Optical system furnished with a device for increasing its depth of field
To increase the depth of field of an optical system, an inversion of the chromatism is produced. This accommodates the fact that, in natural light, users prefer, for their photographs, taking long distance shots outdoors, where the illuminant is mostly composed of blue, and short-distance shots indoors, where the illuminant is mostly composed of red. While optical devices naturally focus the blue components at a shorter distance than the red components, which is unfavorable, with an inversion of the chromatism, a focus more in keeping with what is required is restored.
1. An optical system processing incoherent light divided into at least two spectral bands, a first spectral band for long wavelengths and a second spectral band for short wavelengths, a center wavelength value of the first spectral band being higher than a center wavelength value of the second spectral band, comprising:
an optical transfer function;
a lens to focus the incoherent light received from an object onto an image plane;
a storage device to store a representation of the light images due to said at least two spectral bands received in the image plane; and
a device to increase a depth of field of the optical system adapted for shots taken outdoors with an exterior light and shots taken indoors with an interior light, the device comprising an optical element interposed between the object and the image plane, the optical element having a longitudinal chromatic inversion function such that a focal length for a blue light is greater than a focal length for a red light; and
wherein the inversion function has the effect of modifying the optical transfer function of the optical system such that the optical transfer function value is higher in both frequency bands with the optical element than without the optical element for an object placed in a first distance range relative to the lens and for exterior light.
2. The optical system of claim 1 , wherein the device is adapted for shots taken at distances greater than 10 meters outdoors with the exterior light and wherein said device is adapted for shots taken up close indoors with the interior light.
3. The optical system of claim 1 , wherein the device is adapted for increasing the depth of field for conditions corresponding to shots taken outdoors with type D65 light or shots taken indoors with type A light.
4. The optical system of claim 1 , further comprising a processor connected to storage device for restoring the representation of the light image received in the image plane, said processor filters the image to provide a sharpness to a spectral band as measured in another spectral band.
5. The optical system of claim 1 , wherein the optical transfer function value is higher in both spectral bands with the optical element than without the optical element for an object placed in a second distance range relative to the lens and for interior light.
6. The optical system of claim 1 , wherein the optical element is a diffractive, refractive or hybrid mask.
7. The optical system of claim 1 , wherein the optical element is placed in a location of a main plane of the optical system, between the object and the storage device.
8. The optical system of claim 1 , wherein the optical element is formed of a doublet with the lens, or by several refractive lenses; and wherein said optical element is made of glass, plastic or a light-refractive material.
9. The optical system of claim 1 , wherein the optical element is placed in a location of an image plane of a main plane of the optical system, between the object and the storage means.
10. The optical system of claim 1 , wherein the storage device comprises a charge transfer device, a film, or a video camera.
11. The optical system of claim 4 , wherein the processor comprises a digital filter.
12. The optical system of claim 1 , wherein a focal length for short wavelengths in the optical system is greater than a focal length for long wavelengths.
13. A method for increasing the depth of field of an optical system for shots taken outdoors with an exterior lighting and shots taken indoors with an interior lighting, comprising the steps of:
processing incoherent light divided into at least two spectral bands by the optical system, a first spectral band for long wavelengths and a second spectral band for short wavelengths, a center wavelength value of the first spectral band being higher than a center wavelength value of the second spectral band, the optical system comprising an optical transfer function, a lens to focus the incoherent light received from an object onto an image plane, and a storage device to store a representation of the light images due to said at least two spectral bands received in the image plane;
interposing an optical element between the object and the image plane, the optical element comprising a longitudinal chromatic inversion function such that a focal length for a blue light is greater than a focal length for a red light, the inversion function having the effect of modifying the optical transfer function of the optical system such that the optical transfer function value is higher in both frequency bands with the optical element than without the optical element for an object placed in a first distance range relative to the lens and for exterior light.