The retina has a static contrast ratio of around 100:1 (about 6 1/2 stops). As soon as the eye moves (saccades) it re-adjusts its exposure both chemically and by adjusting the iris. Initial dark adaptation takes place in approximately four seconds of profound, uninterrupted darkness; full adaptation through adjustments in retinal chemistry (the Purkinje effect) are mostly complete in thirty minutes. Hence, a dynamic contrast ratio of about 1,000,000:1 (about 20 stops) is possible. The process is nonlinear and multifaceted, so an interruption by light nearly starts the adaptation process over again. Full adaptation is dependent on good blood flow; thus dark adaptation may be hampered by poor circulation, and vasoconstrictors like alcohol or tobacco. Equivalent Resolution
Roger N. Clark estimates human vision resolution to be equivalent to 576 megapixels (24000 x 24000 pixels) for a 120 degree field of view. Extensive background, assumptions, and calculations are available at http://www.clarkvision.com/imagedetail/eye-resolution.html However, it must be noted that the human eye itself has only a small spot of sharp vision in the middle of the retina, the fovea centralis, the rest of the field of view being blurry. The angle of the sharp vision being just few degrees in the middle of the view, the sharp area thus barely achieves even a single megapixel resolution. The experience of wide sharp human vision is in fact based on turning the eyes towards the current point of interest in the field of view, the brain thus preceiving an observation of a wide sharp field of view.
The narrow beam of sharp vision is easy to test by putting a fingertip on a newspaper and trying to read the text while staring at the finger tip – it is very difficult to read text that's just some centimeters away from the finger tip.
The eye is the organ of sight. This complex structure works by capturing light and transforming it into impulses that the brain can interpret as images.
To understand visual perception, it is important to know the functions of the parts of the eye. The eye includes the eyeball and all structures within and surrounding its almost spherical mass. This delicate organ is nestled within the bony socket of the skull. A layer of fat cushions the socket; the eyebrow, eyelashes, and eyelid provide a barrier against incoming irritants.
Lining the inside of the eyelid and continuing over the exposed surface of the eyeball is the conjunctiva, a thin protective membrane. Tears released from the lacrimal glands in the upper eyelid moisten the conjunctiva and keep the eye clean. The sclera is the tough, white, outer layer of the eyeball. The sclera covers the entire eyeball, except for the circular area in front that admits light, which is covered by the transparent cornea. The choroid layer contains blood vessels that nourish the eye.
Light enters the eye through the cornea. The curved cornea helps to focus the light inward. Behind the cornea is a pigmented structure called the iris. The iris surrounds an opening known as the pupil. The iris changes the size of the pupil, depending on the amount of light present in the environment: If the surroundings are relatively dark, the pupil is enlarged to admit more light; if the environment is bright, the pupil is made smaller.
Behind the iris is the lens, a transparent structure held in place by elastic, muscular-type tissue. The tissue can change the shape of the lens to finely focus the incoming light rays onto the light-sensitive cells that line the back of the eye.
Between the cornea and the lens is a space, the anterior chamber, which is filled with a fluid called aqueous humor. Aqueous humor contains nutrients that nourish the cornea and the lens. The fluid also allows light rays to pass through easily.
The chamber of the eyeball behind the lens holds a clear jelly called vitreous humor. In the retina (the layer of light-sensitive cells that lines the back of the eyeball) are the specialized cells, called rods and cones, that convert light focused from the cornea and lens into electrical impulses. Sensitive nerve endings then transmit these impulses to the brain via the optic nerve, which extends from the rear of the eyeball to the brain.
Because of the screen size and incredible detail on an IMAX image, the quality of computer-generated effects must be perfect to work on an IMAX screen. For example, the dinosaurs in "T-REX" have five times the detail of the dinosaurs in the "Jurassic Park" movies. This means that it takes five times more computer power to render each "T-REX" image, and five times the storage space.
According to Lewis, one of the key challenges when making any IMAX film has to do with the film size. The size of the film means three things to a director:
The camera is immense. It weighs 240 pounds (109 kg), so it requires special supports and rigging to move it around. A typical 35-mm movie camera, by comparison, weighs only 40 pounds (18 kg).
The size of the film means that the camera can hold only a three-minute spool, and it takes 20 minutes to reload.
The incredible detail available with a film size this large means that everything about the shot must be perfect, and each image must be stunning. The audience sees every flaw, and a lackluster image totally wastes the potential of the IMAX medium. According to Lewis, "The cost and complexity in every segment of physical production is an order of magnitude greater with IMAX." In addition, "There are only two IMAX 3-D cameras in the world, so if you have a breakdown, you are standing around spending $100,000 a day on production costs."
On a normal film, 10 setups a day is normal. With IMAX, "Three or four a day is moving at lightspeed", according to Lewis. The camera is also very noisy -- it sounds like a chain saw when it is running. Actors and crew are all affected by the noise.
The screen size and clarity mean that every frame of an IMAX film must be perfect. "In 35 millimeter, you can use lots of cheats in visual effects -- things like rain and darkness," says Michael Lewis. "In IMAX you see everything, and everything is photo-real. There are 100+ IMAX screens in museums, so things must be as accurate as possible. With a dinosaur, you have to worry about things like nostril slant and tooth decay. When placing a dinosaur on the ground, the eye instantly knows if something is not perfect."
The technical challenges mean that an IMAX film, which is normally just 40 or 50 minutes long, costs just as much to make as a normal film for theatrical release. For example, the "T-REX" film took five months for a feasibility study to prepare for filming, 40 days for the shoot and then 12 to 13 months to complete the film in post-production. The CG effects and dinosaurs consumed approximately 4 terabytes of disk space.
A typical IMAX film's production costs fall somewhere in the range of $3 million to $8 million for a 2-D feature, and $8 million to $15 million for 3-D, with 3-D films involving CG running at the high end of the scale. Films can either be funded by IMAX or self-funded by studios like Michael Lewis' L-Squared Entertainment.
Despite the challenges, the unique experience of the IMAX theater makes IMAX films a compelling medium for directors. With the number of theaters increasing worldwide, and with a rapidly growing audience for the IMAX experience, it is likely that a wide variety of films will be created for this venue in the years to come.
