Projector throw distance and screen size

Throw ratio is a multiplier, and it needs two faces to multiply between#

A throw ratio is the distance from the lens to the screen divided by the width of the image produced there. It is dimensionless, so it works in inches or metres without conversion, and every projector datasheet quotes it. What no datasheet says clearly is where the two ends of that distance are.

The far end is the screen surface, the actual fabric or vinyl, not the wall behind it. A fixed frame screen carries its material 2 to 3 in (5 to 8 cm) proud of the wall. An acoustically transparent screen with the left, centre and right speakers behind it sits 18 to 24 in (46 to 61 cm) off the structure, and that is 2 ft of throw distance that vanishes if you measure to the plaster.

The near end is the front element of the lens, not the mount plate and certainly not the back wall. Home cinema projector bodies are 12 to 20 in (30 to 51 cm) deep, with the lens at the front, and behind the body you need another 4 to 6 in (10 to 15 cm) for the HDMI and power tails, which have a minimum bend radius. So the wall to screen distance is throw plus 16 to 26 in (41 to 66 cm) of hardware. On a 20 ft (6.10 m) room that difference is the whole margin.

The vocabulary here is worth pinning down, and the glossary of interior dimensions carries the definitions used across this site.

Diagonal is the number you buy, width is the number you calculate with#

Screens are sold by diagonal and projected by width. For a 16:9 image the conversion is fixed geometry: the diagonal of a 16 by 9 rectangle is sqrt(16^2 + 9^2), which is 18.358, so

width = diagonal x 16 / 18.358 = 0.8716 x diagonal

height = diagonal x 9 / 18.358 = 0.4903 x diagonal

The 0.872 factor is where most throw calculations go wrong. Multiply a 1.5 throw ratio by a 120 in diagonal instead of the 104.6 in width and you get 180 in instead of 157 in, an error of 23 in (58 cm), which is more than enough to put the projector inside the back wall.

Throw distance for the screens people actually buy#

Read the 2.0:1 column against a real room. A 150 in screen at 2.0:1 puts the lens 21 ft 10 in (6.64 m) from the fabric, and once the body and cables are added the projector needs 23 ft 6 in (7.16 m) of room. Very few domestic rooms have it, which is why large screens in houses are almost always driven from the short end of the zoom. The same arithmetic in reverse, from a measured room to a screen size, is what the projector throw calculator does.

Lens shift, offset, and where the projector is allowed to sit#

Lens shift moves the image up, down or sideways optically, by moving the lens relative to the imaging panel. It is quoted as a percentage of image height or width, where 100 per cent vertical means the image can be pushed until its bottom edge is level with the lens axis.

Work an example. A 120 in screen with its bottom edge 31.2 in (79 cm) above the floor has its top edge at 90 in (229 cm) and its centre at 60.6 in (154 cm). On a 9 ft (2.74 m) ceiling with a short bracket, the lens axis lands around 96 in (244 cm). The gap between lens axis and image centre is 96 minus 60.6, which is 35.4 in (90 cm), and 35.4 divided by the 58.8 in image height is 60 per cent. A projector rated at 60 per cent vertical shift just reaches. One rated at 105 per cent has 26 in (66 cm) of spare travel, which is what lets it sit on a shelf at the back instead of on the ceiling.

Projectors with no shift have a fixed offset instead: the image sits a set percentage of its height above or below the lens axis, commonly 8 to 16 per cent. On the same 120 in screen that is only 5 to 9 in (13 to 24 cm) of displacement, so the projector has to be almost level with the top of the image, which for a ceiling mount means an extension pole 24 to 30 in (61 to 76 cm) long hanging in the middle of the room.

Brightness sets the largest screen you can actually use#

Screen luminance in foot-lamberts is the lumens arriving at the screen, multiplied by screen gain, divided by the screen area in square feet. SMPTE uses 16 fL (55 cd/m2) for a darkened cinema, and high dynamic range material wants 30 fL or more on the highlights.

Those are lumens at the screen, at a gain of 1.0. A rated ANSI lumen figure is measured in the brightest picture mode with the lamp at full power and the zoom wide, and a calibrated cinema mode typically delivers around half of it. Doubling the calculated number is the honest way to read the table: a 150 in screen for HDR wants a projector rated near 4,000 lumens, not 2,000.

Ambient light changes the answer again. Every surface in the room reflects light back at the screen, which is why the lux targets and matt dark finishes in media room lighting matter as much as the projector does, and why the general lumen arithmetic in how many lumens a room needs has to be turned on its head here: in a theatre you are budgeting how little light to put in the room, not how much.

Scope screens change the width, and therefore everything#

A 2.35:1 scope screen built to the same height as a 120 in 16:9 image is 58.8 in (149 cm) tall and 58.8 x 2.35, which is 138.2 in (351 cm), wide. Its diagonal is 150 in (381 cm). Because throw is a multiple of width, the throw distance for the same lens setting rises from 156.9 to 207.3 in, a jump of 50.4 in (128 cm). A constant-height scope setup therefore costs about 4 ft (1.2 m) of room length compared with the 16:9 screen of the same height, which is exactly the length most rooms do not have.

Two ways round it. A lens memory system zooms the image wider and taller, then lets the extra height fall off the top and bottom of a wider screen, which costs no room length but uses the whole panel and dims the picture slightly. An anamorphic lens stretches the image horizontally at constant height, which keeps the brightness but adds 6 to 10 in (15 to 25 cm) to the front of the projector and moves the lens datum forward by the same amount, so the throw distance has to be remeasured from the new front element.

Mounting, services and the equipment behind it#

Ceiling brackets add 3 to 6 in (8 to 15 cm) at minimum and extension poles run 6 to 36 in (15 to 91 cm). Whatever the drop, the projector needs a power outlet and a signal path at the mount, not trailing across the ceiling, and the recessed box depths and conduit sizes for that are the same family of figures as in TV outlet, cable and conduit heights. A 1.5 in (38 mm) conduit will take one HDMI cable with its connector; 2 in (51 mm) is the size to install if a future cable might be thicker.

Everything the projector is fed from lives at the other end of the room, and the depth, ventilation and rack unit arithmetic for that cupboard is set out in media console and equipment rack dimensions. The screen height you settle on here feeds directly into theatre seating rows and risers, because the bottom edge of the image is the focal point every sightline is drawn to, and the room length it demands is worked through in home theatre room dimensions. If the ceiling is being lowered to hide the projector or a duct, check the result against ceiling heights and datum lines before the plasterboard goes up, and if you are still choosing between a screen and a panel, start from screen size and viewing distance rather than from the projector. Everything on this page belongs to the wider media room and home theatre set of figures.