Introduction to the Camera

The camera is made up of three basic parts

A two dimensional light sensitive imaging plane, where the image is formed via projection.
The shutter (which controls the amount of time the light sensitive imaging plane is exposed to light).
The lens (which focuses the light onto the imaging plane).

The Imaging Plane
An imaging plane can be of various sizes, it can be digital or film, both perform the same basic function, and both have a scale to rate how sensitive they are to light, called ASA or ISO.

The Lens
The lens is a collective term for a group of lenses coupled in a housing, and these days with motors and IC's to provide focus, and information about the lens such as focal length, aperture etc.

The individual lenses are commonly known as lens elements, some lenses are known to have over 15 elements, lens elements are generally organised into groups, where focus and zoom controls can manipulate these groups (move them closer or further apart).

All lenses will have a focal length, and a maximum aperture, lenses can either be fixed focal length (prime lens) or variable focal length (zoom lens), you can also get a parafocal lens, which is important for video/motion picture film.

Focal Length
Focal length is measured in distance, usually mm (though for some lenses its measured in cm, especially for Large Format equipment, it means the same thing though, 1 cm = 10mm).

Focal length governs the angle of view (angle of view is measured in degrees), the smaller the focal length, the wider your angle of view is, the larger the focal length, the narrower your angle of view is.

To put this into context, a wide angle of view is good for landscapes and scenic pictures, basically anything wide or with a large view, where as a narrow angle of view is good for wild life as wild life.

Focal length has a huge impact on depth of field, and perspective.

Here is a picture to demonstrate this.

Fixel Focal Length
A fixed focal length lens, or prime lens, is a lens that you cannot change the focal length on, and thus, cannot change the field of view, thus you will need to use a different lens if it isn't suited to your task.

Advantages of prime lenses are that they are generally very sharp (high resolution), meaning they are high quality, have a large maximum aperture, even really old prime lenses are of high optical quality, which sell for very cheap these days and can be adapted to many digital (and film) SLR camera systems, which is another benefit.

The down side is that they are not as convenient as variable focal length lenses.

Variable Focal Length
A variable focal length lens, or zoom lens, is a lens that you can change the focal length on, and thus can change the field of view, this may allow you to use one lens to shoot both landscapes, and portraits, and possibly even wild life.

Advantages of zoom lenses are is that they are convenient and a great everyday lens, a good lens to leave on the camera as they give you the oppurtunity to get once in a life time shots which you would miss by changing lenses (such as aliens landing and abducting someone in a matter of seconds, happened to me once and I only had prime lenses and was trying to change but kept fumbling because of the surrealism of it all, I swear!).

The down side is that they have small maximum apertures (unless you have money for a really expensive zoom lens) which can make shooting hand held in low light and indoor situations highly difficult, image stabilisation helps with this in some circumstances.

Aperture
Inside a lens is a diaphragm or iris (the same as the pupil/iris in your eye), this is commonly known as the aperture, the aperture is a number of blades (usually from 5 to 9, some lenses can have 15 or more, or two apetures) inside the lens that creates a “circle” (not really a circle) in the middle of the lens elements where light can pass through, the size of this circle can be adjusted.

The size of this circle restricts the amount of light hitting the imaging plane, the bigger the circle, the more light, the smaller the circle, the less light.

Aperture affects depth of field and exposure (and well, lens resolution, but thats outside the scope of this particular article).

The maximum aperture, is the widest the iris can be opened, this is usually stated as an f-stop.

F-stop
The f-stop is a way of stating what a lens' aperture is (whether maximum aperture, or what was set to to take a picture), this is stated as f/x (this is actually the aperture ratio, but commonly referred to as just the aperture), where x is the f-number, and f is the focal length.

The lower the f-number, the physically bigger or wider the aperture is, the larger the f-number, the physically smaller or closed down the aperture is.

For example, a lens with a focal length of 50mm and an aperture of f/2, has a physical aperture of 25mm, where as a 100mm lens with an aperture of f/2 has a physical aperture of 50mm, which is 4 times the physical area of light entering the lens, however f/2 will give the same exposure as f/2 on any other focal length, as in our example, the 100mm lens will have a smaller angle of view (approximately half that of the 50mm lens, which would be equivalent to the 4x different in physical aperture area in exposure terms) than the 50mm, and thus a narrower path of light entering the lens in the first place.

This is why we use f/x, as it allows us to easily translate exposure from one lens to the next.

Focus
Focus determines what is in focus.

Simple enough, anything that isn't in focus, is out of focus, anything that is out of focus, is blurred.

Selective focus is a use of depth of field.

Depth of Field
Depth of field, is your range of focus.

For example, an image that is in focus from the front to the back has a wide or large depth of field, an image that has a very small or narrow depth of field will have everything in front and behind the subject blurred.

Focal length has an effect on depth of field, the smaller the focal length, the greater the depth of field, the greater the focal length, the narrower the depth of field.

Likewise, aperture has an affect too, the smaller the aperture (larger f-number), the greater the depth of field, the larger the aperture (small f-number) the narrower the depth of field.

Perspective
We all know what perspective is, objects closer will appear larger to us, objects more distant will appear smaller to us, our eyes have a lens in them, which has a certain perspective to it.

Different focal lengths have different perspectives, a wide angle lens (smaller focal length) will have a greater perspective than a telephoto lens (larger focal length).

What I mean by 'greater perspective' is that, close objects will appear huge, distant objects, miniscule, a telephoto lens will compress this perspective (as opposed to wide angle exaggerating it), which is, the difference in which they appear large (close) and small (distant) is less.

Shutter
The shutter inside a camera (normally, in some larger cameras it can be in the lens), is basically what it says, a shutter, think of vertical blinds, that you can open or close, a shutter is similar to that, it opens and closes, it is either open or closed, the amount of time it is open is generally called shutter speed, in motion picture film, it is called shutter angle.

Shutter Speed
Shutter speed is the amount of time the shutter is open, it is measured in fractions of a second, (30/1 or 300/10 both being fractions, and both equalling 30 seconds, for example).

It can be either very fast, or very slow.

If a shutter is open for 1/100th of a second, the image formed on the imaging plane, records everything that happens in 1/100th of a second, though it may look like everything is frozen in time.

If a shutter is open for 1 second, it records everything that happens in 1 second, if the camera is on a tripod to eliminate any camera shake and a car drives past the camera (into and then out of the frame) in that 1 second, the image will show a sharp, non-blurred road, but a blurred car (in the direction it was travelling) as everything in 1 second is being recorded.

Difference in shutter speed can be expressed in stops, eg. Someone might ask you to use a faster (or slower even) shutter speed of 1 stop.

Shutter Angle
Shutter angle is expressed in degrees, it relates to motion picture film cameras, and some very high end digital cinema cameras.

If a camera is employing shutter angle to control it's shutter speed a few things are happening.

Firstly the shutter is a round disc that spins and not something that opens and closes, part of this disc will be a mirror, part of this disc will be missing (the shutter angle).

As the mirror spins in front of the light path it reflects the image into the viewfinder, as the disc continues to spin around to the section where there is nothing, that is when the image gets recorded onto the film or sensor.

The reason shutter angle is used is because with motion picture cameras that dont have electric view finders (and still today as big budget movies still use film), you still need a view finder for obvious reasons, now this is where the disc comes in, the shutter speed changes with which the speed of the disc rotates, which of course is linked to the frame rate, without getting into shutter phase and the shutter being out of phase etc, I'll give you a simple example.

A movie is being shot, the camera is a film camera, it is shooting 24 frames per second, it is using a shutter angle of 180 degrees, 180 degrees is half a circle, so half is the mirror, and half is nothing, allowing light to exposure the film for half the revolution of the disc, the disc performs one revolution per frame of film, thus it performs 24 revolutions per second, and 1 revolution in 1/24th of a second, since the film is being exposed for half the time (180 degrees), the shutter speed is 1/48th of a second (which is basically 1/50th).

Sensitivity
Sensitivity of the imaging plane is measured in ISO/ASA, without going into the differences between different types of film and their response to under and over exposure, these numbers are interchangeable with digital.

The sensitivity of a digital still camera is generally expressed in ISO, you can change the sensivitiy of a digital still camera (more amplification), with film you can either under or over expose it and have it developed as a different sensitivty film (push and pull developing), or using a different rated film and expose it as normal.

Sensitivty in digital cameras is actually measured in decibels, but expressed (normally) in ISO rating, because a sensor A's nominal sensitivity (0 dB of gain) will have a different ISO equivalent to sensor B @ 0 dB (or 6 dB vs 6 dB etc etc), it's much easier to work out exposure with ISO ratings.

This also explains why even ISO 100 on small digital point and shoot cameras is crap at full view, because ISO 100 on these cameras is already using a large amount of amplification.

Decibels to Stops
6 dB is equivalent to 1 stop, this scales acurately as both are a logarhythmic scale.

So if you're using a video camera and move from 0 dB of gain to 6 dB of gain, this like moving from ISO 100 to 200.

This helps you work out the exposure latitude of digital cameras, as it is often expressed in dynamic range which is measured in decibels, websites reporting dynamic range in stops is technically incorrect, but they are giving you the exposure latitude anyway, and it is easier for photographers to work in and understand stops than decibels.

Exposure
Exposure is measured in stops, exposure governs how “bright” your image is, whether it is under or over exposed either accidentally, or on purpose.

The more exposure, the “brighter” the image will be, the less exposure, the “darker” the image will be.

An image shot at f/4, ISO 100, 1/100th of a second, has the same exposure as an image shot at f/8, ISO 100, 1/25th of a second.
How you ask?

The difference between f/8 and f/4 is 2 stops (8/4), the difference between 1/100th of a second and 1/25th of a second is 2 stops, there is no difference in the sensitivity, thus the 2 stops less in aperture has been balanced by the 2 stops more in shutter speed.

How is the difference between 1/100th of a second and 1/25th of a second 2 stops? 1 stop is twice the difference, 2 stops is four times the difference, it doubles for each stop, the difference between those two shutter speeds is four times the difference, this is the same for sensitivity (ISO 100 to ISO 400 is 2 stops), but not for aperture, aperture is already measured in stops.

I have 1/100th of a second shutter speed, whats 3 stops faster? 1/900th of a second? No! It's not 3 squared, it's 2 to the power of 3, the rule is 2 to the power of stops, which is 8 times, or just count, as the stops double each of that before it (1=2, 2=4, 3=8, 4=16).

Image Stabilisation
There are two types of image stabilisation.

That which is built into a lens, and that which is built into a camera body.

Both achieve the same end result, they can counteract a certain amount of camera shake or vibration which would normally affect the image if not using a fast enough shutter speed.

Camera body stabilisation is great. It allows you to have stabilisation with any lens, thus do not have to fork out a lot of money for a lens with stabilisation built into it, even old adapted film primes.

Lens based stabilisation is great. It allows yous to use image stabilisation on a film SLR (as well as a digital SLR), so you can shoot film, and still benefit from this 'modern' technology.

Lens based stabilisation came before camera body stabilisation, as the technology was around before digital SLRs came along, body stabilisation is digital only.

Which is better? If you intend to shoot film as well as digital, and at a decent proportion (not as in a tiny amount of film) then lens stabilisation is better, otherwise if you want to shoot digital only or barely any film, body stabilisation is definitely the choice (unless of course you want a full frame/35mm digital SLR, of which only Nikon and Canon have to date, and both currently do not have any bodies with body stabilisation).

Most companies claim their stabilisation lets you use 1 to 3 stops slower shutter speed than normal hand-held, this is generally true, unless the shutter speed then is not fast enough to 'freeze' a subject, stabilisation only counteracts camera shake or vibration, thus a wider maximum aperture is generally preferebable.