Exposure: Part 1, Shutter speed, aperture and ISO value ~ Brightness

Controlling Exposure

In case you just need the key points, they are:

• You need a brighter image: use a bigger aperture i.e. smaller F-number, longer exposure time, and higher ISO.
• Larger the aperture opening brighter the image.
• Larger the exposure time brighter the image.
• Higher the ISO value brighter the image and more the noise.
  
• The aperture also determines the depth-of-field i.e. over what distance do the things appear to be in-focus. This can be used to blur-out the background for e.g. as in this picture. You can read more about this here.   
• You need to keep your hand stable for the duration of the exposure time else the final picture will be blurry. This blur caused due to camera shake can be used creatively for e.g. as in this picture. Use a tripod. Longer exposure time can be used to create silky water effect, get lovely night pictures etc. 
• At high ISO noise can be an issue. 

Now the illustrated story 

When you press the button to take a picture light falls on the sensors and it is recorded. The brightness (I will also call it the exposure) of the final image is therefore dependent on three things:

1. How big is the opening through which light is coming in i.e. aperture value
2. For how long does light fall on the sensors i.e. shutter speed or exposure time
3. How sensitive are the sensors to light i.e. ISO value

The aperture size also determines the distance over which everything looks sharp and in-focus. This is typically called the depth-of-field. I won't address that part here. You can read about it in one of the previous posts. 

Below I will show you some pictures which will hopefully explain to you the use of the three values (aperture, shutter, ISO) in determining the final exposure or brightness of an image. Lets deal with them one at a time.

How big is the opening through which light is coming in: Aperture

The aperture is the opening through which light passes and then reaches the sensors. Obviously larger the aperture more light comes in, and smaller the aperture less light comes in. You can think of it like a window in your room. If the window is bigger more light can come in. The aperture value is typically mentioned as the F-number.  What you need to remember is this:

• Smaller the F-number <==> Larger is the aperture opening <==> More light comes in
• Larger the F-number <==> Smaller is the aperture opening <==> Less light comes in

Also as I mentioned above, aperture also determines the depth-of-field.

Consider the six pictures shown below in Figure 1. All the six pictures were taken with a Nikon D40 mounted on a tripod. For all the six, the shutter speed or exposure time was 1/10 sec. and the ISO value was 200. However a different aperture value was used for each of them. This makes it easier to see the effect of aperture on the final exposure. The aperture values that were used are mentioned in the bottom right corner of each individual picture.

Figure 1: The effect of aperture on brightness. The aperture values for individual pictures are mentioned in the bottom right corner. The same shutter speed (1/10 sec.) and ISO value (200) were used in all the six cases. 

What you will notice from the six pictures shown in Figure 1 is that as the F-number increases from 4 to 25 the pictures become darker. That is because as the F-number increases, the aperture opening becomes smaller and thus less light makes it to the sensors for the same exposure time.   

How long does light fall on the sensors: shutter speed or exposure time

The light will pass through the aperture and be collected by the sensors for the duration commonly called the shutter speed or the exposure time. This is mentioned in seconds. What you need to remember is:

• Shutter speed or exposure time of for e.g. say 1/10 seconds means that light will be captured for 1/10 seconds.  
• So with a shutter speed of 1/10 seconds you will capture more light than with 1/100 seconds. 
• Also, say for e.g. you use a shutter speed of 1/10 second. Then light will be captured for 1/10 second and the camera needs to be stable for that 1/10 second else the final picture will be blurred. This blur caused due to camera shake during the exposure time is typically undesirable but can also be used creatively for e.g. in this picture. 

Anyway. Consider the six pictures shown below in Figure 2. All the six pictures were taken with a Nikon D40 mounted on a tripod. For all the six, the aperture value was F/5.6 and the ISO value was 200. However a different shutter speed was used for each of them. This makes it easier to see the effect of shutter speed on the final exposure. The shutter speeds that were used are mentioned in the bottom right corner of each individual picture.

Figure 2: The effect of shutter speed on brightness. The shutter speed for individual pictures are mentioned in the bottom right corner. The same aperture (F/5.6) and ISO value (200) were used in all the six cases.

What you will notice from the six pictures shown in Figure 2 is that as the shutter speed goes from 1/4 sec. to 1/250 sec. the pictures become darker. That is because as the exposure time, or the time for which light falls on the sensors, decreases the picture becomes less and less bright for the same sized aperture.

How sensitive are the sensors: ISO value

The light comes in through an opening, the size of which is the aperture value, and falls on the sensors, for duration equal to the shutter speed or exposure time value, and then falls on the sensors. So by altering the sensitivity of the sensors you can control the final brightness. The sensitivity of the sensors is mentioned in terms of the ISO value. What you need to remember is this:

• Lower the ISO value <<==>> Lower is the sensitivity of the sensors <<==>> Less is the noise in the final image
• Higher the ISO value <<==>> Higher is the sensitivity of the sensors <<==>> More is the noise in the final image

I will come to "noise" in a minute. First consider the four pictures shown in Figure 3. All the four pictures were taken with a Nikon D40 mounted on a tripod. For all the four, the aperture value was F/5.6 and the shutter speed was 1/8 sec. However a different ISO value was used for each of them. This makes it easier to see the effect of ISO value on the final exposure. The ISO values that were used are mentioned in the bottom right corner of each individual picture.

 Figure 3: The effect of ISO value on brightness. The ISO values for individual pictures are mentioned in the bottom right corner. The same aperture (F/5.6) and shutter speed  (1/8 sec) were used in all the six cases.

What you will notice by comparing the four pictures is that the one with ISO 200 is darker than the one with ISO 1600. This is because at 1600, the sensors are more sensitive and thus the picture is brighter even though the same shutter speed and aperture value were used. Actually, the sensors don't get magically more sensitive, but lets just leave that issue.   

Typically you would want to use the least ISO value possible but at that value the noise is least. However for e.g. if shooting in the low-light conditions you might want to increase the ISO. Now something about noise.

Consider the four pictures shown in Figure 4 below. Only the ISO values (as mentioned in the bottom right corners) were changed and the shutter speed (1/50 sec) and aperture (F/5.6) were the same for all the four pictures.

 Figure 4: The effect of ISO value on noise. Shutter speed: 1/50, F/5.6

Well, this is not a very good example but you should be able to see the "grainy" noise in the picture with ISO 1600. This picture is shown separately below in Figure 5.

  Figure 5. ISO: 1600, hope that you can see the noise

Happy shooting. 

In summary

• You need a brighter image: use a bigger aperture i.e. smaller F-number, longer exposure time, and higher ISO.
• Larger the aperture opening brighter the image.
• Larger the exposure time brighter the image.
• Higher the ISO value brighter the image and more the noise.
• The aperture also determines the depth-of-field i.e. over what distance do the things appear to be in-focus. This can be used to blur-out the background for e.g. as in this picture. You can read more about this here.   
• You need to keep your hand stable for the duration of the exposure time else the final picture will be blurry. This blur caused due to camera shake can be used creatively for e.g. as in this picture. Use a tripod. Longer exposure time can be used to create silky water effect, get lovely night pictures etc. 
• At high ISO noise can be an issue.