How to work out the ratio and width of screen of a projector lens, and the difference between short throw, on and offset axis
Professional projectors use interchangeable lens and can be exchanged to provide for different screen sizes and projector throws.
Lens are expressed for example 0.8:1 and 1.2-1.8:1. Lets look at the different types and what they are used for, to keep it simple the numbers we will refer to in feet.
The first digit represents the distance of the screen to lens front, and the second digit represents the image width.
0.8:1 is classed as a short throw lens. It enables the projector to be sited closer to the screen whereby you need 0.8 foot to create a 1 foot wide image. For a conventional 8' x 6' screen this equates to a distance of 6'4" to the front of the lens. This is the minimum distance and generally a few inches +- may be required to get the desired size taking into account keystone. Most professional lens' of this sort are classed as on axis. The reason for this is that when you look at the lens it looks very much like a fisheye lens and must be sited centrally to the screen both vertically and horizontally to create an clean image free of lens distortion. If you move the lens off the centre point distortion starts to occur and will result in keystone adjustments.
Short throw lens do not have zoom functions as the optic correction is factory set to keep the image distortion free, they do however have focus to ensure a sharp image.
A short throw offset axis lens enables the projector to be mounted either at the top of the screen, or at the bottom and projects up or down as necessary, this can be useful for front project where it is required to have a short distance to the screen from lens, however is unpractical for a on axis to have the projector dead in the middle of the screen. The lens features some very clever optics with when viewed looks very odd but the lens and often mirror create a perfect image with little or no adjustment as long as it is aligned to the edge of the screem.
Standard Throw Lens
1.2- 1.8:1 Is a standard throw lens and because it has two digits at the beginning indicates it has zoom. These zooms can either be manually or electronically by motor, the later proves popular when mounted high on trussing out of easy reach enabling quick adjustment, generally the focus control follows the same pattern of manually or electronically. The two sets of figures mean that on the lower figure of 1.2 the image will be 1.2 times as wide as the depth of the screen from lens front. If you were to keep the projector in the same place and then adjust the lens to the other extreme of 1.8 the image will appear 1.8x bigger. In real life terms back to a 8'x 6' screen the smallest distance the projector lens to screen would need to be 9'6" and largest distance it could be away from the screen would be 14'4" away. A benefit of a standard throw lens is that the projector can be mounted higher or lower than the screen and alignment is less important as most projectors have keystone and only needs little adjustment.