Creative Exposure Part Two: Exposure Reciprocity

May 13, 2017

Creative Exposure Part Two: Exposure Reciprocity

In our last post, we talked about the exposure triangle. There we saw that exposure is a function of three components:

Creative Exposure Part Two Exposure Reciprocity

Aperture size

The aperture is the opening in your camera's lens. The size of this opening determines the amount of light that gets into your camera. A larger opening lets more light in and a smaller opening lets less light in.

The aperture size is measured in f-stops. The larger the opening in the camera’s lens, the smaller the f-stop number. And vice versa. So an aperture of f/2.8 is larger than one that’s f/16.

Shutter speed

The shutter is a small door or curtain in your camera that opens and closes to allow light to strike the image sensor. The speed with which it opens and closes controls the amount of time the camera sensor is exposed to light. A longer shutter speed lets in more light than a shorter shutter speed.

Shutter speed is stated as a measurement of time. So shutter speed is typically expressed in terms of seconds, and can range anywhere from fractions of a second to whole seconds.

ISO

The ISO is the third point on the exposure triangle. It represents the sensitivity of the camera’s image sensor to light. As the ISO increases, the sensor becomes more sensitive to light. So, as the ISO goes up, less light in required to properly expose an image.

Exposure Reciprocity

These three components work together to give an image proper exposure. They need to balance each other out. That means that if one of these elements changes, one or both of the other two needs to also change to keep the same level of exposure.

Creative Exposure Part Two Exposure Reciprocity

Reciprocity is a term used to describe the relationship between shutter speed and aperture. When you make a change in one of these exposure controls, you need to balance that out by making the opposite change in the other control if you want to keep the same level of exposure. This is sometimes also called the Rule of Equivalent Exposures.

This is because the aperture and shutter speed settings move in opposite directions. As the aperture size gets smaller, the time that the shutter is open has to get longer in order to allow the same amount of light through to the image sensor. The opposite is also true.

In other words, using a wide aperture and a fast shutter speed gives the same exposure as a small aperture with a slow shutter speed. There’s a reciprocal relationship between the two. You can get the same exposure if you change both of them equal amounts in opposite directions.

If this is hard to understand, think about how a faucet fills up a drinking glass. If you open the faucet all the way, a lot of water comes out and the drinking glass will be filled quickly. But if you reduce the amount of water coming out of the faucet by partially closing it off, the drinking glass will take longer to fill.

Stops

Changes in the amount of light are usually referred to in terms of stops.

Stop is the word photographers for either halving or doubling the amount of light.

So, of example, a shutter speed of 1/2 of a second lets in twice as much light is a shutter speed of 1/4 of a second (since the shutter is open twice as long) so we say 1/2 is one stop greater than 1/4.

You can halve or double light using the aperture also. But isn’t as obvious when you are working with the aperture since you can’t easily use the different aperture values to tell if the light going through the lens has halved or doubled.

The chart below can help. It shows the progression of aperture values and shutter speeds, with a stop between each. So for both the shutter speeds and apertures below, the values that sit right next to each other are one stop apart.

Creative Exposure Part Two Exposure Reciprocity

Each smaller aperture and each shorter shutter speed lets in half as much light as the aperture or shutter speed before it.

From the chart, you can see that if you change aperture values from f/4 to f/5.6, you have decreased light by 1 stop. If you change the shutter speed from 1/60 to 1/30, you have increased light by 1 stop. If you change the aperture from f/16 to f/8, you have increased light by 2 stops.

By the way, chances are your digital camera has additional shutter speeds and aperture values between those that show in the chart above. These represent fractional stops. In fact, most digital cameras offer apertures and shutter speed values in 1/3rd-stop increments.

Why Reciprocity is so Important to Your Photography

Reciprocity is important in photography for two very crucial reasons.

First, the reciprocal nature of the exposure controls mean that, for any given lighting situation, there are numerous combinations of shutter speed and aperture that will produce the same level of exposure. So you have some choice in which combination to use.

Secondly, the exposure controls–aperture, shutter speed and ISO–do more than control the level of brightness in your photos. Each of these is also a creative control, with the ability to affect the look of your images in a way that goes beyond the level of exposure.

Specifically:

Aperture affects depth of field

As the aperture size gets larger (and the f-stop number gets smaller) the depth of field becomes more shallow. Depth of field is the amount of the image that is in focus.

Shutter speed controls how motion is captured

A fast shutter speed will freeze any movement occurring within the scene. A slower shutter speed will capture the movement as motion blur in your image.

ISO affects digital noise and grain

As the ISO increases, so does the appearance of digital noise and graininess in your image.

So, let’s put this all together.

Since we know that there are lots of combinations of aperture, shutter speed and ISO that will provide your image with the proper exposure, your choice of which combination to use becomes a creative one. What look do you want for your image? What story do you want to tell? Do you want a shallow depth of field or a large one? Do you want to freeze motion or show it?

An Example

Let’s look at an example of how reciprocity works in real life.

Say that you are setting up a shot and the meter in your camera says that you need to use an aperture value of f/16 with a shutter speed of 1/30th of a second.

Creative Exposure Part Two Exposure Reciprocity

But you don’t want to use those settings. Maybe it’s windy and things are moving around. You don’t want to show that movement in your photo. Instead, you want to use a faster shutter speed to freeze that motion and make sure that everything is sharp. So you want to increase the shutter speed to 1/125th of a second.

This change reduces the light from the shutter by two stops. In order to keep the exposure level the same, you will need to increase the light from the aperture by two stops. You will need to change the aperture size from f/16 to f/8.

Creative Exposure Part Two Exposure Reciprocity

Instead, say that you want to create an image that has a very large depth of focus. You are shooting a landscape and you want the scene to be sharp from front to back. To get that large depth of field, you’ll want to decrease the size of the aperture. So instead of using f/16 with a shutter speed of 1/30th of a second, you want to reduce the size of the aperture opening by using a aperture value of f/32.

This change decreases the light from the aperture by one stop. In order to keep the exposure level the same, you will need to increase the light from the shutter by one stop. That means that you will need to change the shutter speed from 1/30th to 1/8th of a second.

Creative Exposure Part Two Exposure Reciprocity

In each of these, we are keeping the balance and, therefore the same level of exposure, by changing both the aperture and shutter speed by equal amounts in opposite directions.

That’s it for today!

Be sure to check back for our next blog post where we will continue our discussion of creative exposure as we look at how shutter speed affects your images.

Until then…

For Further Reading…

Understanding Reciprocity in Photography

The Rule of Equivalent Exposure (With Quick Quiz)




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