Mastering Modes and Metering (or "The magic stuff your camera does")

Now that you've spent some time with your camera, you've probably
noticed that big dial with a whole bunch of symbols on it. Maybe
you've even read the manual (smart cookie) and used some of the pre
programed modes for something like sports or macro.
All of these program modes can be used to simply make your life
easier, or to give you more control over how your photos look. We'll
look at exactly what those guys do a bit futher down.

Camera Modes

There are three other modes available on many compact cameras and all
dslrs which are very useful. These are Av, Tv and M, or respectively
aperture priority, shutter speed priority and manual modes. The first
two are "semi auto" modes. You can fix one or two elements of
exposure, and the camera will automatically adjust the final element to
achieve an exposure based on the ambient light levels. For example in
aperture priority (Av) mode, you would select an F stop, and optionaly
an ISO, and so the camera would select an appropiate shutter speed for
a correct exposure. In shutter speed priority, you would select a
shutter speed, and the camera would adjust the aperture to an f stop
suitable for the ambient light.

There are a few standard modes that you will see on most cameras, including "auto", macro, landscape, sports and portrait.

Auto mode: This basicly turns the camera into a ol' big point and shoot. It's useful for when someone else needs to take a picture with your camera, and doesn't know the first thing about photography, but offers you hardly any control over how or where the camera exposes. Just press the button and the camera figures out the rest. It's ok to spend some time in this mode when you're learning how to use your DLSR, but mastering some of the other modes it gives you will help give you more control over your photos.

Sports mode: gives preference to a high shutter speed to freeze fast
moving objects. It does this by opening up to a wide aperture and
increasing the ISO to allow a typical shutter speed faster than 1/1000
of a second.

Macro mode: sets the preferred focus point to the closest object to the
lens. Useful for capturing fine details of a scene and small stuff like insects or flowers closeup..

Portrait mode: sets the aperture to a maximum value(decreases the F stop number) , to minimise the depth of field. This turns the background into an abstract blur, bringing attention to the subjects face (the focus point) which is sharp and clear.

Landscape Mode: Stops down the lens (increases the F stop number) to maximize depth of field, so you can capture a scene with the foreground and background in clear focus. Think about a mountain scene with some flowers in the foreground - using this mode, you could keep both parts of the picture clear and sharp.

Metering Modes

Aren't these cameras smart? Well if you have ever taken a picture of a
snowy scene you will appreciate how easily they can be fooled. See, to
figure out the exposure, the camera uses one of a few
different tricks called "metering modes". The simplest metering mode
is an average metering, where all the brightness of all points in the frame are just
averaged and the exposure set to make that average a shade of
"middle" (18%) grey. This works pretty well for a lot of scenes, but
is pretty easily fooled. For example, a person set against a white
wall will have a much lighter average value than the 18% grey. So what
will the camera do? Try to underexpose the picture so the white is
much closer to that 'middle grey' value. Of course, we really want an
exposure based on the subjects face, and the wall to be much brighter
than middle grey. So what are we going to do?

The easiest solution in this situation is to use another metering mode
called " spot metering". This metering mode bases the exposure on a
very narrow feild, a roughly circular 'spot', in the center of the
frame. Using this metering, the exposure can be set to the subjects
face, letting the wall be exposed to white. Voila.

Another very common metering mode is "Centre weighted", which is basically a hybrid of spot metering and average metering. The brightness in the middle of the frame is considered 3 or 4 times more important than the parts around the edges of the frame when determing the exposure. Again, in most situations this will ensure the subject has a good exposure, but will also try to not horribly over or under expose the other parts of the image. Either this or spot metering is a good option when your subject is going to have a very different exposure than its surroundings.

Perhaps the most commonly used metering mode is "Matrix Metering". No, it's not related to the enslavement of the human race. The camera measures the exposure at several points in the scene, then does it's little magic tricks to figure out the best exposure based on those points. Thats a pretty vague explaination, but it can vary from camera to camera in exactly how it determines that exposure, and can do so somewhat unpredicataby. However, it will tend to produce the most accurate exposures under most situations if you're not sure where to base your exposure around. Think of it as the "Auto" mode of the metering systems, you don't really think about exactly how it figures it out for you, but it will get it right most of the time.

One other thing worth knowing about the automatic metering modes is the EV or "exposure compensation" function. Sometimes, you'll notice the camera consistantly under or over exposes some types of scenes, even in matrix metering. The classic example is a family shot against a snowwy background - the camera sees all that bright white snow, and tries to dull it down. Problem is, now you've gone and underexposed your subjects.

EV allows you to tell the camera "you need to expose this scene so it's one stop brighter than you would normaly" by dialing in 1 stop of EV. A -1 EV would darken the cameras automatic exposure one stop. On low end DSLRs adjusting EV is often quicker and more intuitive for some people than changing metering modes, so do consider it as an alternative when adjusting exposure.

There you have it, you now know how to control your cameras automatic expsoure functions, and pick the best modes for your subject matter. Pretty neat, no? Now get out and take some pictures, but don't forget to keep on licking.

Elements of Exposure: Digital camera basics

Exposure is one of those photographic skills that is fairly simple to learn, but hard to master. You will have most likely heard people talk about exposure in the past, probably in realation to a picture being under or over exposed. Exposure is essentially the amount of light recorded by the sensor, which will essentially determine how light or dark your image is. Too high an exposure, and you will be left with a blown out, blanket white picture. Too low and you will be left with an empty black image. Modern cameras can determine a proper exposure for a picture automatically, with a nifty little device inside them called a light meter.

But firstly, how do we control exposure? Every camera has 3 elements which are used to determine exposure.

1. ISO (or how sensitive the sensor is to light)

2. Aperture (or how much light is let through the lens)

3. Shutter speed (how long the sensor is exposed to light)

Each of these will have an effect on both the exposure, and the "look" of the picture.

Lets start with...

ISO

ISO is a measure of the sensitivity of a photographic film. ISO itself stands for "international standards organisation", which created a guideline for film sensitivities worldwide. Despite being grounded in film technologies, digital cameras have an adjustable ISO. A higher ISO will make the sensor more sensitive to light, almost akin to a volume control on a stereo. Using this example, turning up the ISO in a low light situation is like turning up the volume for a very quiet song.

However, this has a side effect. A higher ISO introduces more digital noise, or "grain" into a picture. Click on the image to the left to see how grain a high ISO has made it.


High ISO also reduces dynamic range (how dark or light the shadows or highlights can be while maintaining detail). A high dynamic range is particularly useful for salvaging shots with computer software if the exposure is wrong, often called "exposure lattitude".

Sometimes shooting with a high ISO is unavoidable, but generaly I like to shoot with the lowest ISO I can for the clearest shots with the most dynamic range.

Aperture

You can think of aperture as "how big is the hole in the lens", or how much light can get through the lens. It's measured in F stops, where a lower F -stop is equivalent to a bigger hole, and a brighter exposure.

So, for example, a picture taken at F8 will be brighter than one taken at F11 if the other settings are the same.

Aperture is very important for one other reason - it controls the depth of field of the picture, or how much of the picture is sharp. A higher F stop will have a larger part of the picture in clear focus.
See how most of the image is blurred, and only a small part around her mouth and nose is in sharp focus? That's because this photo was taken at F2, a rather low F stop. This gives some cool creative control over pictures, because now we can direct our viewers eye to a small part of the scene, and abstract everything else in background blur. Or we can increase the F stop, and get a larger part of the scene in clearer focus.

Shutter speed

Finally, we can control exposure by controlling the length of time the sensor is exposed to light. The longer we leave the shutter open, the more light is allowed to fall onto the sensor.


Shutter speed can vary from hours to a 10000th of a second, but typicaly are between 1/2 a second and 1/4000th of a second. A photo is never taken in an instant, only ever over the time the shutter is open. If you're shooting at a 4000th of a second, then you're recording everything that happens in that 4000th of a second. This is pretty neat, because not a lot changes in that type of timeframe. So it's possible to 'freeze' fast moving objects with detail we cannot normaly see, like water droplets splashing.
On the other hand, a longer shutter speed allows thing to move during the exposure. A common use of this is to keep the camera steady on a tripod, and record moving objects as a blur.



In this photo, the water is trickling while it's surroundings are stationary. With a half second exposure and the camera resting on a rock, the sensor records the stream as a soft blur while it's surroundings (that are in focus) are tack sharp.

One other thing to note about shutter speed is camera shake. Hand holding a camera with a long shutter speed will introduce blur because of the camera moving in your hands. A good rule of thumb is to keep the shutter speed shorter than 1/(the focal length of your lens). So if you were using a 200mm lens, then anything faster than 1/200s should produce motion blur free photographs. At 1/100 s, you would risk introducing extra blur.

STOP!(s)

Ok, now lets put all this into use in a theoretical situation. Suppose using your cameras inbuilt light meter, you know that you will get a good exposure at 1/250 at F8, ISO 200.

However, being the creative genius you are you decide that you want to have a very narrow depth of field to blur out the background, and want to take the picture at F4 instead. No worries, just change that aperture to f4 and take a picture.... what do you know, it's white and washed out. Looks like we over exposed there...

By changing the aperture and keeping all the other settings the same, you've just let more light fall on the sensor and ruined that lovely exposure you had. So what are you going to do? Well this is where understanding "stops" come in.

Note that these are related to the "F stops" that we were measuring the aperture with, but are NOT the same. A stop is simply a relative measure of twice or half the amount of light falling on the sensor.

So if we have our exposure at 1/250 at F8, ISO200 and we were to decrease the shutter speed to 1/500 of a second, then the sensor would be exposed for half as long as before, so we would have decreased the exposure by one "stop". Halve it again to 1/1000th of a second, now we've dropped it 2 stops.
If we increased the ISO to ISO400, then we would have doubled the sensitivity of the sensor, meaning it is now twice as sensitive to light. Hence, we would increase the exposure one stop.

Aperture and F stops

Because aperture is related to the area of a circle (how big the hole is letting the light it) it's numbers work a little differently. Multiplying by the square root of two (around 1.4) gives you an aperture one stop darker.

So some common aperture values,

F1.4 F2 F2.8 F4 F5.6 F8 F11 F16 F22

(each of these is one stop darker than the one preceding it)

Notice that looking at every second number above, the F stop doubles. So, if you double the F stop (for example F8 to F16) then you have actually decreased the exposure 2 stops.


Going back to our original example, if we've gone from F8 to F4 we have actually increased the exposure 2 stops. That explains why that photo was overexposed... So to get an "equivalent exposure" we need to adjust the ISO and shutter speed to cancel out those 2 stops. We can decrease the shutter speed from 1/250th to 1/1000th of a second, which will decrease the exposure 2 stops. The two will cancel each other out, meaning our sensor is receiving exactly as much light as it was with out original exposure.

Understanding exposure can be daunting when just starting out, but its a fundamental tool in photography. Give it a little while and it'll be second nature when you pick up a camera.