Glossary

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Aperture

One of the three main factors determinimg the exposure of an image (the amount of light captured). The aperture is the opening inside the lens through which light passes before reaching the sensor, and its size varies in order to adjust the amount of light passing through. The size of the aperture is expressed as a fraction of the focal length of the lens (f), e.g. f/8 means the diameter of the aperture is 1/8th of the focal length of the lens. A very large aperture (e.g. f/1.0) lets in the most light, but reduces the depth of field (the amount of the image, in terms of distance from the lens, which is in focus). Conversely, a small aperture (e.g. f/22) lets in far less light, but allows a much greater depth of field.

Field of view

The field of view is the amount of a scene, measured in degrees, which is captured in the image. Stitching a row of images together will increase the horizontal field of view, and stitching a column of images will increase the vertical field of view.

ISO

One of the three main factors determining the exposure of an image (the amount of light captured). The ISO rating is an expression of how sensitive the camera sensor is to light, and can be varied on most digital cameras. Broadly speaking, the sensitivity of camera sensors is increased by increasing the electronic gain of the sensor. For consistency with traditional film cameras, this sensitivity to light is expressed using a scale which was standardised by the International Standards Organisation for camera films. A low ISO setting (e.g. ISO 100) produces a clean, smooth image, but requires the most light. A high ISO setting (e.g. ISO 1600) produces good images in low light conditions, but suffers from image 'noise' (visual interference, or graininess) caused by the high gain of the image sensor. This effect can be desirable with some types of image.

Megapixels

A pixel is simply a dot of light on the screen (or colour on a page) and is the smallest element of a digital image. The size of an image is related to the total number of pixels - the more the better in simple terms.

One megapixel is one million pixels - for example, an image which is 1000 pixels wide and 1000 pixels high. A typical digital camera may produce images of 1 to 8 megapixels, whilst professional digital SLRs are in the region of approx 8 to 16 megapixels. Stitching images together allows much larger image sizes to be produced, with the limits only really imposed by the ability of the software used, and the hardware running the software!

A milestone in digital imaging occurred in 2003 when Max Lyons produced the first known gigapixel image of Bryce Canyon in Utah, USA. This image is over 1000 megapixels in size and was stitched from 196 source images.

Pixel dimensions

The width and height of an image in pixels, e.g. a 6 megapixel image might be 3072 pixels wide by 2048 pixels high, or 3072x2048px.

Print sizes

The size of an image when printed depends on two main factors: (i) the pixel dimensions, and (ii) the density of the printed pixels (or resolution), usually measured in pixels per inch (ppi). A high resolution image has all its pixels packed tightly into a smaller area on the printed page, resulting in a smaller image, but one with greater apparent detail. Therefore, to print an image at a large size and at high resolution (around 250 to 300 ppi), it must have a relatively large number of pixels. So the larger the image in megapixels, the larger it can be printed without neglecting quality.

Images displayed on screen usually appear larger than when printed because screen resolutions are much lower (typically 72 or 96 ppi). Effectively, the pixels are larger when viewed on screen.

Projection type

Cylindrical projection: A panorama with cylindrical projection. This is like taking a cylinder, making a cut from top to bottom, and opening it out flat. This type of projection is often used for landscapes with up to 360 degrees horizontal field of view. The source images are taken by rotating the camera around the 'nodal point' of the lens.

Rectilinear projection: A panorama with rectilinear projection. This is like joining 4 sheets of A4 paper to make one sheet of A2 and is often used for making large images of essentially flat objects, e.g. a distant landmark. Also used to join scanned images where the item being scanned is larger than the size of the scanner bed.

Spherical projection: A panorama with equirectangular projection. This is a method of presenting a spherical surface (such as the earth's crust) as a rectangle. Imagine a beach ball being cut open so that the plastic is laid flat on a table. This wouldn't be easy, but if you stretched the plastic to make a rectangle, that is effectively an equirectangular projection. This technique is most commonly seen on maps of the world - the stretching is obvious when you look at the top and bottom of the map. Spherical panormas are used to capture a full 360 degree horizontal field of view, including a full 180 degree vertical field of view, thus enabling you to see all around as well as up and down. Virtual tours (or panoramic movies) often use spherical panoramas to display the inside of a room, for example.

Shutter

One of the three main factors determinimg the exposure of an image (the amount of light captured). The shutter speed is the length of time that the digital camera sensor is exposed to the light when the image is taken. A slower shutter speed (e.g. longer than 1/60th of a second) captures more light, but moving subjects can appear blurred. On the other hand, a faster shutter speed freezes motion with a sharp image, but more light is required for a correct exposure (image brightness). Shutter speeds typically range from 1/8000th of a second (very fast) to 30 seconds (very slow). 'Bulb' exposures last for as long as you keep the shutter open, but this setting is usually only available on SLR cameras.