ISO & Noise

Besides affecting exposure, ISO also determines how much "noise" (digital term) or "grain" (film term) will be present in an image.

Back in one of the early posts on exposure, I mentioned that only aperture and shutter speed actually control how much light makes it into the camera, while sensitivity (a.k.a. ISO or film speed) determines how the camera responds to that amount of light. This is an important fact we'll need to remember as we discuss noise and grain.

Let's start by talking about noise in a digital image. In order to get a decent explanation, we'll need to say a little something about how the image sensor inside your digital camera actually works. Metaphorically speaking, the image sensor is like a bunch of tiny light-catching measuring cups arranged side by side (just image a field full of Pyrex). These metaphorical measuring cups, called "photosites", each measure the amount of light they receive and convert this into one pixel in the final image. If one measuring cup received a lot of light and is mostly "full", this translates into a bright pixel. If another measuring cup received only a little light and is mostly empty, this translates into a dark pixel. Ok, with me so far?

So, these measuring cups ("photosites") have a certain capacity, that is, it takes a specific amount of light to fill them up half-way, or all-the-way, etc. This is like the "native ISO" of the camera, i.e. the default sensitivity of the image sensor. This is normally the lowest ISO setting available on your camera and is also where you will get the highest image quality. The image below was taken at ISO 100, the "native ISO" of my camera. This is only a very small part of a picture of my dining room, enlarged to show the smooth texture and lack of noise provided by a low ISO setting.

So what happens when you increase the ISO? We haven't actually increased the amount of light coming into the camera (only aperture and shutter speed control that) so how does a digital camera cope?

Let's go back to the measuring cup metaphor. If only a small amount of light is coming in the camera, the measuring cups (photosites) are barely getting full at all. At a low ISO setting, the camera would look at these mostly "empty" photosites and render the overall image too dark. When you increase the ISO, the camera tries to amplify the amount of light recorded by each photosite to make the image brighter.

As an analogy, imagine each of your measuring cups is a 2-cup size and they all have a very small amount of water (light) inside, let's say between 1 - 4 teaspoons. If you wanted to increase the amount of water in each cup, but do it equally so they are all still the same in proportion to each other, you might try to double the amount of water in each cup.

The problem is, it's hard to accurately measure small amounts of water in a big cup, that's not really where they are at their best. Is it 1 teaspoon in this measuring cup? 1.5? 2? You could guess the first time, and be pretty accurate. However, this level of error gets multiplied if you are trying to really increase the amount of water, say 4 times, or 16 times. This is the same problem (metaphorically) that your camera runs into at high ISO, and the mistakes it makes appear as noise.

The shot below was taken at ISO 3200. The graininess and colorful speckles are noise.

Ok, sorry this post is so long, but a quick word about film. For film, sensitivity is normally called "film speed" and is an inherent property of the film that cannot be changed in-camera. Also, film works differently than digital and is made up of chemical crystals instead of electrical sensors. When enough light hits the chemical crystals, they begin bursting and create dark areas on the negative, which will translate into bright areas in the final image.

For films with low film speeds, these crystals are small and hard to burst, resulting in a fine-grained film that has high image quality but low sensitivity to light. For films with high film speeds, the crystals are large, easy to pop, but create a coarse grain that degrades the overall quality of the image. Some people find the look of grainy film appealing, and use this affect for artistic purposes (thus the "film grain" filter in Photoshop). I am not one of those people, but I thought I'd mention it just in case you are.

ISO = done! Next up, exposure mode!

Shutter Speed & Motion Blur

Shutter speed plays a huge role in how motion is portrayed in a photograph.

If the shutter speed is high enough, moving objects will appear to be frozen. This is the case in the image below. The shutter speed was approximately 1/320th of a second, almost fast enough to make the water droplet appear totally frozen. As it is, the water drop appears mostly frozen with just a little blur around the edges. Not a perfect example, but you get the idea.

In the next image, I've slowed the shutter down to one full second. The water drops have now blurred together, looking like a trail of dust or a strand of spider web. Had I turned the water up a little higher, it would be more visible, but I want to compare oranges-to-oranges here and only change the shutter speed. The thing to remember here is that a slow shutter speed blurs moving objects.

Like I said above, a slow shutter speed causes moving objects to look blurry. However, this rule applies to anything that moves, including the camera itself! If the camera moves when you're using a slow shutter speed, everything in the image becomes blurred. This is called "camera shake".

In the top image, the fast shutter speed is "freezing" all motion, whether we're talking about the falling water droplet or my unsteady hands. In the image directly above, a tripod is holding the camera still, so the slow shutter speed isn't causing any camera shake. Only the water is moving, therefore that is the only part that appears blurred. In the bottom image, I am hand holding the camera while using a one second long exposure. Blur city. You can see why camera shake is considered a bad thing...

So how fast does your shutter speed need to be to prevent camera shake? The answer is about 1 to 1.5 times your focal length. We'll cover focal length later, but just file that away in the back of your mind for now.

Ok, so what did we learn? Use a fast shutter speed to freeze action and keep your images free of the dreaded camera shake. Use a slow shutter speed (and a tripod) to convey motion, whether you are shooting traffic or waterfalls. Handhold at a low shutter speed if you are forced to in dark conditions, but be sure to hold it steady as a rock if you don't want a unrecognizable blur instead of a photo.

Up next... ISO.

Aperture & Depth of Field

In the last several posts we talked about how to change the brightness or darkness of an image using three different settings: aperture, shutter speed and sensitivity (a.k.a. ISO). We established that you can brighten or darken a photo equally well by changing any of these three settings. Now the question is, how do you decide which setting to change?

If aperture, shutter speed and ISO only affected exposure, then it wouldn't matter which ones you changed. However, all three of these settings significantly affect your images in many other ways beside exposure. Let's start by talking about aperture and its affect on depth of field.

Simply put, "depth of field" refers to how much of a photograph is in focus. Ever seen a portrait where only the person's face is in focus and the background is just a pleasing, out of focus blur? This is "shallow" depth of field. Ever seen a landscape where the blades of grass a couple feet from the camera and the mountains miles in the distance are all equally in focus? That is very "deep" depth of field.

Got the idea? Ok, now let's revisit the definition above, since its a little bit of an oversimplification. Depth of field does not literally mean what percent of the image area is in focus (as in half the image is in focus, the whole image is in focus, etc.). Depth of field really refers to how quickly in-focus areas of an image transition to out-of-focus areas in relation to distance from the camera.

There's a lot on info in that sentence, so let's use an example. Let's say you focus on an object 3 feet from the camera. Let's also say that in the resulting photo, everything that was between 2.5 and 6 feet from the camera was acceptably sharp, or in other words, in focus. That area, from 2.5 - 6 feet is your "depth of field". With me?

Ok, now let's bring in aperture. Aperture is perhaps the most important setting for determining depth of field. A wide aperture (low f-number) such as f/2, f/2.8, or f/4 gives you shallow depth of field. A narrow aperture (high f-number) such as f/11, f/16, or f/22 gives you deep depth of field. Let's look at some examples:

I shot the above photo with an aperture of f/2. I focused on the second pear from the left, and as you can see that is almost the only thing in sharp focus. The background and foreground are totally blurred, and all the other pears are fairly blurry, especially those farthest from the camera.

The next image was shot at f/4, two full stops difference from f/2. Notice that the depth of field has increased. A deeper area in front of and behind the second pear (where I focused) is now acceptably sharp. The foreground is still pretty blurred, as is the background.

Note: Since I changed the aperture two full stops, I also changed the shutter speed two full stops to keep the exposure level the same. I did the same thing for all the test shots in this post so we can see what we're doing here.

This image was shot at f/8, another two full stops difference. Notice how all the pears and some of the foreground are in focus and the background is starting to be recognizable.

The final image was shot at f/16, a change of another two full stops. Now this is deep depth of field. Foreground to background we are pretty much sharp throughout the whole image.

Bottom line, you can control depth-of-field using aperture, and this is an important consideration when changing your exposure settings. Use a low f-stop such as f/2.8 or f/4 for shallow depth of field to isolate a subject and blur a boring background, or use a high f-stop such as f/16 or f/22 to get front to back sharpness if you want to preserve every detail.

Exposure Illustated

We've talked a lot about exposure, now let's look at some pictures.

In the center of this composite is a more-or-less correctly exposed image. Based on the amount of light in our dining room, I ended up with f/2.8 (aperture), 1/30th of a second (shutter speed) and ISO 800 as my exposure settings to give this result.

Now look at the row of dark images on the left side. All three of these images are one full stop darker than the center image. To get the upper left image, all I did is increase the shutter speed to 1/60th of a second. In the middle-left image, I decreased the aperture to f/4 instead. In the bottom-left image I lowered the ISO to 400 instead.

The point is there are 3 equally valid ways to make an image darker:

1. Increase the shutter speed.
2. Decrease the aperture (remember, this means a higher f-number)
3. Decrease the ISO.

Now look at the right row of images above. These are all one full stop brighter than the center image. In the upper right image, I decreased the shutter speed to 1/15th. For the middle-right image I increased the aperture to f/2 instead. For the bottom-right image, I increased the ISO to 1600.

So these are your three options for making an image brighter:

1. Decrease the shutter speed.
2. Increase the aperture (lower f-number)
3. Increase the ISO.

So if there are always three ways to make an image brighter or darker, how do you choose which exposure setting to change? We'll cover that next.

Exposure Part 3 - Sensitivity

Sensitivity is the last setting that controls exposure.

Sensitivity is also called "ISO" or "film speed", which are much more commonly used, but are somewhat less descriptive. There is also the now-antiquated "ASA" just in case you needed more jargon rattling around in your brain.

Just to set the record straight, "ISO" refers to the International Standards Organization. Its not a unit per se, but just an agreed upon convention about how sensitive a photographic medium is (such as film or a digital image sensor). ASA is the old school American Standards Organization, which actually uses the exact same scale as ISO, it's just different letters. So ignore that crap.

As far as film speed goes, when you are dealing with film, its sensitivity is a fixed property engineered into the film. You can't change it, you can just change which film you are using. Since most of us are shooting digital, at least most of the time, it's important to know that you CAN change the sensitivity of a digital sensor with just the press of a button.

So what is sensitivity? Do we have to watch LifeTime movies now? Bravo? No, fortunately we don't.

Sensitivity is how easily a digital sensor (or film) reacts to light. If the sensitivity is high, it takes less light to get a correct exposure. If the sensitivity is low, it takes more light. As mentioned above, sensitivity is most commonly given as an ISO setting. Here's a progression of ISO settings, all one full stop apart.

100, 200, 400, 800, 1600, 3200

Notice something familiar? The number doubles or half when moving from one setting to the next. Just like aperture and shutter speed, sensitivity operates in "stops", but this time we aren't changing the amount of light allowed in the camera, just how the image sensor reacts to it.

Let's go back to the ever-exciting water analogy. If aperture is the width of the hose, and shutter speed is how long its turned on, sensitivity is the size of the bucket we are trying to fill. ISO 100 is a kiddie pool, ISO 1600 is a coffee cup.

Another familiar example of sensitivity is sun burn. Some people are fair-skinned and burn to a crisp in 30 minutes. They are ISO 1600. Other people are dark as night and take all day to get burned. They are ISO 100. Get it?

ISO, just like aperture and shutter speed, has other important affects on your photography, especially regarding image quality and noise. We'll circle around for that shortly when we get into the creative uses of ISO, but first we'll start applying what we've learned about exposure.

Exposure Part 2 - Shutter Speed

Let's move on to our next exposure setting: shutter speed.

Your brain can have a break on this one, because shutter speed is much simpler to understand than aperture. A shutter is just a light-proof curtain that covers and uncovers the film or digital sensor inside your camera for a specific period of time. Normally, the shutter curtain is closed, protecting the image sensor from light until you are ready to take a picture. When you press the shutter button, the shutter uncovers the sensor for a certain amount of time and then immediately recovers it. The amount of time that the sensor is uncovered and exposed to light is called the "shutter speed".

Fortunately, shutter speed uses normal, familiar units: the second. Shutter speeds are generally given in fractions of a second rather than decimals (i.e. "1/8", not ".125"). This doesn't mean all shutter speeds are less than a second long, in fact some exposures can take several minutes or even multiple hours in special situations, but most of the time you will be dealing with fractions of a second.

Like aperture, the difference between shutter speed settings is talked about using "stops". A difference of one full "stop" (whether we are talking about aperture or shutter speed) is either double, or half, the amount of light. With shutter speeds, this corresponds directly to either doubling the shutter speed or halving it.

For example, here's a progression of shutter speeds, all one stop apart:

1/15, 1/30, 1/60, 1/125, 1/250, 1/500

The longer the shutter is open, the more light makes it into the camera. The shorter the shutter speed, the less light admitted.

Let's revisit the garden hose analogy. If I turn on the hose for 15 minutes, I'm going to get a certain amount of water. If I leave it on for twice as long, 30 minutes, I'm going to get twice as much water. This may be too much water, it may be too little, but the point is I can control how much water I get by changing how long I leave the hose turned on. The goal is to get the correct amount of water for the container I'm trying to fill, whether that's an Olympic swimming pool or a Dixie cup.

The same is true for photography. You want to get the "right" amount of light to create a correct exposure. Next we'll talk about the final piece of the puzzle, sensitivity (or ISO, or film speed).

PS - shutter speed, like aperture, affects other things besides exposure, such as sharpness and whether moving objects appeared blurred or frozen. We'll get to that later on when we discuss how to use shutter speed creatively.

Exposure Part 1 - Aperture

In this post, we'll talk more in depth about one of the three settings that controls exposure: aperture.

"Aperture" refers to the size of the opening inside the lens that admits light into the camera. More specifically, the aperture is a hole in a piece of opaque material that "stops" light. This piece of light-stopping material is called the "diaphragm"; the hole itself is the aperture. If you're getting too caught up in the terms, just think of your eye. Your pupil is the hole in your eye that allows light in, and this is just like the aperture in a camera. Your iris is like the diaphragm, it controls the size of your pupil and can get bigger to let more light in, or smaller to admit less light.

To use another example, think of a fire hose compared to a garden hose. The bigger opening in the fire hose allows more water to pass through in a given amount of time. If I turn on a firehouse for 10 seconds, I'm going to get a lot more water than if I turned on a garden hose for that long. Although this isn't a perfect metaphor (you can change the speed of water, while the speed of light is always the same), the basic principle is the same for the aperture in a camera. Big hole, more light. Little hole, less light. Nothing to it.

When we talk about aperture settings in photography, we use a weird unit of measure: "f-stops". When written, an aperture setting appears as the letter "f" followed by a number. Sometimes there is a slash between the "f" and the number, which I tend to use but is totally optional (example: f/11 and f11 are both fine). If you read this out loud, you'd say "f eleven".

The only weird part about aperture settings is how random the numbers seem. Here's a progression of  aperture settings in "full" stops, then I'll explain a bit more below:

f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16

The lower the number, the wider the aperture, that is, the bigger the hole. f/1.4 is a very large aperture, f/16 is a small aperture. So what's the difference between these settings? If I change the aperture setting on my camera from f/1.4 to f/2, this halves the size of the aperture, letting in only half as much light as before. If I go back to f/1.4, I am now letting in twice as much light as I was at f/2. This halving or doubling is a "full stop".

To wrap things up, let's discuss how aperture ties in to exposure. In order to get a proper exposure, we want to let the "correct" amount of light into the camera, depending on how dark or bright our environment is. One way to control this is to adjust the aperture. Outdoors in direct sunlight, I may select a very small aperture such as f/16. Inside my house I would probably use a larger aperture like f/1.4.

At this point in the discussion, there doesn't seem to be much reason to choose one aperture over another. However, aperture affects more than exposure. Later in the blog we'll talk about how aperture affects sharpness and depth-of-field (i.e. how much of an image is in focus at one time). Also, your choice of aperture will affect the other exposure settings you choose when you set your shutter speed and ISO.

For now, here's the big things to remember:

1. Big, wide aperture is a large hole that admits more light. A small, narrow aperture is a tiny hole that only admits a small amount of light.
2. Low f-number = big hole. f/1.4 is a very wide aperture. Conversely, a high f-number means a small hole. f/16 is a tiny pinprick.
3. Aperture will affect the appearance of your images and impact other camera settings in important ways which we will talk about in-depth later on. Selecting an aperture setting can be either a creative decision or a practical one, and this will be a constant consideration as you begin exploring photography.