Two ways to fit two shapes on one rectangle
Films are not one shape. A modern scope release is around 2.39:1, a flat release is 1.85:1, television and streaming series are usually 16:9 which is 1.78:1, and older material turns up at 1.33:1. A screen is one fixed rectangle. Something has to give, and there are exactly two ways to give it.
Constant image width keeps the picture the same width for everything. A 16:9 screen shows 16:9 content edge to edge; a scope film arrives with black bars baked into the signal and occupies a shorter rectangle in the middle. Nothing has to move, the projector never changes setting, and masking panels covering the bars are purely cosmetic. The consequence is that scope films — the ones shot widest, usually the ones with the most in the frame — are displayed smallest.
Constant image height keeps the picture the same height for everything and lets the width change. The screen is a wide one, scope content fills it corner to corner, and 16:9 content is a narrower rectangle in the middle with bare screen down each side for masking to cover. Now the widest films are also the biggest, which is what cinemas do and why they do it. The price is that the projector has to produce two different image widths, which means a zoom lens with the range to cover both, a stored lens position to recall each one, or an anamorphic lens that moves into the light path.
The arithmetic behind both
For constant height, both images share a height H. The wide image is H times the wide ratio; the narrow one is H times the narrow ratio. The masking each side is half the difference. On a 2.39:1 screen 130 inches diagonal, the viewable area is 119.93 by 50.18 inches. A 1.78:1 image at that same height is 89.32 inches wide, so each side needs 15.30 inches of masking, and the projector has to change image width by a factor of 119.93 divided by 89.32, which is 1.343.
For constant width, both images share a width W. The narrow image is W divided by the narrow ratio in height; the wide one is W divided by the wide ratio. The bar at top and bottom is half the difference. On a 16:9 screen 130 inches diagonal, the viewable area is 113.30 by 63.73 inches, a 2.39:1 image inside it is 47.41 inches tall, and each bar is 8.16 inches.
| Constant image height | Constant image width | |
|---|---|---|
| Screen shape | Wide, 2.35:1 or 2.39:1 | 16:9 |
| Which content fills it | Scope film | 16:9 series and flat film |
| Masking runs | Down the sides | Along the top and bottom |
| Projector must change size | Yes, by the ratio of the two shapes | No |
| Largest picture goes to | The widest films | 16:9 material |
| Brightness of scope content | Lower, because the area is larger | Higher, because the area is smaller |
The part that catches people out
Constant image height makes the scope picture bigger, and bigger means dimmer. With a 1.78:1 narrow shape and a 2.39:1 wide one, the wide image is 2.39 divided by 1.78 times the area, which is 1.343 — 34 percent more square feet for the same lumens to cover, so about 26 percent fewer foot-lamberts on exactly the films most people built the room for. The projector did not change. The picture got bigger.
This is the argument for anamorphic lenses that people often mangle. An anamorphic lens does not simply magnify the image sideways. It lets the projector use its full panel height for a scope film instead of blanking the top and bottom rows, so the light that was being thrown away on black bars is put on the screen instead. That recovers some of the brightness and some of the vertical resolution. Whether it is worth the cost of the lens and the sled that moves it depends entirely on how much scope material gets watched, and a zoom-based approach with a lens memory achieves the geometry for nothing while keeping the brightness penalty.
Building the masking
Fixed masking is a pair of panels made and left where they are, which suits a room that watches mostly one shape. Manual masking is panels that slide, hinge or drop into place. Motorised masking is the same thing with a motor, a controller and a mains supply, and it is the version that turns a piece of joinery into an electrical job.
Whatever the mechanism, the panel size is the travel plus an overlap so that no sliver of light escapes at the join. The calculator adds the overlap you specify to each panel in both directions. The material wants to be genuinely matte and genuinely black — black velvet is the traditional choice because it absorbs light at grazing angles rather than glinting, and a satin black paint at the edge of a bright picture does glint.
Anything hung on a wall above head height needs fixings into structure, and a motorised panel adds moving parts and a power supply to that. New mains wiring in a wall is licensed work everywhere, and where a panel or a screen is mounted over seating the fixing is carrying a load over people. The wall mount fastener load calculator works out what a bracket puts into its fixings, and the stud wall calculator covers laying out blocking where there is no stud in the right place.
Deciding before you buy the screen
The screen shape is the decision that cannot be revisited cheaply, so it is worth being honest about what actually gets watched. A household that watches series, sport and streaming will spend most of its evenings on 16:9 material and is better served by a 16:9 screen. A room built for films, where the majority of what plays is scope cinema, is the case constant image height was invented for.
Run the numbers both ways in the calculator before ordering. Look at the physical width each arrangement demands on the wall, at the brightness figure for the shape you watch most, and at the zoom range the projector would need. The geometry side of the same install is the projector throw distance calculator, and how far back the seats want to be for a given image size is the viewing distance calculator.
Questions people ask
What is the difference between constant image height and constant image width?
Constant image height keeps the picture the same height for every film and lets the width change, which means a wide screen, scope films filling it corner to corner, and masking down the sides for 16:9 material. Constant image width keeps the picture the same width, which means a 16:9 screen, series and flat films filling it, and letterbox bars along the top and bottom for scope. The practical difference is which content gets the biggest picture and whether the projector has to change setting between titles. Constant height gives the widest films the largest picture and requires the projector to change image size; constant width gives 16:9 material the largest picture and requires nothing from the projector.
Do I actually need masking, or is it just cosmetic?
On a constant image width setup it is cosmetic, and worth doing anyway. A projector cannot produce true black, so the area where the letterbox bars are is painted a faint grey, and a matte black panel over it gives the eye a hard edge that makes the picture look deeper than it measures. On a constant image height setup masking covers bare screen material rather than projector light, and the effect is the same: the edge of the picture becomes definite instead of fading into a lit border. Neither changes anything a meter would record about the image itself. Both change how it looks, which is the point.
Does an anamorphic lens make the picture brighter?
It can, and the mechanism is worth understanding because it is often described wrongly. Without one, a projector showing a scope film on a constant height setup blanks the top and bottom rows of its panel and throws away that light. An anamorphic lens lets the projector use the full panel height and squeezes the image horizontally instead, so light that was being wasted on black bars ends up on the screen. That is a real gain in both brightness and vertical resolution. Whether it is worth the price of the lens and the sled that moves it in and out depends on how much scope material you watch, and a zoom-based approach with stored lens positions gets the same geometry for free while keeping the brightness penalty.
What zoom range does a constant height setup need?
The ratio of the two content shapes, at minimum. Going between 1.78:1 and 2.39:1 means the image width has to change by 2.39 divided by 1.78, which is 1.343, so the lens has to cover at least a 1.34 to 1 zoom range at your throw distance and you want margin beyond that. The calculator turns your throw distance into the two throw ratios directly, which is the form the projector specification is written in. Check both against the published range for the model, and be wary of a lens that reaches one end only at the extreme of its travel, because that is where zoom lenses are least sharp and where setting up repeatably is hardest.
Why is the scope image dimmer on my constant height screen?
Because it is bigger. Brightness is lumens divided by area, and a 2.39:1 image at the same height as a 1.78:1 image has 34 percent more area. The same projector output spread over 34 percent more square feet is about 26 percent fewer foot-lamberts. This is the built-in cost of the constant height arrangement, and it lands precisely on the films the arrangement exists to serve. An anamorphic lens recovers some of it by using the projector panel fully. Otherwise the answer is to size the projector output for the scope image rather than the 16:9 one, which the projector brightness calculator does directly.