Exposure Reciprocity

Thank You

During the year, in the rush of events, we tend to overlook the important friendships that are the true basis of business relationships. One of the great pleasures of the New Year is the opportunity to exchange cordial greetings with those whose friendship and good will we value so highly.

In this spirit it is our pleasure to say “Thank You” and extend our sincere appreciation for the very pleasant association we enjoy with you. Please have a very happy and joyous New Year.

Exposure Reciprocity

Let’s start the year out with one of the foundation topics in photography and that is exposure reciprocity. We assume you know the basics of ISO, Shutter Speed and Aperture. Those are the three controls that help you manage exposure. Together, they control the total amount of light that is allowed to hit the sensor.

Reciprocity in photography defines an inverse relationship between the intensity of light and the duration of light. The intensity of light is controlled by the aperture while the duration of light is managed by the shutter speed. 

Once you have set a correct exposure for the scene you are photographing, exposure reciprocity allows you to make adjustments in aperture and shutter speed, all while maintaining the same exposure. For example, let’s say you are photographing a river with a correct exposure of 1/15 of a second at f/2.8. You can photograph the same scene at any aperture on your camera as long as you make an opposite and equal adjustment to the shutter speed. In this example, if you change your aperture to f/22, you’d increase your shutter speed to 4 seconds to compensate for the lower intensity of light due to the aperture being closed down to f/22. 

 While maintaining the exact same exposure, you can photograph a scene from one end of the aperture scale to the other as long as you make an equal adjustment in your shutter speed.

While maintaining the exact same exposure, you can photograph a scene from one end of the aperture scale to the other as long as you make an equal adjustment in your shutter speed.

What about ISO?

 To smooth out the water in the fountain, I used an 8-stop variable neutral density filter. The resulting exposure was 3.2 seconds at f/22.

To smooth out the water in the fountain, I used an 8-stop variable neutral density filter. The resulting exposure was 3.2 seconds at f/22.

Now the overall exposure is determined by three controls, aperture, shutter speed and ISO. Just like aperture and shutter speed, the ISO can also be used to make reciprocal adjustments - and the same rules apply. It’s just that in day to day practice, photographers usually select a working ISO and then don’t change it too much. Certainly not on a shot by shot basis. 

It’s important to note that a control like the aperture does not need to do all the heavy lifting by itself. You can also use filters on your lens to adjust the intensity of the light. One of the most common examples of this is the Polarizing Filter. This filter will reduces the intensity of the light getting into the sensor by about two stops. 

In the case of the water fountain in the photo, I wanted to get a very slow shutter speed but on a bright sunny day I just couldn’t slow the shutter speed down enough without help. For this image, I used an 8-stop variable neutral density filter on the lens. This allowed me to get a shutter speed of 3.2 seconds at f/22 giving me nice smooth water in the fountain.

Finally, the question to answer is why? Why change the exposure? The reason is that each control has creative results. As you vary the aperture, the depth of field changes. When you vary shutter speed, the amount of motion blur changes.