Understanding Exposure One Pixel at a Time

(This is lesson 3 of my how it’s done series.)

You’ve made it to lesson 3. The reward is we’re going to actually talk about cameras in this lesson. This is the first lesson of three parts where we will be discussing exposure. Exposure is the heart of taking pictures. Once you understand exposure you can take control of the photographic process. Understanding exposure is the first step in becoming a creative photographer.

 

So what exactly is exposure? Exposure is how much light you allow into your camera when you take a photo. The two controls a camera has over exposure are aperture and shutter speed. That’s it – only two controls! Sounds pretty simple right? It is simple once you understand how aperture and shutter speed work together to create the correct exposure. “But wait!” you might be saying to yourself right now, “why does my camera have ten separate exposure modes if there are only two controls over exposure?” That’s an excellent question, and we will answer later. For now, I’m going to break down exposure to its basics. After you understand how aperture and shutter speed relate to each other we’ll delve into all those exposure modes on your camera and also talk about light metering and how to get the correct exposure. But, we have to start at the start. Here we go.

 

We’ll start at the very smallest level – one pixel. Your image sensor is made up of millions of light sensitive pixels. This extremely sensitive array of pixels lives most of its life in total blackness inside your camera. When you press your camera’s shutter release the shutter opens for a set amount of time and exposes the image sensor to light. Each pixel records how much light it was exposed to. Your camera collects the information from all these millions of pixels, processes it, and produces an image that you can recognize. It’s pretty amazing, but there is a catch. Your camera’s pixels are only sensitive to a a certain range of light. If too little light exposes a pixel it records only black. If too much light exposes a pixel it records only white. Between the two extremes of total black and total white is where your camera can record detail. A correct exposure exposes your camera’s sensor to just enough light to stay within the range from black to white. (Are you starting hear echoes of lesson two’s discussion of white and black points?)

 

I like to use a cup of water as an analogy to explain exposure. Imagine each pixel in your camera as a measuring cup, and imagine light as water filling that measuring cup. To make the analogy complete we have to make one slight change — imagine the first tick on the measuring cup is slightly higher than the bottom of the cup. We aren’t going to count the first few drops of water that fill the cup. We start our measuring slightly higher than zero.

 

OK, let’s look at this pixel/cup of water analogy starting at black. Black is what your camera records when not enough light fills the pixel to get above the lowest tick mark on the measuring cup. The first photons (particles of light) that fall on the bottom of the cup are too few and far between to be considered meaningful and the camera processor says, “hey, I’m not counting you. You are just some random photons and not good information.”  Below is an illustration of this idea.  The yellow circles represent light particles, and there is a scale on the right from black to white that shows what your camera sees as the cup fills up.

Now, we’ll fill the cup a little more. Between black and white is the area where the amount of light can be recorded. In this example the cup is about half full. This pixel would record as medium gray.

In this next example the cup is full to the highest measuring line. This pixel would record as pure white. When the cup is full the image processor says, “That’s white!”

What if we over fill the cup? The camera will record the pixel as white. Once you put more water in the cup than it can measure everything beyond measures as white only.

Let’s take a look at a sample black and white photo and discuss how the this theory relates to an actual picture. Why black and white? Because for now color is too complicated. In reality all the pixels in your camera have either a red, green, or blue color filter covering them. Each pixel is measuring the lightness or darkness of one color, and from that information your camera’s processor builds a color image. For the purpose of this discussion it is easier to imagine your image sensor is recording only shades of gray.

This is a photo of an elk skull I found in Yellowstone National park. The elk was most likely a victim of hungry wolves. There are piles of bones everywhere in the back country of Yellowstone. It looks like an animal battlefield. You can see that the elk’s antlers are cleanly sawed off. That’s likely the work of a park ranger trying to prevent someone from making a high value souvenir out of the skull. Anyhow, I placed a bunch of sample points in this photo and it is up to you to guess how full the pixel cup is at each sample point.

 

Starting at sample point one – this point is nearly white. The pixel cups in sample point one would be almost completely full.

 

Sample point two is in an area that is gray, but not very dark gray. The pixel cups in sample point two would be about three quarters full.

 

Sample point three is pretty dark gray, but not completely black. The pixel cups in sample point three would be about one quarter full.

 

The area in sample point four is in a dark shadow. It is one of the darkest spots in the photo. The pixel cups in sample point four would be empty, or nearly so. Sample point four is black.

 

By now you should be getting the hang of how pixels sense light. The correct exposure is really a Goldilocks scenario. When the light is too little the pixels record black. If the light is too much the pixels record white. If the amount of light exposing the pixels is just right the image sensor records detail that creates a photograph. But, I still haven’t explained how you as a photographer control the amount of light entering your camera. That brings us back to shutter speed and aperture.

 

We’ll talk about shutter speed first. Shutter speed is simply the amount of time your camera’s shutter is open. A shutter speed of 1/250 means your shutter will be open for 1/250 of a second. A shutter speed of 1/800 means your shutter will be open for 1/800 of a second. The slower the shutter speed the more light is let into your camera to expose the sensor. Therefor a shutter speed of ¼ lets a lot more light into your camera than a shutter speed of 1/500. Lots of cameras don’t display shutter speed as a fraction. They only display the divider and that can be somewhat confusing. My camera displays the shutter speed 1/250^th as 250 and 1/800^th as 800. A new photographer might look at 800 and think that 800 is bigger than 250 therefor it means more light exposing the sensor. That is backwards. Remember you are dealing with fractions and your camera may be only displaying the divider. Refer to your camera’s manual to find out how your camera displays shutter speed.

 

A helpful way to think of shutter speed is to refer back to the measuring cup example. Imagine that you are filling the measuring cup from your kitchen faucet. The shutter shutter speed is the amount of time you leave the tap open. The longer the tap is open the more water goes into the cup. The longer the shutter is open the more light gets light into your camera. Simple enough, right.

 

But, not all faucets are created equal. Some faucets have a big wide mouth and others are narrow. Which brings us to the other control we photographers have over exposure: aperture. Aperture is the diameter of the smallest opening in your lens. The aperture is constructed of movable metal blades that open and close depending on the orders given by your camera. A narrow aperture restricts light while a wide open aperture lets more light into the camera. Going back to the measuring cup and faucet analogy, think of a wide aperture as a really big faucet (like a fire hose) and a small aperture as squirt gun sized faucet. Which one would fill the measuring cup faster? The fire hose would of course. How long do you think it would take for a fire hose to fill a small measuring cup? I’d guess it would take about 1/250 of a second. What about a squirt gun? Maybe if you had a super soaker you could fill the measuring cup in about 30 seconds. Do you see where I am going with this? A small aperture is going to require a slower shutter speed than a wide aperture.

 

Aperture is expressed in F-stops. This is a concept that will be covered in a later lesson so I am not going to go too deep into it yet. For now all you need to know is that the smaller the F-stop number, the wider the aperture. An aperture value of f2.8 is wider than f5.6. F11.0 is wider than f16.0. This is another set of counterintuitive numbers that seems to throw off a lot of new photographers. A small number F-stop lets in more light than a large number. Why do camera manufacturers make things so confusing? It’s a long answer. Here is the short version: the F-stop is a ratio between the width of the aperture to the focal length of the lens. A wider aperture means a smaller ratio – hence the smaller number for a wider aperture. Don’t ask me to go any deeper than this… You don’t need to know. The answer involves pi, and not the sweet tasty type of pie. I’m talking about pi – the 3.142 variety. I don’t want any of my more intellectually challenged readers to have unpleasant flashbacks to 10^th grade geometry class.

 

We are almost done with this incredibly long lesson. Congratulations to those of you with the attention span to stick it out this far. The entire internet is just a mouse click away and here you are sticking by my side. I am so proud of you. Here is a question for my star students: What part of the measuring cup and faucet analogy have we left out? We talked about how long you leave the tap open (shutter speed) how big the faucet is (aperture) but, we left out one important element. Is there a plumber in the audience? Can you think up another factor that will affect how fast the cup fills with water? The answer is water pressure. If you have no water pressure it doesn’t matter how big your faucet is because you won’t be able to fill your measuring cup without water. Likewise, if you have a very small faucet, but have incredibly strong water pressure (like a pressure washer) you would be able to fill your measuring cup in a fraction of a second. Now how does water pressure relate to photography in this analogy? Water pressure is the same as your light source. A weak light source (low water pressure) like a candle in a cathedral means you will need a longer shutter speed, or bigger aperture, or both. A bright light source (high water pressure) like high noon in July in death valley gives you the opportunity to use a faster shutter speed, or smaller aperture.

 

Does that all make sense to you? Good. This lesson is over. Now that you understand exposure we’ll be able to talk about more about how to set the right shutter speed and aperture in upcoming lessons.

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