we are taking pictures of a neighbor basketball game.  We again set the ISO to 400 on the camera. We decide that we will start by using our shutter priority mode of the camera and set the shutter speed to 1/500 to freeze  the action on the court. The camera sets the aperture to F-11.0.  This aperture provides us with a fairly deep depth of field where all the players are coming into focus.  We take the picture and find that we did freeze the action and are happy with the overall exposure of the image.   We would like to focus on a particular player on the court and make the other players less in focus. 

In order to accomplish this, we decide to change the aperture  from F-11.0 to F-5.6 (opening up two stops – 4 times as much light to sensor).  Now. we need to offset this increase in light to the sensor to keep our nice original exposure.  Since we do not want to change the shutter speed that is effectively freezing the action, we decide to adjust the ISO speed to ISO 100. This reduces the sensor sensitivity 2 stops(4 times less sensitive).  So ISO 400/aperture f-11.0/ shutter 1/500 equals  ISO 100/aperture f-5.6/ shutter 1/500  as far as exposure goes.

The main points to remember is that these settings are all interrelated when making an image(exposure).  It is up to us as photographers to creatively choose these settings to produce the desired results we want in our images.

One of the most valuable resources, that assisted me to kick starting my understanding of exposure is the Understanding Exposure book by Bryan Peterson.   I strongly recommend it to those that are trying to take their photography to the next level  beyond the automatic exposure modes of their camera.   He thoroughly explains these settings and also provides a guide  on when to use certain values to achieve the results that you are looking for.