panoguide - Glossary

Barrel Disortion

Convex/Concave Disortion

Depth of Field

The amount of the picture (in terms of distance from the camera) which is in focus. For landscape shots typically the depth of field is maximum so as to get the entire scene all the way to the horizon in sharp focus. For portrait photography the depth of field is normally very small so that the subject is in focus and all of the unwanted background is thrown out of focus (the effect is to emphasise the subject). Depth of field is affected by the aperture set when taking a picture: for large depth of field the aperture should be small (e.g. F22) and for portrait work, etc, it should be large (e.g. F4).

Field of View (FOV)

35mm cameras produce rectangular pictures (36 x 24mm negatives) and therefore the field of view is greater for the longer dimension. Generally when someone refers ambiguously to the field of view of a 35mm image they are referring to the field of view along the longer of the two dimensions, regardless of which way up the camera was (portrait or landscape).

Film plane

Same as the focal plane.

Fisheye lens

A wide angle lens that captures the scene exactly as it appears, thereby incorporating curves which the human eye does not perceive because the focal length of the human eye is approx 50mm. Fisheye lens images tend to cause all straight lines to appear curved. See also rectilinear lens.

Focal Length

The distance between the rear principal point of the lens and the rear focal point when the lens is focused on infinity.

Focal Plane

The area perpendicular to the axis of the lens which intersects the rear focal point is known as the focal plane, which is the same as the film plane

Focal Point

When parallel light rays enter a lens a perfect lens will cause the light rays to converge on a single point before fanning out again to reach the film. This point is the 'rear' or 'image' focal point. There is also a 'front' or 'object' focal point.

HFOV

Short for "Horizontal Field of View". The horizontal angle in degrees that a particular image represents of the world. The wider a lens the greater the angle of view. When using a 35mm camera the HFOV is calculable from the focal length as follows:
if camera is used portrait
HFOV = 2 * tan^-1(12 / focal_length)
if camera is used landscape
HFOV = 2 * tan^-1(18 / focal_length)

Linear Disortion

A measure of the amount of disortion a lens exhibits causing straight lines to appear curved. If a picture is taken of a square that almost fills the film area, the edges of the box are rarely perfectly straight and often the centre of each of the vertices will bow slightly inwards or outwards. The percentage shift one way of the other is expressed as linear distortion (positive means inwards to the centre; negative outwards). Zoom lenses may typically exhibit a positive linear distortion at their shortest focal length and a negative one at the longest focal length.

Nodal Point

A light ray that enters a lens crossing the front nodal point will emerge from the rear nodal point at exactly the same angle relative to the lens's axis as it had on entering the lens. Nodal points are identical to the principal points of a lens, except when a camera is used in unusual surroundings (e.g. underwater). Hence generally the terms 'nodal point' and 'principal point' are the same.

Pano head

Short for "panoramic head". An additional piece of equipment to put on top of a tripod which can be small or very large. Allows the camera to be positioned so that one of the nodal points of the lens is perfectly above the point of rotation. By using a pano-head the camera can be rotated to take a set of pictures for making a panorama without suffering from "parallax". There are various manufacturers. See, for example, Kaidan (www.kaidan.com).

Parallax

If you shoot a sequence of images for creating a panorama without using a pano-head and tripod you will suffer from parallax. It may be barely noticeable, or it may completely ruin the panorama. The most important factors are: how far off-centre the camera lens's nodal point is when turning, the angle of view of the lens (wider is worse), and how short a distance there is from the camera to the nearest object (shorter is worse). See What is Parallax? for more detail, example pictures and a quick demo you can try for yourself to see what parallax is.

Principal Point

A simple lens will have two principal points which will be somewhere within the lens element. Most lenses these days have many elements with the result that the principal points can be just about anywhere (including outside the lens). The principal points occur at exactly one focal length from the lens's focal points, in the direction of the lens. It is possible for the rear focal point to be 'in front' of the front focal point.

Rectilinear Lens

A lens which produces an image in which lines that appear straight to the human eye appear straight in the image. Most lenses fall into this category and all that are 35mm and above will simply because they are within the range of our own eyes. However wide angle lenses capture more than our eyes can see and inherently produce large amounts of distortion. It's difficult to describe further without examples (and I only have rectilinear lenses). When a rectilinear image is stitched the warping process re-creates the distortion you might otherwise expect from the lens. Compare the straight lines in the individual source images of East Croydon Railway Station with the stitched result in the Technique section.
A rectilinear lens can cause objects close to the lens to appear distorted. A picture of a square grid (e.g. paving) will look a little odd in the foreground: some of the stones may appear more like parallelograms than squares or rectangles. This is because the lens distorts the light so as to ensure the image contains straight lines not curves.
Because of the complex optics of wide angle rectilinear lenses it is not possible to buy a 180 degree rectilinear lens. Generally rectilinear lenses are available down to a focal length of around 14mm (approx 114 degree VFOV, portrait).

VFOV

Short for "Vertical Field of View". The vertical angle in degrees that a particular image represents of the world. The wider a lens the greater the angle of view. When using a 35mm camera the HFOV is calculable from the focal length as follows:
if camera is used portrait
VFOV = 2 * tan^-1(18 / focal_length)
if camera is used landscape
VFOV = 2 * tan^-1(12 / focal_length)