Measure the image and the available position
Choose a lens after establishing the visible image width and the permitted projector positions. If a conventional lens cannot fill the image from those positions, assess a shorter-throw alternative. Do not order “a short-throw projector” without checking the actual lens range, image height and space occupied by the projector itself.
Think of the room as three linked areas: the image, the projector and the route taken by the beam. Furniture may leave a place for the equipment while people still walk through the light. A ceiling position may clear that route but introduce mounting, access and venue-approval requirements.
The first calculation
Throw ratio is approximately projection distance divided by image width, using the same units. For example, a lens positioned 4.2 metres from a 3-metre-wide image needs a ratio near 1.4:1. This is a useful shortlist calculation, not an instruction to place the projector casing at that distance.
Manufacturers define reference points and usable ranges for each model and lens. Epson's distance tables show why aspect ratio, zoom and lens position belong in the check. Use the chosen equipment's own drawing for the final layout.
| Position | Potential benefit | What to confirm |
|---|---|---|
| Behind the audience | Equipment can stay outside the presentation area | Lens reach, beam height and rear access |
| Closer to the screen | May fit a shallower space | Suitable lens, image geometry and separation from visitors |
| Above the audience | Can clear circulation at floor level | Approved mounting, access and cable routes |
| Behind the screen | Separates the beam from the audience | Rear-projection material and sufficient backstage space |
Worked example: a lens that almost fits
Imagine a 3-metre-wide presentation image. The only agreed lens position is 3.3 metres in front of it, giving a required ratio of 1.1:1. A hypothetical available zoom lens covers 1.2–1.6:1. At that width, its approximate distance range is 3.6–4.8 metres. It does not fit the agreed position.
There are three sensible proposals to review: find a compatible lens covering the required ratio; obtain approval for a position at least 3.6 metres away; or reduce the image width. At 3.3 metres and a ratio of 1.2:1, the geometric width would be about 2.75 metres. That smaller image must still pass the audience reading test.
Do not assume digital correction can turn the first lens into the second. Also avoid using the edge of a zoom range without confirming installation tolerances. A changed screen position, a scenic panel or a protective enclosure can remove the margin shown on the first drawing.
Lens shift and image correction are separate decisions
Lens shift repositions the image within a supported optical adjustment range. Epson documents it separately from projector distance in its lens-shift instructions. Ask the supplier to confirm the permitted combination of horizontal and vertical adjustment for the selected system.
Give the installer both the screen bottom height and proposed lens height. “The projector will be above the screen” is not enough to verify that the image lands correctly. If image correction is part of the proposal, agree a test of fine text and edge detail using the final source material.
Measurements to send before booking
Visible image width, height, aspect ratio and height above the floor.
Permitted projector locations, dimensions of the surrounding space and any obstructions.
A plan showing audience, presentation area, doors and regular walking routes.
Mounting or support options already confirmed by the venue, with access times.
Screen type, sample content and who will approve a practical alignment test.
Common mistakes are measuring from the rear of the casing, choosing by diagonal instead of width, forgetting projector depth and assuming every lens fits every chassis. Photograph the available space with a labelled measurement rather than sending an unscaled room image.
Include these measurements with your projector rental enquiry. If mounting and scenic construction also need coordination, ask for them in the complete AV scope.