Camera Basics (System and processes)

Camera Basics

Exposure Controls

When you take a picture, you "expose" a film or sensor to light. The two parts which work together to control your exposure are the APERTURE and SHUTTER. Some "Point and Shoot" cameras select these automatically, but more expensive digital cameras enable you to set these manually, or to "program" them for certain shooting conditions.

Shutter Speed

In still cameras, the term shutter speed represents the time that the shutter remains open when taking a photograph. Along with the aperture of the lens (also called f-number), it determines the amount of light that reaches the film or sensor. Conventionally, the exposure is measured in units of exposure value (EV), sometimes called stops, representing a halving or doubling of the exposure.

Multiple combinations of shutter speed and aperture can give the same exposure: halving the shutter speed doubles the exposure (1 EV more), while doubling the aperture (halving the number) increases the exposure by a factor of 4 (2 EV). For this reason, standard apertures differ by √2, or about 1.4. Thus an exposure with a shutter speed of 1/250 s and f/8 is the same as with 1/500 s and f/5.6, or 1/125 s and f/11.

In addition to its effect on exposure, the shutter speed changes the way movement appears in the picture. Very short shutter speeds can be used to freeze fast-moving subjects, for example at sporting events. Very long shutter speeds are used to intentionally blur a moving subject for artistic effect. Short exposure times are sometimes called "fast", and long exposure times "slow".

Adjustment to the aperture controls the depth of field, the distance range over which objects are acceptably sharp; such adjustments need to be compensated by changes in the shutter speed.

Standard Shutter speeds:

• 1/1000 s
• 1/500 s
• 1/250 s
• 1/125 s
• 1/60 s
• 1/30 s
• 1/15 s
• 1/8 s
• 1/4 s
• 1/2 s
• 1 s

Aperture

In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are, which is of great importance for the appearance at the image plane. If an aperture is narrow, then highly collimated rays are admitted, resulting in a sharp focus at the image plane. If an aperture is wide, then uncollimated rays are admitted, resulting in a sharp focus only for rays with a certain focal length. This means that a wide aperture results in an image that is sharp around what the lens is focusing on and blurred otherwise. The aperture also determines how many of the incoming rays are actually admitted and thus how much light reaches the image plane (the narrower the aperture, the darker the image for a given exposure time).

An optical system typically has many openings, or structures that limit the ray bundles (ray bundles are also known as pencilsof light). These structures may be the edge of a lens or mirror, or a ring or other fixture that holds an optical element in place, or may be a special element such as a diaphragm placed in the optical path to limit the light admitted by the system. In general, these structures are called stops, and the aperture stop is the stop that determines the ray cone angle, or equivalently the brightness, at an image point.

In some contexts, especially in photography and astronomy, aperture refers to the diameter of the aperture stop rather than the physical stop or the opening itself. For example, in a telescope the aperture stop is typically the edges of the objective lens or mirror (or of the mount that holds it). One then speaks of a telescope as having, for example, a 100 centimeter aperture. Note that the aperture stop is not necessarily the smallest stop in the system. Magnification and demagnification by lenses and other elements can cause a relatively large stop to be the aperture stop for the system.

Sometimes stops and diaphragms are called apertures, even when they are not the aperture stop of the system.

The word aperture is also used in other contexts to indicate a system which blocks off light outside a certain region. In astronomy for example, a photometric aperture around a star usually corresponds to a circular window around the image of a star within which the light intensity is summed.

The aperture stop of a photographic lens can be adjusted to control the amount of light reaching the film or image sensor. In combination with variation of shutter speed, the aperture size will regulate the film's or image sensor's degree of exposure to light. Typically, a fast shutter speed will require a larger aperture to ensure sufficient light exposure, and a slow shutter speed will require a smaller aperture to avoid excessive exposure.

A device called a diaphragm usually serves as the aperture stop, and controls the aperture. The diaphragm functions much like the pupil of the eye – it controls the effective diameter of the lens opening. Reducing the aperture size increases the depth of field, which describes the extent to which subject matter lying closer than or farther from the actual plane of focus appears to be in focus. In general, the smaller the aperture (the larger the number), the greater the distance from the plane of focus the subject matter may be while still appearing in focus.

The lens aperture is usually specified as an f-number, the ratio of focal length to effective aperture diameter. A lens typically has a set of marked "f-stops" that the f-number can be set to. A lower f-number denotes a greater aperture opening which allows more light to reach the film or image sensor. The photography term "one f-stop" refers to a factor of √2 (approx. 1.41) change in f-number, which in turn corresponds to a factor of 2 change in light intensity.

Aperture priority is a semi-automatic shooting mode used in cameras. It allows the photographer to choose an aperture setting and allow the camera to decide the shutter speed and sometimes ISO sensitivity for the correct exposure. This is sometimes referred to as Aperture Priority Auto Exposure, A mode, Av mode, or semi-auto mode.

Typical ranges of apertures used in photography are about f/2.8–f/22 or f/2–f/16, covering 6 stops, which may be divided into wide, middle, and narrow of 2 stops each, roughly (using round numbers) f/2–f/4, f/4–f/8, and f/8–f/16 or (for a slower lens) f/2.8–f/5.6, f/5.6–f/11, and f/11–f/22. These are not sharp divisions, and ranges for specific lenses vary.

ISO Speed

ISO is the measure of a photographic film's sensitivity to light, determined by sensitometry and measured on various numerical scales, the most recent being the ISO system.

Relatively insensitive film, with a correspondingly lower speed index requires more exposure to light to produce the same image density as a more sensitive film, and is thus commonly termed a slow film. Highly sensitive films are correspondingly termed fast films. A closely related ISO system is used to measure the sensitivity of digital imaging systems. In both digital and film photography, the reduction of exposure corresponding to use of higher sensitivities generally leads to reduced image quality (via coarser film grain or higher image noise of other types). In short, the higher the film speed, the grainier the image will be.

Metering

We talked in some detail about exposure in three earlier sections so by now you'll understand how shutter speeds, apertures and ISO all combine to affect the brightness of an image. Naturally, different conditions will lead to differing amounts of light being available and therefore a decision must be made regarding which shutter speed and aperture combination to use - for this an exposure meter is used.  This device, be it a dedicated device or one built into your camera, simply measures the amount of light available which is then converted into aperture and shutter speed.

Different Types of Light Meter

Light meters may be in-camera light meters or handheld light meters. All modern cameras have built in light meters but there can be advantages to using handheld light meters on occasion. We'll cover in-camera light meters and their various options in a moment but first we'll touch upon the different types of handheld light meters. Handheld light meters typically fulfil one or more of the following functions:

• reflected light metering
• incident light metering
• flash metering

Reflected Light Metering

The meter that is performing a reflected light reading is measuring the light reflected off your subject. You point the light meter at your subject and it tells you what the exposure would need to be in order to render that subject at a mid grey (typically referred to as 18% grey). This type of meter will give you different exposure results for a black subject or a white subject as it only tries to make an exposure to make things grey.  It has no way of knowing if your subject is black, white or somewhere in between so it just measures the light coming off the subject and assumes the elements of the image will average out to be grey.

At its most basic level this is how an in-camera meter operates.

A special type of reflective light metering device is called a spot meter. This measures the light reflecting off a very small area of your subject, a small 'spot' if you will. To use a spot meter you aim it at a part of your subject that you wish to appear to be mid-grey and set your exposure as it tells you. Alternatively you can point it at a darker or lighter portion of your scene and adjust the exposure accordingly (more on this later).

Incident Light Metering

With an incident light meter you measure the strength of the light source. To do this you would hold the meter in front of your subject and point the meter at the source of light to take a measurement. This type of metering avoids the complication caused by your subject being of a reflective nature other than mid-grey as it is measuring the light totally independent of the shade or colour your subject is.

Flash Metering

A flash meter is a special type of incident light meter. A flash meter works in just the same way as described above except that it will automatically trigger your studio flash, take a reading while the flash is illuminated and tell you what aperture is needed for your currently selected ISO (shutter speed does not matter as it will be described in the 'Flash Photography' section of this book).

In-Camera Light Metering

Number 16 is the cameras built in EV meter.

As mentioned, an in-camera light meter is taking reflective light readings through the lens from the scene the camera is pointed at. If your camera is in an automatic mode the light meter will enable your camera to automatically select the exposure based on the scene it is measuring.  If it is in manual mode the light meter will tell you if it thinks the picture will be under-exposed or over-exposed. Modern cameras come with at least one of the following metering modes (not to be confused with exposure modes - metering modes usually reside somewhere in the menu system):

• centre weighted average metering
• intelligent metering (somtimes called evaluative, matrix or multi metering)
• partial metering
• spot metering

Your camera may come with all of these modes. This is what they do.

Centre weighted average metering measures the light across the whole scene but pays more attention to the brightness of the scene in the centre. This was the most common type of metering on film SLR cameras and many people continue to use it as it is easy to understand and predict when it may be fooled into providing an incorrect meter reading.
Intelligent metering is called something different by different manufacturers. Canon call it evaluative, Nikon call it matrix.  It works by evaluating the whole scene and using an algorithm to try to identify what the scene is and therefore what the exposure should be. Whilst this mode is generally quite successful, some users choose to avoid it as the manufacturers don't publish how it makes its decisions and therefore it can be a little harder to predict.

With Canon's implementation they bias the exposure reading to the currently selected focus point as they assume that the focus point will be placed over the main subject of the scene.
Partial metering measures a smaller part of the scene than the previous two but a greater part of the scene than spot metering. Partial metering takes its reading from a circle covering approx the centre 9% of the frame.

Partial metering can be useful where there is a wide dynamic range in your scene and you want to ensure a certain part is exposed correctly, for example a spot lit actor on a dark stage.
Spot Metering measures a small part of the scene typically 1% to 5% and is useful for being precise about where exactly in the scene you want your light meter reading to come from.  This is always taken from the centre of the frame except on some professional level cameras when there may be an option to have it taken from the selected focus point.

How to read your in-camera light meter.

Whilst most of the following discussion will apply to evaluative/matrix type metering there may be times where that type of metering may behave differently than other metering modes.  As mentioned above, this is because this type of light metering uses unpublished and probably fairly complex algorithms to decide on a correct exposure. For the most part the following information will be fine for that type of metering mode but if your camera meters in a way that isn't explained here while in that type of mode it's simply because it is trying to be clever!

Most, if not all, of the time you will be using your camera's in-built light meter. As we have mentioned this type of light meter is carrying out reflected light metering and is trying to render the scene so that it averages out to a mid-grey tone.  It is surprising how often this works out just fine but this can mean scenes containing very bright or very dark elements can fool a camera meter into under or over exposing.

If the scene you are photographing contains very bright elements it may cause your camera to think it needs less exposure than it really needs for example:

• a snow scene
• a sunny day on a beach
• a bride in a white wedding dress
• a back-lit subject (i.e. where the light is behind the subject)

In these examples the camera will be trying to render your subject grey whereas they really need to be white or close to white. This will cause your meter to suggest the wrong exposure and if you listen to it you will end up with an underexposed picture. To rectify this you will need to tell your camera to overexpose by one or two stops. In manual mode to simply select an exposure where your camera meter is telling you it will be overexposed by one or two stops (depending on how much whiter than mid-grey your subject is), in an automatic mode you will need to use exposure compensation (see exposure part 4).

If the scene contains very dark elements your camera may over expose as it thinks there is less light than there is. Remember, it is trying to make your subject grey.  If your subject is darker than that it will turn out too bright in your photograph as the camera tries to compensate. For example:

• a close-up of a person wearing dark clothes
• a person against a dark background such as a person lit by spotlights on stage.

In these circumstances you can adjust for your light meter's misreading of the situation in the same way as above. The only difference is this time you need to tell the camera to underexpose.

Grey Cards

Because light meters can be fooled by dark or light subjects, photographers sometimes use grey cards. These are, like the name suggests, cards that are grey. To be more specific they are cards that are designed to be the shade of grey that your light meter is looking for. Therefore if you take a light meter reading by pointing your camera at the grey card you should get the correct exposure. To make this work, the grey card needs to be in the same lighting conditions as the subject so you may ask your model (if you are taking a portrait) to hold the grey card close to his or her face while you take a spot meter reading from it to set your exposure.

If you don't want to carry your grey card around with you, you can use your hand. Take a reading off your grey card outside your home then take a reading of your hand. Your hand and grey card are likely to give different exposure results but as long as you know this then you can compensate in the field. For example, a white person may find that their hand and their grey card give a one stop different exposure with the hand apparently requiring one stop less exposure. This means that in the field someone like this can take a meter reading off their hand and adjust the exposure so the camera appears to overexpose by one stop.

What if none of these systems are working?

Natural light, as well as many scenes with artificial lighting, is predictable, so that exposure often can be determined with reasonable accuracy from tabulated values.

Exposure values (ISO 100 speed) for various lighting conditions

Advanced Techniques - The Zoning System

The Zone System is a photographic technique for determining optimal film exposure and development, formulated by Ansel Adams and Fred Archer in 1939–1940.

The Zone System provides photographers with a systematic method of precisely defining the relationship between the way they visualize the photographic subject and the final results. Although it originated with black and white sheet film, the Zone System is also applicable to roll film, both black and white and color, negative and reversal, and to digital photography.

Sources 

http://www.azuswebworks.com/photography/camerabasics.html

http://en.wikipedia.org/wiki/Shutter_speed

http://en.wikipedia.org/wiki/Aperture

http://www.creativephotobook.co.uk/pg04007.html

http://en.wikipedia.org/wiki/Zone_System

http://en.wikipedia.org/wiki/Exposure_value#CITEREFANSI_PH2.7-1973