Using Shutter Speed, Aperture, and ISO to Create an Image

Now that we know what Shutter Speed, Aperture, and ISO are (also known as the Exposure Triangle), how do we put them to together and create an image? At the heart of this answer lies the light meter.

The Light Meter

Every camera has a light meter built inside that reads the amount of light that is reflecting back to the camera. When on Auto mode, the camera uses this information to select the camera settings it deems fit to make a correct exposure. However, cameras aren’t very smart, and often don’t choose the right settings when left to think for themselves. So we can use the reading on the light meter to make our own choices regarding the camera settings.

So where do we find the light meter? Depending on your camera, it can be in a few different spots. Here’s where mine are on my camera.

The back screen:

The top screen:

In the viewfinder:

When you push the shutter button halfway down, a little notch will appear on the meter (seen in the first image). If you’re shooting in manual, this meter gives you an estimate of whether your settings will yield a properly exposed image or not. If the notch is to the right of 0, the picture will be overexposed. If it’s to the left, underexposed. This is how you know how much to adjust your settings to get the proper exposure.

A word of caution: light meters that are built into cameras read reflected light. This means that if your subject is in shade, but the background is in full sun, the camera meter will see all of the sun behind your subject and think it needs to account for that, leaving the background properly exposed but your subject dark. In these situations, you have to be smarter than the meter. I always expose for a person’s skin, so if it’s a situation like this, I’ll try to fill the frame with just my subject’s face to get a proper reading, adjust my settings until the meter reads zero, and then back away and reframe my shot. When you back away, the light meter will see all of the light and tell you that you’re picture is going to be overexposed, but remember: you’re smarter than it, and you know that the subject will be perfectly exposed.

Choosing Your Settings

Every time I go to take a picture, I have to decide for myself what the most important setting is. For example, if I’m shooting a portrait of someone, I know I will want a wide aperture, so I’ll set this setting first. Then, I need to decide the second most important setting. Usually this will be ISO, and I’ll pick this depending on how much light is available. Lastly, I’ll adjust the shutter speed until my meter reading is right where I want it, and CLICK. Done.

Let’s look at a different example. Say I’m photographing my dog running around. She’s super quick so I’ll want a fast shutter speed. I’ll also want a relatively small aperture (maybe f/3.5) so I have a deeper depth of field which allows for some focusing error (meaning she won’t move out of the plane of focus as fast). That leaves ISO to fall wherever need be to get a proper exposure?

Priority Modes

Obviously, shooting in manual mode is much harder than shooting in auto. But it yields much better results! A happy medium is shooting in one of the two Priority Modes on your camera: Aperture Priority (A or Av) and Shutter Priority (S, Sv or T, Tv). A quick Google search tells me that these acronyms stand for Aperture Value and Shutter/Time Value. Hence the letters “v” and “t.” :)

By picking a priority mode, you can choose which is more important based on your situation, and let your camera choose the rest of the settings. If you’re currently shooting in Auto, this is a GREAT way to jump in a little deeper without going full manual.

Have any questions? Let me know in the comments!



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