Metering Mode pt. 2 - Center-Weighted and Spot Metering

OK, so yeah, I've been gone for a month.  Not out-of-town gone, more the wife-had-surgery-work-went-bananas-at-the-same-time type of gone.  But regardless, I'm back, and we need to continue our photo education, so lets get to it.

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In the last post we talked about matrix metering, which involves some pretty complex decision making on the part of the camera.  Now let's get into center-weighted and spot metering modes, which are much simpler and more predictable.

Center-weighted metering behaves the most like the classic "average luminance" metering we discussed in the first post on metering.  There is one difference however.  As implied by the name, center-weighted metering cares more about the center of the frame than the edges.  When I say the center, I mean a circle or oval taking up about 50-70% of the frame (the actual area being measured depends on your camera).  Center-weighted metering will suggest exposure settings that render this circle / oval area a medium gray, with some minor consideration given to very dark or very bright areas at the periphery of the frame.

There are two great thing about center-weighted metering:

1. It's predictable.
2. It prioritizes the center of the frame (where your subject tends to be located a lot of the time).

Center-weighted metering is good in situations where the center of the frame needs to be properly exposed and the edges don't matter.  For example, a portrait where the sun is setting right behind your subject, or a still-life shot of a leaf, sitting in a pool of light on the otherwise dark forest floor.  Makes sense right?

Now let's move on to spot metering.  Spot metering only measures a tiny portion of the frame (normally 2-5% of the total image area).  This spot is always in the center of the frame, although in some newer professional cameras it is possible to move the spot around.  Spot metering will suggest exposure settings that will make the object inside the spot appear a perfect 50% medium gray.

The two advantages of spot metering are:

1. It's almost perfectly predictable.
2. You can meter very small areas of the frame separately from each other.

Spot metering is good if you need a very precise meter reading for one tiny portion of the image and want to ignore everything else.  Now remember, when you spot meter an object it's going to give you exposure values that will make that object medium gray.  If you're spot metering a light-toned object you'll have to overexpose compared to the meter reading.  If you're spot metering a dark object, you'll need to underexpose it somewhat.  Spot metering also works great with auto-exposure lock, another feature we will come to shortly.

Metering Mode pt. 1 - Matrix Metering

OK, let's talk some more about metering. In the first post on metering, we oversimplified things a little too much. Don't worry, it's not really going to get more complex, but we need to refine the details a little.

We started with the understanding that your in-camera meter uses "average luminance" (i.e. brightness) to determine what exposure settings to use. That meant the camera's meter measured the total amount of light coming in through the lens, and assumed this light was spread perfectly evenly throughout the frame. The meter then suggested exposure settings that would make this average amount of light come out a medium gray tone in the final image.

This IS how light meters started out, however photography has come a long way and we now have more than one "metering mode" that controls how the meter behaves. There are basically three standard metering modes: matrix, center-weighted and spot.

"Matrix metering" is the smart(-ish) metering mode that is standard for almost all modern cameras. I'm going to use "matrix metering" in this post, but this is technically the Nikon term. I believe Canon calls it "evaluative metering" and the generic may be "zone metering", but I learned on Nikons and therefore I'm rolling with "matrix".

Matrix metering divides the frame into several different "zones" and measures the light in each zone separately. It uses this information to try and "guess" what kind of scene you are shooting and make exposure recommendations that would be best for this type of scene. Before this week that's basically all I knew about it, but thanks to Ken Rockwell's website (google him) I learned a bit more.

Let's use our old example of a snowy pasture to show how matrix metering works. Let's say our composition consists mostly of a snowy field, with a couple clumps of dark trees on the left side and the bright sun in the upper-right corner of the frame. Matrix metering is smart enough to know that there is nothing brighter than the sun that it can reasonably expect to run into. So when it measures something as bright as the sun in the frame (i.e. which is always the sun itself) it pays special attention to that. Matrix metering also knows that direct sunlight is very consistent and always produces the same amount of light. So now, matrix metering will pick exposure values that are suitable for subjects lit directly by the sun, and we will get realistic tones in our image that match what we see with our eye. The sun will be pure white, the snow will be a pretty bright white, and the dark trees will be black. Good job matrix metering!

Matrix metering has a lot of other logic included that helps it make good decisions. It is good at measuring the brightest and darkest areas of an image and trying to distribute these tones within the dynamic range of the camera. This means it tried to keep the brightest areas bright, and the darkest areas dark and strike a compromise somewhere in the middle. Here's a good example of it doing that:

See how it picked exposure settings that kept the dark areas from being pure black and the white areas from being pure white? It did a decent job of retaining detail in both these areas, and this looks reasonably like the scene did in real life (I know, a picture of the leg of my couch, how interesting).

That being said, as smart as matrix metering is, it is definitely not perfect and can be fooled. Let's go back to the snowy pasture example and pretend it is an overcast day instead. The meter is not going to see anything bright or dark enough to give it a good idea of what you are shooting. So it goes back to being a normal light meter. Most of the frame is snow, so it's going to make that medium toned (i.e. gray). If there are enough dark trees in the frame, its going to take them in consideration too and maybe pick slightly brighter exposure settings. In this case the snow may be a slightly lighter gray, but still not the white we are looking for. Sorry matrix metering, we know you tried.

Final thought: matrix metering is a good way to go most of the time for decent pictures. Its fairly smart and makes a lot of reasonable decisions. The problem is it still a machine, not a mind reader, and doesn't know your creative intent. Another problem is that it is not very predictable. If you are trying to over or underexpose something, its hard to know what the meter is already doing. Maybe it knows to overexpose the scene already (like the sunny snow day), or maybe it is hopelessly lost and shooting for a compromise (the cloudy snow day). If you are making your own adjustments, you have to guess how the meter already feels about this scene, which can be a mess.

That's the complex world of matrix metering, now let's move on to center-weighted and spot metering, which are much simpler!

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!