The Basics of Exposure – Aperture, Shutter Speed, ISO

The Basics of Exposure

Have your camera handy so you can follow along and look at the settings on your camera as we talk about each of them. 

Exposure, exposure is getting the perfect amount of light onto your digital sensor or film. We want to have as good exposure as possible so that we have detail in both shadow and bright areas; there is contrast (The range from dark to light) and good color. We’ll talk about the 3 things that control exposure and then how to use each of those to make every photograph you take artistically interesting and also how to balance the 3 for a perfect exposure.

 Here is a well exposed photograph

 Let’s break down the three things that affect exposure: Aperture. Shutter Speed and ISO

 Aperture 

Aperture is the adjustable opening in your lens that allows light to pass through to the digital sensor or film. It goes from very small to almost as large as the lens glass itself. To express the size of the opening, aperture is enumerated in “f stops” With the largest opening being the smallest number and the smallest opening being the largest number. Yes that seems backwards but not so much so when you know that it is actually a fraction or ratio just like ½ is larger than ¼. 

A typical range of apertures would look like this:

f/1.4, f/2.0, f/2.8, f/ 4.0 f /5.6, f/8.0, f/ 11, f/16, f/22 from largest to smallest.

 Here is a graphic representation that may help to make it clearer

 
Your camera may have more numbers in between but the above are known as whole stops. A whole stop represents a halving or doubling of light. f/2.8 lets in twice as much light as f/4. Most cameras change f stop in 1/3 stop intervals

Shutter Speed 

Shutter speed is how long the shutter and a given aperture is open to allow light to hit the digital sensor or film. The longer your shutter is open the more light will hit the sensor. Shutter speed is enumerated in seconds or parts there of. 1, ½, ¼ 1/15, 1/30, 1/60, 1/125, 1/250, 1/500^th. But on your new digital camera the 1/ of the fraction is dropped so 1/125^th is expressed as 125. Each doubling or halving of speed, indicates a doubling or halving of light

 ISO

ISO is the sensitivity to light of the digital sensor or film. The higher the number, the more sensitive the sensor will be to light. ISO is expressed as 100, 200, 400, 800, 1600, 3200 etc, with each doubling in number representing a doubling in sensitivity. 400 is twice as sensitive as 200.

Okay, that’s nice technical mumbo jumbo now how do I use all of that?

Excellent question. Now that we know what the different pieces are, how can we use them to make better photographs and not just snapshots. 

Artistic use of Aperture

So what does aperture control in the creative and artistic process? Aperture controls Depth of Field or DOF as it is known. What is Depth of Field? Depth of Field is the amount of distamce in our image that is in focus or the field of focus. A shallow Depth of Field would have the subject in focus but everything in front of it and behind it out of focus, isolating the subject. A deep depth of field would be in focus all the way from the foreground out to infinity.

 Different types of photography use different DOF. Typically portrait photography will use a shallow depth of field to isolate the subject/person and keep the focus on them rather then a distracting background. Also a still-life may use a shallow DOF for the same reason. Landscape will often use a very deep DOF  from a foreground subject all the way out to the horizon at infinity because in a landscape image everything is important and needs to be recognized and in good  focus. 

Now HOW do we use aperture to control DOF? The larger (smaller f number) the aperture, the shallower the DOF. The Smaller the aperture (bigger f number) the deeper the DOF.

So if we were shooting Wildflowers and we wanted to isolate them from the background we would use a large aperture such as f2.8 or f4.

If we instead were shooting a meadow of flowers extending out to beautiful mountains in the background we would use a small aperture such as f11 or f16.

Artistic use of Shutter Speed

Shutter speed is used to either stop motion or to show motion. 

If we want to stop motion, such as an Athlete in their sport or a race car going by or even stop the motion of a bird flying, we would use a High or Fast shutter speed, usually 1/500^th or faster. This will stop the motion of our subject and can “Freeze objects” such as a baseball just before the batter hits it with his bat.

 But there are also times that we want to show motion; Dancers dancing, to show their movement, or, to silken the water in a waterfall to show the movement of the water. To do this we would use a Low or Slow shutter speed, 1/30^th all the way down to as many as 10 seconds long. Now one word of caution here, if you are going to use a slow shutter speed, you need to mount your camera on a steady tripod. Or you will not only see the motion of the object but the movement of the camera in your hand. This will lead to un-sharp and blurry pictures. So while we want to see movement we only want to see it in our subject.

 While we are on the subject, there is a range of shutter speeds that are appropriate for hand holding without a tripod. Otherwise, you will not have sharp clear images. Now this depends how steady your hands are but as a rule of thumb, most people are able to hand hold a camera at 1/ over the focal length of their lens. So if you are shooting with a 200mm lens, your minimum shutter speed should be 1/200^th of a second, anything slower than that and you should be on a tripod or at least a monopod. 

Artistic use of ISO 

Now if we remember from the earlier in this chapter, ISO is the sensitivity to light of either our Digital Sensor or Film. Now you may ask, if we can make the sensor more sensitive why not always use a higher ISO. Well the problem with that is the side effect of higher ISOs is noise. This will look like grain or tiny blocks in your image. So we want to use the lowest ISO possible for the light conditions we have. 

On a bright sunny day outdoors there is always enough light that we can set our cameras on the lowest ISO, usually 100 or 200. As we move into some highly shaded areas we may need to move up to ISO 400, especially if we have the need for a higher shutter speed or can’t use a tripod. As we move indoors, depending on the lighting we now need to move up to ISO 800 and higher to capture our images again when it is not possible to use a tripod or if we need to stop action. 

Shooting scenes at night on the street would also call for using a higher ISO. But say we can use a tripod and don’t have to worry about stopping motion of an object. In that case it would be better to use a tripod and a slow shutter speed along with a low ISO. You will still capture the image and have much less noise to deal with in your image 

This image was shot with just the street lights and was hand held

The Triangle of Exposure

As you read more and more about exposure you will often hear  the three parts that we just discussed as the “Triangle of Exposure” This is a very good way to think about it because the final thing that we need to know about exposure and ; Aperture, Shutter Speed and ISO, is that they all have an effect on the other. If you have the same light in your scene but make changes to one part of the “Triangle” you will have to make a equal but opposite adjustment to at least one of the other parts of the Triangle.

Say you are taking a Photo of a person and you have a good exposure and your settings are Aperture f/8, Shutter speed, 1/200 and ISO 100. But you decide you want to use a “Larger” aperture of f/5.6 to have a shallower DOF and isolate that person. You will have to make an adjustment to the shutter speed in the opposite direction to keep that perfect exposure you started with. So your new setting would be f/5.6, 1/1400 and ISO 100. 

Now this all may seem confusing still to you. When we discuss Shooting in Manual Mode in an upcoming blog post it hopefully becomes more clear when we show you how to use your camara’s meter