Aperture

Aperture is “The size of the opening in the lens when a picture is taken.”

When you hit the shutter release button of your camera a hole opens up that allows your cameras image sensor to catch a glimpse of the scene you’re wanting to capture. The aperture that you set impacts the size of that hole. The larger the hole the more light that gets in – the smaller the hole the less light.

Aperture is measured in ‘f-stops’. You’ll often see them referred to here at Digital Photography School as f/number – for example f/2.8, f/4, f/5.6,f/8,f/22 etc. Moving from one f-stop to the next doubles or halves the size of the amount of opening in your lens (and the amount of light getting through). Keep in mind that a change in shutter speed from one stop to the next doubles or halves the amount of light that gets in also – this means if you increase one and decrease the other you let the same amount of light in – very handy to keep in mind).

One thing that causes a lot of new photographers confusion is that large apertures (where lots of light gets through) are given f/stop smaller numbers and smaller apertures (where less light gets through) have larger f-stop numbers. So f/2.8 is in fact a much larger aperture than f/22. It seems the wrong way around when you first hear it but you’ll get the hang of it.

The main reason photographers change the aperture is to control Depth of Field.

Depth of Field (DOF) is that amount of your shot that will be in focus.

Large depth of field means that most of your image will be in focus whether it’s close to your camera or far away (use small aperture).

Small (or shallow) depth of field means that only part of the image will be in focus and the rest will be fuzzy (use large aperture).

Choosing the correct aperture (f-stop)

Aperture f/5.6 and lower

This will isolate the subject from the background. The main subject will be in focus while the background is blurred. This is best used for portrates or close up shots of objects such as flowers and birds.

Aperture f/8 to f/16

Used when depth of field does not matter because the distance between the main subject and the background is small. This is generally used for photo’s taken where all objects are in the immediate surroundings and you want the entire scene in focus. Examples would a dog playing in the garden or a model sitting the the bonnet of a car .

Aperture f/16 or greater

Used when a large depth of field is important. Generally used for landscape photography where objects can be miles apart but you want the entire scene to be in focus. An example would be flowers in the foreground and a mountain range in the background. Many photographers don’t recommend going above f/22, if you do a tripod is recommended as a slower shutter speed is required to allow enough light to reach the sensor.

Bracketing

Many modern DSLR and some high end point-and-shoot camera’s have a Bracketing option. This instructs the camera to take a series of shots at different exposures. Usually three shots are taken, one using the current settings, the second 1 f-stop below and the third, 1 f-stop above.

Depending on your camera you may need three presses of the shutter release to capture the three shots. However some provide an automatic option where one press will take the three differently exposed shots one after the other.

Bracketing is useful in difficult lighting conditions were contrast can be difficult. By taking the shot with three different exposures the best can be chosen later on. Because the camera cannot take three shots simultaneously Bracketing doesn’t work for fast action shots. However this all depends on the ‘drive’ speed of your camera.

Cameras allow the Bracketing width to be adjusted. For RAW images one stop is usually acceptable. However if shooting JPEG’s it’s a good idea to make the gap closer as there is less opportunity to adjust later on.

Focus

AE-L/AE-F button Vs Half Shutter Press

By default most camera’s will only lock focus with a half shutter press. This means when recomposing a shot with a half shutter press the camera could adjust exposure. Pressing AE-L/AE-F button will lock both exposure and focus even when recomposing. However mainy DSLR camera’s such as Nikon’s allow this functionality to be changed so a half press will lock exposure as well.

ISO

 

ISO Stands for International Standards Organizations. In photography its a standardised way of measuring sensitivity to light.

With traditional photography it is used to define how sensitive the film is to light. It is measured using numbers typically 100, 200, 400, 800, 1600 etc. Lower numbers mean the film is more sensitive than higher numbers.

In digital photography the image sensor replaces the film. The ISO value now indicates the sensor’s sensitivity to light. Because the image sensor is controlled by software the ISO values can be changed using the camera’s controls. Depending on the type of camera the ISO range will vary.

By selecting a lower value the image sensor is less sensitive to light and produces a finer grain. In situations where there is plenty of light it’s a good idea to keep the ISO setting as low as possible (100 is appropriate for most cameras). This will ensure noise is kept to minimum. However if using very fast shutter speeds such as 1/1000 or greater, images may appear very under exposed (dark). Raising the ISO value will increase sensor sensitivity and produce a correctly exposed shot. Night shots with slow shutter speeds may also require higher ISO values for correct exposure.

High ISO values a generally used in low light conditions to get faster shutter speeds.

100 ISO is generally classified as ‘normal’ and produces a clean, crisp shot.

Many camera’s provide an automatic option for ISO levels. The camera will analyse the current light intensity and attempt to keep the value as low as possible.

By using manual ISO settings you have more control over aperture and shutter speeds. However it can be more difficult to balance ISO, aperture and shutter speed for a well exposed shot.

Selecting the correct ISO involves good assessment of the shot required. You need to take into account lighting levels, whether the subject is moving, are you using a tripod and how much picture noise is acceptable.

It best to keep ISO levels low. However there are situations where a higher level is acceptable such as museums, concerts and indoor sport events.

Light Metering

Modern digital cameras are very good at selecting the correct exposure metering automatically. However most DSLR and high end Point-and-shoots have multiple metering modes which many people never use. Switching from the default mode can have a dramatic effect on the end result.

Digital cameras use something called a reflective meter. This means they measure light reflected from objects in the scene. Dedicated handheld meters can measure reflective and ambient light (light falling on the object rather than reflected).

Many digital cameras contain a built-in database of images with associated exposure settings. When the camera measures exposure in metering mode it compares the scene with this database. The results allow the camera to work out the best settings for a correct exposure.

For most cameras, the exposure meter will only see in black and white and to obtain the correct exposure it uses what we call 18% grey as a reference. This is a mid-point grey that has been worked out from the average light levels of thousands of different scenes.

Unfortunately the light reflected from your scene may be darker or brighter than 18% grey. This could lead to incorrect exposure. The built-in image database goes some way to resolving this issue, however it’s not infallible. This is where selecting the correct metering mode will help.

Matrix Metering

This is the default for most DSLRs. The camera breaks the scene down into zones. Each zone is independently metered and the average exposure is used for the entire scene. For most shots this form of metering is ideal.

Centre Weighted

As the name suggests centre weighted metering measures predominantly from the centre of the image. The rest of the scene is also measured but when calculating the exposure more weight is given to the centre measurements.

This metering method works well where the main subject is in the centre of the frame, such as a person in a field or on a road.

Spot Metering

When in this mode the camera only measures a very small area in the centre of the view finder, usually 5%. Some DSLRs allow this value to be adjusted. The main use for this mode is perfect exposure on your subject, for example preventing a backlit subject being too dark.

When in spot metering mode, you direct the exposure point at your subject, lock the exposure, usually with a half press of the shutter or with the exposure lock button if your camera has one, and then recompose the image. Remember, to return to centre weighted or matrix modes when finished as spot metering in general will cause wild fluctuations in your exposure if you leave it on by mistake.

Some cameras provide more metering options; many can now employ 3D metering. This uses the distance information from the lens to determine what the subject should be and exposing accordingly

Shutter Speed

Shutter speed is the time is takes for the shutter to open and close.

With film based cameras it was the amount of the time the film is exposed to light. For digital cameras its the amount of time the sensor can see the sceen.

Shutter speed is measured in seconds, usually fractions of seconds. A larger denominator gives a faster shutter speed i.e. 1/100 is slower than 1/1000.

Shutter speeds slower than 1/60 will result in image blur unless the camera is held steady using a tripod or something similar.

Apature and shutter speed go hand-in-hand as they both control the amount of light reaching the sensor. You need to take this into account when getting the right exposure.

Very slow shutter speeds such a 1 or 2 seconds are usually used in very low light conditions.

Shutter speed can be used to give the impression of motion. A fast speed will freeze the image, however a slow speed introduces blur which gives the illusion of motion.

Photography Terms

Vignette – Any process by which there is loss in clarity towards the corners and sides of an image, usually caused by the lens.

Color Fringing – Also know as chromatic aberration. This creates halos of solid color around edges of objects. The halos are usually purple in color.

Barrel and Pincushion Distortion – The former gives the subject an expanded look, which the latter make the object appear pinched.

Color Space

Most DSLRs and many high end compacts have an option to switch color space between sRGB and Adobe RGB. By default cameras are generally set to sRGB. But what’s the difference?

Basically Adobe RGB contains more colors than sRGB. The amount of data in the image stays the same but the colors are represented differently by the monitor or printer reproducing the image. If an Adobe RGB image is displayed on sRGB monitor the colors are made to fit. This makes bright colours look less saturated. However when using a monitor with compatible Adobe RGB colour spaces the image will display more accurate colour levels.

When shooting RAW the cameras colour space setting doesn’t effect the image, it’s only a concern with JPEGs. A RAW image will allow the colour space to be selected during post processing. Many image editors such as Photoshop and Lightroom can easily convert between the two during image export. The JPEG however will use the cameras selected colour space as part of the image capture process and cannot be changed afterwards.

Most printers and monitors use sRBG images so its usually best to sick to this colour space. However if producing images for clients it’s worth checking if they have a preference.

Exposure

What is exposure?

Wikipedia (http://en.wikipedia.org/wiki/Exposure_(photography)) states the following:

In photography, exposure is the total density of light allowed to fall on the photographic medium (photographic film or image sensor) during the process of taking a photograph. Exposure is measured in lux seconds, and can be computed from exposure value (EV) and scene luminance over a specified area.

There are three elements which affect the exposure of a shot.

1. ISO
2. Aperture
3. Shutter Speed

Each of these elements changes the way light hits the film/digital sensor. A change to one element will affect the remaining two. Therefore when adjusting one element you will always need to consider the effect on the others. Modern digital cameras do provide assistance by allowing manual control of one element while automatically adjusting the other two for a good exposure.

Mastering exposure takes a lot of practice. The best way to learn is to take lots of pictures with different settings and study the results. The great thing about digital cameras is you can take as many pictures as you like at zero cost.