The Exposure Triangle consists of Shutter Speed, Aperture and ISO Speed. These topics represent the three variables that create the perfect exposure. These three pieces of the Triangle work together, balancing and compensate for each other. If your camera is in an Auto setting, as one changes, the other two react to keep everything in balance. If you are shooting in manual, you will need to consider the effect changing one of these three settings will have on the other two. Sound complicated? Not really, once you understand how they work together.
I have covered each topic of the Exposure Triangle separately:
If you haven’t read these posts you may want to before we continue on, it is important to understand the separate components of the Triangle and the effects each has on the photograph before you can understand how they are completely intertwined.
The Balancing Act
Balancing Exposure is a balancing act between the amount of light entering the lens (Aperture) and the amount of time that light is allowed to hit the camera’s sensor (Shutter speed). You can get a perfectly exposed picture with a large Aperture balanced by a short shutter speed, or a small aperture balanced by a long shutter speed. Recall my analogy of filling the water glass in my Aperture Part 1 Post.
So, if Aperture and Shutter Speed are a balancing act, where does ISO Speed come into play? Remember in my ISO Post, I mentioned that the camera always tries to use the lowest ISO speed for the lighting conditions and the shutter speed/aperture settings. So, if you are shooting in a bright sunny day and you have the camera set on a wide aperture to have a shallow depth of field, the camera will select a fast shutter speed to balance the fact that you are flooding the sensor with a great deal of light. It will also select a low ISO Speed because the lighting conditions will allow that. All is good in the world.
However, what if you want to shoot in a low light situation, and need a relatively fast shutter speed because you want to stop the motion –as in a soccer game your kid is playing at dusk (I remember those days!). You have the camera set on a fast shutter speed to stop the motion; the camera will open the Aperture as wide as it can go to let in as much light as possible trying to balance the exposure. But, what if that is still not enough to get a good exposure? That’s where the camera will crank up the ISO Speed to make the sensor more sensitive to light. It will compensate for the fast shutter speed you are trying to use in a low light situation.
Think of it as on a seesaw, if one person is much heavier than the other, remember how you could adjust the length of one side to compensate? This works exactly the same way.
Conversely, if you are shooting on a bright day, and want to use a long shutter speed to show the blur of motion, and want to have a shallow depth of field -as if you were taking a portrait in front of a waterfall on a bright day and wanted to blur the background (wide Aperture) and soften the movement of water to give it a gossamer effect (slow shutter speed). The camera will dial down the ISO Speed as far as it can go to get the right balance.
Remember, though, while ISO Speed is the great balancer and safety net, there is a trade off: The higher the ISO Speed, the more digital noise, or grain you will have in your picture. Sometimes, it’s worth that to get your shot; sometimes it is not –you have to be the judge. It may be a good idea to use a variety of settings and look at them later on the big screen to choose the one you like best (the small screen of your camera is not good enough to see the minute details).
Back to the Balancing Act
Here is the great thing about Shutter Speed, Aperture and ISO Speed: Each change in setting either doubles, or halves the amount of light hitting the sensor.
Shutter Speed: 1/125 of a second lets in twice as much light as 1/250 of a second
Aperture: f-5.6 lets in twice as much light as f-8.
ISO Speed: ISO 100 is twice as sensitive to light as ISO 200
All Together Now
This means that for the lighting conditions, if you use 1/250 sec, f-8 and the camera selects ISO 200 to get the perfect exposure, then you change the shutter speed to 1/500 sec, you can change the aperture to f-5.6 to let in more light. Or if you keep it at f-8, the camera will adjust the ISO speed to 400 to keep the exposure in balance. If you change the shutter speed to 1/125, and keep it at f-8, the camera will adjust the ISO Speed to ISO 100.
See how that works? It’s a beautiful thing.
How the Settings Work Together
Shutter speed Aperture ISO
1/250 f-8 200
1/500 f-8 400
1/125 f-8 100
1/250 f-5.6 100
1/500 f-5.6 200
All these settings will give you the exact same exposure. You may want the various shutter speed settings to control the amount of motion in the picture; or the various aperture settings to control the depth of field in the picture. The camera adjusts the ISO Speed to keep the exposure in balance.
Take your camera out and experiment. Change the shutter speed and keep the aperture the same and see what it does to the ISO speed. Change the Aperture and keep the shutter speed the same and see what it does to the ISO setting. If you are adventurous, you can manually set the ISO Speed and see what you need to set the Shutter Speed and Aperture on to get a good exposure; then manually change the ISO Speed and see how you need to adjust.
Set it and Forget it!
I always recommend setting the camera’s ISO Speed to Auto and leave it there. It’s much easier and less mind boggling. Especially when you are just learning, it’s difficult enough to understand what the shutter speed and aperture is doing, how to adjust them and what it does to the picture. Let the camera figure out the ISO setting; let it be your great safety net. Just set it and forget it (for now).
Tell me about your experiments, your successes and failures.