Exposure Triangle and Exposure Values
Many people starting out in photography get confused by the relationship between aperture, shutter speed and ISO. Commonly known as the exposure triangle.
I think that one reason why photographers struggle with the exposure triangle, is that, nobody talk about EV, Exposure Value(s).
The exposure triangle refers to the relationship between aperture, shutter speed and ISO when metering, and photographing a subject or scene.
EV is determined by your camera’s meter or by a hand-held meter and is based on the overall brightness of a scene or subject, it is expressed as a number. Although, you never get to see this number with most modern digital cameras.
To help you understand more about the exposure triangle, here is an image of a Sekonic L-208 TWINMATE Analog Light Meter, you can see that at ISO 100 and an EV of 5.5, the light meter shows the combination of aperture and shutter speeds that can be used to obtain a correct exposure. This is important to understand and explains how your camera’s light meter, ISO and combination of aperture / shutter speed work together to create produce an exposure.
Bright scenes have higher numbers 15-20 or more, dark scenes have low numbers which might even be as low as -6 to 0.
So we can have arrange of exposure values from -6 or less, through 20 or more.
EV’s -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1 5 16 17 18 19 20
When you meter a scene, your camera meter assigns the scene an exposure value, and depending the shooting mode set on your camera, you or the camera will do the following:
Because there is a correlation between aperture and shutter speed for any given EV, there are number of aperture / shutter speed combinations you can use when determining your exposure.
(If you refer back to the image of the Sekonic light meter above, you can see the combination of aperture and shutter speeds that can be used @ ISO 100 and an EV value of 5.5.)
In the days of analogue film, if you were shooting in low light, you would buy a film that had a higher ISO rating, making it more sensitive to the light and enabling you to ensure that you could expose the scene properly.
In principle, with digital cameras, ISO functions in much the same way. However, the main benefit of being able to adjust ISO is that it can help you
- Keep the noise in your images as low as practical.
- Use the appropriate aperture / shutter speed for your subject in various lighting conditions by raising or lowering your ISO.
- Avoid under or over exposed (usually the former) images when the scene is too dark or too bright for your chosen aperture / shutter speed combination.
Back to EV,s and the Exposure Triangle
Understanding the concept of Exposure Values (EVs) when metering a scene, helps you make sense of the exposure triangle and how to use each element of the exposure triangle to your advantage when using your camera.