Monday, March 15, 2010

How Aperture, Shutter Speed, and ISO All Work Together (Exposure part 4)

This is part six in an ongoing series of things I'm learning in my photo 1 photography class. You can find the first five posts here:

-Photo Basics - Basic Definitions
-Photo Basics - Camera Controls and White Balance
-Aperture and Depth of Field (Exposure part 1)
-Shutter Speeds and Capturing Motion (Exposure part 2)

-ISO (Exposure part 3)

Anyone new to photography or finally venturing into the manual setting on their DSLR knows that getting properly exposed photos is no easy task. Sometimes the pictures come out way too dark (or underexposed) and other times way too light (overexposed). The temptation to just change the dial to "program" can be high, but with some basic understanding of how the camera works, and a lot of practice, perfectly exposed photographs are within reach. While I can’t practice for you, my goal is to give you some of that understanding in this post.

First off, please read my previous posts on aperture, shutter speeds, and ISO. While in a practical sense this post will be more useful, since understanding how these things work in conjunction is something you’ll use every time you take a photograph, having a basic understanding of how each of these three things works on its own is a must. (Otherwise how can you understand how they work together?)

I will also be repeating myself a lot in this post, but I can not stress enough how important exposure is to photography. In fact, I would say exposure is the number one most important technical aspect to any photograph. No one is gonna be too interested in your one of a kind picture of a spotted skunk or crazy tsunami if your picture is blown out and over exposed.

Exposure is controlled by three things: aperture, shutter speed, and ISO.

Remember, the amount/intensity of light allowed to enter your camera will determine how exposed your picture is. If too much light enters the camera then you have an overexposed picture that will be very white. If not enough light enters the camera then you will end up with an underexposed picture that will be really dark/black. The aperture controls the intensity of light. Your shutter speed controls the time interval it does so and ISO determines how sensitive your film/sensor is to the light.

Overexposed = too much light.
Underexposed = not enough light.

One thing that aperture, shutter speed, and ISO all have in common is that moving to the next full stop either halves or doubles the light. Even if the numbers aren’t actually doubled, the light still is. An example is the aperture values f/8 and f/5.6. Even though the number 8 is not the double of 5.6, changing your aperture setting from f/5.6 to f/8 still doubles the light. Same as how moving from f/8 to f/5.6 halves the light.

Got it?

When you double/halve the light in one place, you’ll need to also double/halve the light some place else in order to compensate for the extra/less light. This is how aperture, shutter speed, and ISO all work together.

My point is that pictures taken at 1/60, f/8, ISO 400 and 1/125, f8, ISO 800 will have the same exposure.

Let’s use this picture as an example:


Besides cropping, this is straight-out-of-camera (SOOC) and was shot at 1/250, f/4, ISO 100. As you can see, the bottoms of Holden’s shoes are out of focus. This is because they are closer to my camera and therefore not within my depth of focus. The way to fix this would be to close down my aperture to f/5.6 (the next full stop). Now here is where the give and take comes in. Since I just halved the amount of light by closing down my aperture, I will need to compensate for that loss of light by either using a slower shutter speed (which will let more light in) or using a higher ISO (which will make my camera more sensitive to light and therefore need less light for exposure). Since I can’t make my shutter speed any slower without causing blur, my only option is to change my ISO to 200. Now I didn’t actually have to do this since I liked the picture the way it was, but my point is that the exposures of 1/250, f/4, ISO 100 and 1/250, f/5.6, ISO 200 are the same.

Did that make sense?

Just think about aperture, shutter speed, and ISO as three bowls full of stones on one side of a scale with exposure on the other side. In order to have good exposure both sides of the scale must remain even. You can move the stones around the three bowls as much as you want, but if you remove a stone from one bowl you have to add it to another to keep things even. So remove light from your camera’s shutter (by using a faster shutter speed) and you’ll throw off your exposure unless to add that light someplace else (by using a higher ISO or opening up your aperture). Make sense?

Here are some SOOC photographs illustrating my scale/bowl metaphor. What I did was start with a base picture and then doubled or halved the light without compensating for the difference. You can easily see how quickly everything gets overexposed/underexposed:


Shutter speed:


You can do the same thing yourself by first taking a photograph in “program” mode to get a base photo to start with (I'd recommend setting your camera down or using a tripod so all the shots are the same). Then jump over to manual and see how quickly your exposure gets thrown out of whack by only changing one of the three settings.

This, in a nutshell, is how aperture, shutter speed, and ISO all work together. Knowing how these three elements work together will allow you to have at least a basic idea of what to do when you quickly turn on your camera, snap a picture, and end up with something like this:

If you still aren't sure where to start, I'd recommend shooting your photos in "aperture priority" mode. This allows you to select your ISO and aperture while the camera handles the shutter speed. It doesn't give you the control like the manual setting does, but it is a great place to start.

And that's it for exposure (finally). We've moved onto film in my photography class so I think my next post will be after we start on photoshop.

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