By saying its not a trick, i meant it’s not something that a post-production house works on. You don’t see lucas giving some film to ILM and say “add some DOF blur. cheers”
are you sure about motion cameras that can capture completely focused images? I mean, not “i think i’ve seen a movie that loooked pretty focused”, but REALLY sure? I ask, because the DOF effect IS DUE to the fact cameras use lenses. I mean, any book on optics will tell you this, and i do not belive that they use some camera system (if it does, in fact, exist) to film entirely in focus images.
I could probably even draw an ascii diagram of the effect. you have a point source irradiating light in
all directions. more than a single photon beam is going to to find its way through the appature, and you NEED the lens to converge all this light into a single point. But you can ONLY do this for a single plane from the camera. All other planes will have a refractive error. The error for a point is given:
refractive error = | 1/d + i - a |
er, i’ll just cut and paste bits of my thesis here. Although I’m talking about bad eye-sight, its just (!) and exaggerated form of what happens in a lens system:
The majority of cases, poor eyesight can be attributed because the optical propor-tions
of the eyeballs are awry . The eye is much like a camera. It has a pupil
that corresponds to the camera’s aperture; a cornea and lens that corresponds to
the camera’s lens; and a retina that corresponds to the camera’s film. The distance
between the retina and the lens is the focal length of the lens system.
Nature abhors the exact geometry required for a perfect optics system. In
the case of myopia, or short-sightedness, the eye is longer than normal. Distant objects are inevitably out of focus, but near objects can be brought into focus
by ‘pulling out’ the optical system, ie. so the retina is further away from the
lenses . In the case of hypermetropia the eye is shorter than normal. The eye
can compensate for this by utilising some of its internal focusing power usually
reserved for near-vision to reduce the focal length of the system for a clear retinal
image; but this leaves less reserve for focusing on near objects, and thus the eye
is long-sighted .
A simplified optical system is illustrated in Figure 4.4. The ability to focus
light from a point source to a sharp point on the retina (or focal plane) by an
optical system is known as accommodation. Those with poor eye-sight have a
reduced range of accommodation. In these circumstances, the eye tries ‘as best it
can’ to project the ‘focused’ 1 plane onto the retina with the least amount of blur.
The smallest disc obtained by the converging light is known as the circle of least
There are two techniques for simulating refractive errors. In the first, an op-tical
system designed to blur is placed directly in front of the observer’s eyes.
Depending on the observer’s accommodation range, the eye may be able to bring
the image into focus. This technique is known as the “observer” method. The
second technique is to defocus an image as it is projected onto a screen by placing
an optical system in front of the user. The eye is unable to bring the image into
focus because of the intervention of the screen. This technique is known as the
A metric for the level of defocus in , is the defocused point-spread function.
The level of defocus is measured as the size of the image it forms on the retina,
known as the defocus blur disc (or simply ‘blur disc’). In practice, the light dis-tribution
is affected by aberrations, diffraction, the refractive error and the pupil
! = D L (4.1)
If the complexities of factoring aberrations and diffraction into the light distribution function are ignored, then the size of the defocus disc is cited in Ef-fectDefocus
as equation 4.1. Where D is the entrance pupil diameter and L is
the refractive error for any level of accommodation. The result, !, is the angular
diameter of the blur disc, ie the angle subtended from the eye to the blur disc.
L = j
d + i
didn’t come out well, but check out:
Chan C, Smith G, Jacobs RJ 1985: Simulating refractive errors: source and
observer methods. American Journal of Optometry and Physiological Op-tics,
yes, you do need to track the plane the user is accomodating (the a in the eqn above). we used a cursor system because we didn’t haev eye-tracking h/w