In a sense, a projector reverses the process in a camera.

With a camera, light comes from the scene being photographed into a lens, where it is bent to focus on a plane some small distance inside the camera. At that plane is a film or CCD or some such, which is used to record it.

A projector, however, shines a light through the image on the film or digital panels. This light passed through a nearby lens which focuses it on a projection screen some distance away.

But despite the apparent complementary nature of these two processes, there remain very important differences. These in turn place a different emphasis on the adjustments for cameras and projectors, particularly when it comes to aperture.

WHAT IS IT?
Trivially, an aperture is a hole. In optics, it is the hole that controls the amount of light entering or leaving the optical system.

Look in the lens of an SLR camera. Set it to manual aperture and then turn the ring on the lens marked with F-stops (numbers in a sequence like 2, 2.8, 4, 5.6, etc). Inside the lens you will see an iris or diaphragm opening and closing. The higher the F-stop, the smaller the hole. Indeed, each of those numbers represents a halving of the area of the aperture over the previous number.

All this is vitally important in photography, and understanding it is important for competent photography. Obviously, changing the size of the aperture changes the amount of light admitted into the system. But in photography it changes other things too: most importantly, the depth of field. The smaller the aperture (i.e. the higher the F-number), the fewer light rays shooting in haphazard directions are admitted.

The technical word is that the light is more highly collimated – the beams are closer to parallel.

Since the more randomly directed light rays are excluded by a smaller aperture, the focus is more precise over a wider range of distances. The practical effect is that there is a greater depth of field: things at a greater range of distances are focused.

If you want an arty close-up with a fuzzy background, open up your camera’s aperture wide (i.e. choose a low F-number). If you want everything in focus, from the stuff close to you to the furthest distances, go outside where there’s plenty of sunlight and close up that aperture to as small as it will go.

At the extreme, a pinhole camera has a tiny aperture (thus the name), and an enormous depth of field. But a very long exposure time, of course. A tiny aperture means that very little light actually gets in.

BUT A PROJECTOR IS NOT A CAMERA
The optical job of a projector is not the same as that of a camera. It is not even a true reversal.

That’s because a projector does not have to cope with depth of field at all. Rather, its film panel, or its LCD, DLP, LCoS or whatever panels, are all in planes at fixed distances from the lens. The projection screen is also in a plane at a fixed distance (after installation) from the lens. Since there is no depth at either end, there can be no need to worry about depth of field.

With no depth of field issues, all that is of interest with apertures is the amount of light they allow to pass.

You may think that this makes things simple. All that’s required is the lowest possible F-number for the greatest possible aperture, thereby allowing the greatest amount of light to be emitted. And you’d largely be right that this is important, but it is far from the end of matters.

In fact, brightness control is just as important, because it is vital in the effective operation of many projectors.

BRIGHTNESS AND CONTRAST
There is great demand for bright projectors. The bigger the screen and the more ambient light there is falling on the screen, the greater value brightness.

Obviously with a larger screen the light produced by the projector has to be spread over a larger area, so each bit of the screen isn’t quite as bright as with a smaller screen.

But ambient light is also important. One of the two or three characteristics most important in a high quality projected image is contrast: the difference between the light and dark parts of the image. A wide spread between these, and the picture will seem rich and involving. A narrow gap and it will either seem very dull, or quite washed out.

The floor under how dark parts of the picture can get is established by how dark the screen itself is. If light is shining on it from a window, then it clearly won’t get very dark at all. The only way to stretch that contrast is upwards: by allowing the projector to produce a lot of light to make its whites much brighter than the rest of the screen.

To get greater brightness a more powerful lamp helps, but so does a low F-number. A low F-number means a bigger aperture and so more of the lamp’s light makes it out to the screen. But the fact is the F-Number is merely one input into brightness. Sometimes it is not even reported in the official product specifications. Instead, the actual measured brightness a projector is capable of producing is listed (using ‘lumens’ as the unit). That is, in a practical sense, the important figure.

GETTING DARK
But that doesn’t mean that the aperture has become unimportant. In many projectors it helps with contrast in another important way: by allowing the picture to go dark.

Back around 2004, the likes of Panasonic and Epson found themselves at a technical disadvantage in the home theatre projector market. They were using LCD as their projection technology, but some of their competitors (typically, more expensive ones) were using DLP. Those DLP projectors of had contrast ratios of 2,000:1 or thereabouts, while LCD struggled to get much above 1,000:1. This difference was pretty obvious on screen, particularly in a dark room. The blacks from LCD were really quite grey, to the point that in dark scenes, as well as looking unimpressive, important detail in the darker registers practically disappeared in the general grey glow.

As a result, Panasonic and Epson introduced a new way to control the light output from their projectors: dynamic irises.

Previously, many projectors had an adjustable iris which could be used to open up or close down the aperture, but Panasonic and Epson brought a motorised version under the control of their electronics.

Frame by frame the projector’s brain would examine the picture data. If the overall scene was dark, it would simultaneously close down the dynamic iris to restrict light output, and recalibrate the brightness of each individual pixel based on the lower light level.

This was a game changer. LCD projectors – remember, these were generally lower in cost than DLP ones – immediately jumped to ‘dynamic’ contrast ratios of 2,000:1 and higher.

On tests such as chequerboard patterns (in which full black and full white are shown on-screen at the same time), their measurements were not particularly good. But this didn’t matter, since the human eye isn’t particularly good at seeing how black an area is in close proximity to a bright white. What mattered were subjective levels of brightness, which took a huge leap forwards. In one year of model changes LCD projectors changed from flawed devices that you put up with because they were relatively affordable, to performance leaders. Many were suddenly cinema quality.

AND SO IT GOES
Since then the native (non-dynamic) contrast ratio of the various display technologies has improved significantly, to the point where even LCD can be natively above 4,000:1. But aperture control – aka the dynamic iris – has continued and improved, and indeed spread.

Nowadays many projectors based on all kinds of home theatre technologies employ dynamic iris, including DLP, and dynamic contrast ratios have stretched to the hundreds-of-thousands to one.

All, though, intelligently controlling the size of the projector’s aperture.