A Quick Word About the Zone System

This is an afterthought to the last post on dynamic range, but let's take a quick look at the Zone System. This is something Ansel Adams and his buddy Fred Archer put together in the 1940's, originally designed for black and white film.

Basically they created their own 10-stop scale of tones from pure white to pure black (11 tones in all). This scale was supposed to represent the full range of black and white film and the paper it was ultimately printed on. Just to make things confusing, they labeled these zone "0" (zero) through "X" (roman numeral 10).

The idea was, the meter will always give you a reading for the middle gray tone "V" (5). Want to render whatever you just metered as zone "VII" (7, a light tone)? Overexpose by 2 stops. Want something to be zone "II" (2, a very dark tone), underexpose by 3 stops. Got it?

So why aren't we using the Zone System to talk about exposure? Well, first of all this scale represents the theoretical dynamic range of black and white sheet film, but all I care about the actual dynamic range of the digital camera I am using.

Second, I hate the numbering system. Roman numerals suck and are irritating to use. Also this doesn't match up to anything you see in modern cameras. Do I ever see a "VIII" or "IX" on my in-camera meter? No, I see "-2, -1, 0, +1, +2". Zero is metered value, not "V". Negative numbers are underexposure (darker), positive numbers are overexposure (brighter). That's pretty intuitive, right?

So that's why I more or less skipped the Zone System. This may make old school photography teachers want to cry, but I'm trying not to waste your life here. If you're shooting black and white sheet film and hand developing and printing it, learn the Zone System in depth, there are tons of books on it. If you're the other 99.99% of the population you can safely ignore it.

Tonality & Dynamic Range

In the last post, we discussed how your in-camera meter uses the average "luminance" (a.k.a. brightness) of a scene to suggest correct exposure settings. The problem we ran into was that the meter wants to treat everything like its medium gray. Its operating on the assumption that the light and dark areas in your scene fall neatly on either side of this "average luminance" and that this will be a good predictor for how you want your image exposed. However, not all scenes are average! The meter struggles with subjects that are supposed to be very light (snow) or very dark (a black cat). This is where we begin our discussion of tone.

Let's start by saying that your digital camera records two different types of information about light: luminance (amount of light) and color (you know what color is!). The camera takes the amount of light it receives (luminance) and renders this into different tones, that is, light or dark areas on the final image. Pure white, light gray, medium gray, dark gray, pure black, these are all examples of tones. We are ignoring color entirely here, for this post it doesn't matter if the medium gray was a medium red, green or blue, we're just talking about how light or dark this area is.

Now let's get down to brass tacks. If you are photographing something dominated by light tones (say a snowy pasture), you now know that the meter will suggest exposure values to make the scene medium toned (i.e. gray, dingy looking snow). Since we want bright white snow, we also know we have to overexpose this image compared to the meter's recommendation, but the question is, how much? 1 stop? 2? How do you know?

This is where dynamic range comes into the picture. Look at the picture below.

I took this series of test shots with my Nikon D200 for this post, to show the range of tones actually recorded by my camera. Slightly to right of center, you see the swatch of gray that says "0.0, 55%, metered value"? This is a piece of white typing paper shot at exactly the exposure settings recommended by the meter. It ended up being 55% gray (one would expect 50% gray, but hey, this is a real world test). This is a medium tone, which is what we should expect now that we know how the meter works.

In all of the swatches to the right, I overexposed according to the meter, first by one stop (+1.0), then two (+2.0), then three (+3.0). One stop overexposed brought me to 75% gray (a light tone), two stops produced 95% gray (almost white), and three stops overexposure gave me a featureless pure white.

Going to the left, one stop underexposure gave me a 35% gray, a fairly dark tone. Two stops underexposure created a 20% gray (even darker), three stops under was a 12% gray (pretty damn dark), four stops under equalled 5% gray (almost black) and five stops under exposed took me to 0% gray (pure black). By the way, I should probably be saying "0% lightness" instead of "0% gray", but you get the idea.

Did you notice something? It took only three stops OVER exposure to get to pure white, but five stops of UNDER exposure to get to pure black. Interesting huh? This range, from pure white to pure black, is called the "full range" of this camera, which happens to be approximately 9 full stops. On the other hand, the range from almost pure white to almost pure black (equivalent to 5% and 95% on the illustration above) is called the "dynamic range", which is approximately 7 full stops.

The important thing I learned from the above experiment was how much brighter or darker my camera makes an image with a one, two, or three stop adjustment from the metered value! We also learned that this is NOT linear, which is very useful. Intuitively you'd expect there to be a certain percent change in brightness for each full stop, say 20% brighter per stop. Go ahead and throw that idea out the window. Exposure isn't linear, its supposed to be logarithmic, but even that doesn't perfectly correspond to how a real camera works in a real situation. The important thing to know is how YOUR camera responds to changes in exposure, so you can create the kind of images YOU want!

To wrap things up, let's go back to our snowy pasture example. More specifically, let's say we are photographing one lonely sapling sticking up out of the snow. Almost the entire frame is filled with white snow, the thin black stick of the sapling is hardly contributing to the meter reading at all. So how much should I overexpose this? It depends. If I want a white, gleaming snow with no detail probably 2 stops, maybe even 1/3 or 2/3 stops higher than that. What if I want to really capture the texture in the snow itself but still render it a light tone? I'd probably go 1 stop over, or 1 and 1/3.

Ultimately I'd make some educated guesses, look at the photos on the back of the camera and at home on my monitor, and try to remember how I would have done it differently the next time I'm out shooting! The point is, it takes practice to learn how your camera renders different tones, and also how your meter evaluates different scenes. This post should give you a good starting point of what to expect so you don't have to feel overwhelmed. In the end your knowledge will always be fine tuned by experience, so go out and start experimenting!

Metering

In the previous post, I talked about how your camera will make change your exposure settings for you in certain exposure modes. So how does it do that? How does it know how much light is present in the scene you are photographing?

The answer is: by using a light meter. A light meter is a device that (like its name implies) measures light. There are two basic types of light meters, "incident" and "reflective".

We'll start with incident meters because we won't spend much time on them. Incident meters are handheld devices with a white globe on top (example picture below). If you've ever watched part of a fashion shoot on TV, you may have seen one of the photographer's assistants sticking this in front of the model's face repeatedly as they adjust the lights. This is because incident meters measure the light that falls on them directly, so they have to be placed where the subject is to get a useful reading. This works well for portraits, product shots, and anything where the subject is close at hand. Incident meters are not practical for landscape shots, sports, or anything where the subject is far away or moving.

To use an incident meter, you set the desired ISO on the meter, place it in front of the subject (white globe facing the camera), and press the button to make it measure the incident light. The meter then gives you a f/stop and shutter speed setting that should result in a correctly exposed photograph. The meter will also have a wheel where you can change either the aperture or shutter speed setting it suggested. When you do this, it will automatically adjust the setting you didn't choose in the opposite direction so the exposure will still be correct. This is useful when you need to use a certain aperture or shutter speed for creative reasons.

Now on to reflective metering. Reflective light meters measure the light that is being reflected off objects in the scene and enters the camera. While there are handheld reflective light meters, almost all modern cameras have built in reflective light meters. This type of built-in meter is often referred to as TTL metering (through-the-lens), since the meter actually measures the light that is entering the lens of the camera. Learning how to use this in-camera meter is absolutely essential to taking well exposed photographs.

So how does your camera's meter work?

Well, in its simplest form, the meter uses "average luminance". This just means it measures the total amount of light coming in the lens, and assumes that this light is evenly distributed over the entire frame. It then determines the exposure settings that would make this average light level come out a medium gray (more or less half-way in between pure black and pure white). The camera designers know that the the world isn't medium gray, but they are shooting for the middle ground to get you a decent exposure.

So how do you use your in-camera meter? If you are in Manual exposure mode, you should see a bar like this:

Ok, this is the entire control panel from the top of my D200, but see the bar in the middle, with the plus on the left, zero in the middle and minus on the right? That's what we're talking about. This should be visible somewhere on one of your camera's control panels, in the manual exposure mode if not others.

What this bar tells you is how over or under-exposed the meter thinks your image will be at your current exposure settings. In the picture above, the bigger hash marks stand for whole stops, the smaller marks for 1/3 stops. The "plus" side of the meter is overexposure (too bright), the "minus" side is underexposure (too dark). So at 125th of a second, f/5.6 and ISO 800 (ISO not shown here), the meter thinks whatever my camera was pointed at (the dining room wall?) would be 1 and 2/3 stops underexposed. Ok here comes the important part so pay attention:

The meter thinks you want everything (on average) to be medium gray. But what if your subject is very dark or very bright and you want to keep it that way? If you take a landscape of a bright snow scene, you want the snow to be white instead of dingy gray, right? If you take a picture of your black cat on your black leather couch, you don't want a gray cat on a gray couch either.

So how do you deal with that? If you know you want your scene to appear light-toned (bright), pick exposure settings that the meter thinks are over-exposed (towards the plus sign). If you want to make your scene dark, select exposure settings that the meter thinks are under-exposed (towards the minus sign).

THAT'S IT! This is something a lot of photographers never really get a handle on, so learn it now! There's some practice involved to get this figured out, so go experiment!

Exposure Mode

So far in this blog we've established that only three camera settings control how bright or dark a photograph is: aperture, shutter speed, and sensitivity. Absolutely every time you take a picture, all three of these exposure settings are set to a certain value. So the question is: if you haven't been setting your exposure settings, who has?

The short answer is your camera. A better answer is, it depends on your "exposure mode".

Exposure mode is the setting that determines how your exposure settings are controlled. Most cameras that have adjustable exposure settings also have several exposure modes to choose between, depending on the situation you are shooting in. We'll cover the big four in this post: Manual, Aperture Priority, Shutter Priority, and Program.

So far in this blog we have been assuming you are using "manual" exposure mode. In manual exposure mode you must set your aperture, shutter speed and ISO yourself using the switches, dials or menus on your camera. If you don't change it, it doesn't change. The big advantage of manual exposure mode is you have total control over your exposure. The bad news is every time the light changes, you'll need to adjust your exposure settings. Sun goes behind a cloud? Change. Sun comes back out? Change. Want to photograph something in the shade instead? Change. And changing settings takes time. Sometimes you may miss the moment fiddling with the camera, or forget to change your settings and get a poorly exposed image. That being said, manual exposure mode is very powerful because you are calling all the shots and have total creative control. A lot of professionals and artists only shoot in manual mode for this reason. Manual exposure mode is available on all professional cameras and on many higher quality "point-and-shoot" (consumer) cameras.  It is normally designated with a capital "M".

Now let's talk about Aperture Priority. With this exposure mode, you are only responsible for one exposure setting: aperture. You get to pick the aperture and the camera sets your other exposure settings (to be more accurate, most cameras will only compensate by changing your shutter speed and won't alter your sensitivity [ISO] setting). Aperture priority is very useful if you're concerned about controlling depth of field for an image and don't have enough time (or energy) to deal with manual mode. If you have Aperture Priority exposure mode available on your camera, it will probably be denoted with an "A" or "Av" symbol.

Shutter Priority is just what it sounds like and also the opposite of aperture priority. You pick the shutter speed, the camera adjusts the aperture. Like with aperture priority, most cameras do not adjust your ISO while in shutter priority mode. Shutter priority is great when motion is a big part of the image and you either want to blur or freeze the moving parts of the image, but aren't particularly worried about depth of field. Shutter priority is normally represented with an "S" or "Tv" (for "time value" if you were wondering).

The last exposure mode I'll talk about, Program Mode, perhaps should have been called Sensitivity Priority. In program mode, you set only the ISO and the camera adjusts the aperture and shutter speed as needed. This is a good choice when you need to concentrate on your timing to capture a moment and neither depth of field or motion blur are incredibly important. Program mode is generally represented with a "P". Also it is worth mentioning that some cameras adjust ALL your exposure settings in Program Mode, including your ISO. This is sometimes billed as "intelligent Program Mode" or something similar.

Ok, so what if you have other modes on your camera, and maybe don't even have any of the ones I mentioned above? Isn't "portrait" (perhaps a silhouette symbol) or "macro" (the flower symbol) or "landscape" (maybe a mountain symbol) an exposure mode? Well kinda. These are really "scene modes", similar to an "intelligent Program Mode" where the camera sets all of your exposure settings for you. But these scene modes also tend to control other camera functions such as focusing, metering and tonal rendering (don't worry, we'll cover those concepts later). Is this a bad thing? Not necessarily, but for the purposes of controlling your exposure settings, all of these function like an intelligent Program Mode where all the exposure settings are adjusted for you. If you aren't sure which exposure modes are available on your camera or exactly how they function, be sure to check your camera manual.

Ok, now we know how to control who is making your exposure setting decisions and why we might want to change that depending on the situation we're shooting in. Next we'll talk about how to choose the right exposure settings depending on the amount of light in the scene using a very important concept: "metering".

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.