Forum Übersicht - A-Bajonett-Objektive » Patente für das 2.8/135 [T4.5] Smooth Trans Focus

Liebe Mitglieder,

ein weiteres bislang einmaliges Objektivdesign hat Minolta 1998 vorgestellt: Das 2,8/135mm [T4,5] Smooth Trans Focus, das mithilfe eines speziellen Apodisationsfilters im Strahlengang für ein optimales Bokeh vor und hinter der Schärfeebene sorgt, ohne daß man dafür Abstriche bei der Schärfe machen müßte. Obwohl das Objektiv für das AF-System rund um das A-Bajonett gedacht ist, unterstützt es keinen Autofokus.

http://www.mhohner.de/sony-minolta/onelens/af135f28stf Minolta 2,8/135 [T4,5] Smooth Trans Focus
http://www.mhohner.de/sony-minolta/onelens/af135f28stfs Sony Alpha 2,8/135 [T4,5] Smooth Trans Focus (SAL-135F28)

http://www.dyxum.com/columns/articles/lens...5F28_review.asp

In der folgenden Patentschrift wird u.a. das konkave Grauglaselement in seiner Anwendung als Apodisationsfilter zur Bokeh-Optimierung beschrieben, wobei der Begriff Bokeh damals noch nicht geläufig war:

US-Patent 3843235, angemeldet von Minolta am 1972-10-24, erteilt am 1974-10-22:

http://www.google.com/patents?id=Li84AAAAEBAJ
ZITATIMAGE FORMING OPTICAL SYSTEM WHEREIN DEFOCUS IMAGES ARE IMPROVED

Abstract

An optical system, such as that used in photographic cameras and microscopes, which forms images of objects, having an optical element of appropriate transmission characteristics for obtaining suitable out-of-focus (defocused) images.

Inventors: Mino; Masayuki (Osaka, JA), Okano; Yukio (Osaka, JA)
Assignee: Minolta Camera Kabushiki Kaisha (Minami-ku, Osaka, JA)
Appl. No.: 05/299,802
Filed: October 24, 1972

[...]

BACKGROUND OF THE INVENTION

The present invention relates to an image forming optical system, and more particularly to such a system having appropriate characteristics for obtaining suitable out-of-focus images.

In the photographic camera art, images of objects not in the plane of best focus may be recorded as blurred on the film, such images being known as out-of-focus or defocused images. Also, in most cameras, an image of a point source not in the best focus plane becomes a disk or circular patch of uniform illumination, namely, a circle of confusion, the defocused image being a mass of circles of confusion. Obviously, such a defocused image often produces an imperfect photograph.

In the past, methods have been devised for avoiding the imperfect defocused image by means of selecting a proper lens surface curvature, or by providing a specially ground portion for the lens surface, or by determining the distances between lens components which may be different from optimum values for in-focus images. With these methods, however, defocused images may be improved for those objects in the proper range, for example in front of the best focused object, but it is impossible to improve all the defocused images. Furthermore, the defocused images of objects in other than the proper range are more difficult to improve so that the best focused image may even be disturbed.

SUMMARY OF THE INVENTION

The primary object of the present invention is to provide an optical system by means of which suitable defocused images for all objects may be obtained even though they may not be in best focus plane, without disturbing the best focused images.

Another object of the present invention is to provide such an optical system by means of which suitable defocused images may be obtained not only for beams parallel with the optical axis but also for beams oblique thereto.

Other objects and a fuller understanding of the present invention may be had by referring to the following description of the preferred embodiments taken in conjunction with the accompanying drawings.

[...]

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The image forming optical system, such as a photographic lens system, is in general a compound lens system of which its aperture is of circular shape or the like determined by an aperture stop provided in the system, the transparency through the aperture being uniform as illustrated in FIG. 3.

In FIG. 1, assuming that an aperture of a lens system L is of circular shape, and that the lens system L has no vignetting and is free from aberration, rays from point source P form a disk of uniform illumination, namely, a circle of confusion, on a plane S which is disposed at a distance. Delta. Z from the conjugate point P' perpendicularly to the optical axis. Since an object is considered to be a collection of point sources, an image of the object on plane S through lens system L, namely, a defocused image, is a mass of circles of confusion, and the imaging property of the lens system for the defocused image is shown as a curve (a) in FIG. 2 which represents an optical transfer function in relation to spatial frequency. The curve (a) shows that the optical transfer function of the conventional lens system for the defocused image has negative value in a certain range of spatial frequency, which shows an inversion of the image. Moreover, the curve (a) shows that there is large difference between the optical transfer function in the spatial frequency S.sub.1 as compared to that in the spatial frequency S.sub.2. This shows that a defocus image of an object through the lens system L is not clear and that the image is unsuitable to produce a good photograph. This is chiefly due to the reason that the transparency T® (see FIG. 3) is uniform in the aperture of the conventional lens system which indicates a transparency distribution of the aperture in a radial direction.

On the other hand, if the transparency distribution of the aperture is made as having the property of decreasing exponentially from center toward the circumference and as being symmetric with the center as shown in FIG. 4, it can be expected that the distribution of illuminance in the circle of confusion may have a similar property to that of the transparency distribution. In this case, a defocused image of a point source is not a circle of confusion with uniform illuminance but of illuminance which is maximum at the center and decreases in proportion of the distance from the center. The image forming property for such an image with a circle of confusion with the non-uniform illuminance is shown as a curve (b) in FIG. 2 when the transparency distribution of the aperture is shown by a formula as follows:

T (r) = (1 - r.sup.2).sup.2

in which T(r) is the transparency and r is the distance in the radial direction.

The should be noted that curves (a) and (b) of FIG. 2 were arrived at under similar conditions regarding the diameter of the aperture and the focal length of the lens system, and the distance. Delta. Z.

The highly favorable condition of the image as defocused in accordance with the present invention is expressed as follows from the standpoint of the optical transfer function:

1. The optical transfer function is a monotonously decreasing function in accordance with an increase in spatial frequency.

2. The optical transfer function does not become negative.

Again, according to FIG. 2, the negative value of the optical transfer function in the curve (b) is small, which means that the response characteristics represented by the curve (b) fulfill the requirements (1) and (2) and can satisfy the aims of the present invention.

On the basis of those facts, the present invention provides an optical system having an optical element with such light absorbing properties that the transparency distribution in the optical system will be as mentioned above. In this manner, it is possible to obtain a highly favorable condition of a defocus image.

In these cases it can be understood from FIG. 1 that if the distance. Delta. Z is increased or decreased, only the radius. Rho. of the circle of confusion changes without the distribution of illuminance. Consequently, as can be seen by curves (a) and (b) in FIG. 2, which represent the response characteristics for the defocused image, the form of curves (a) and (b) and the relation between them do not change in spite of change of the distance. Delta. Z since they only extend or contract in the direction of the lateral axis with such change.

Therefore, the present invention is able to provide favorable defocused images of objects existing in various distances from the best focus plane.

The curve (b) in FIG. 2 has a higher value than the curve (a) in the low frequency range until the curve (b) meets the abscissa, which indicates that the defocused image according to the present invention is less fuzzy than that of the conventional lens system.

However, for the purpose of the present invention, the property of transparency distribution need not be limited within the form as shown in FIG. 4. Various forms as shown in FIGS. 10 to 12 are available which depict responses to the desired state of various defocused images insofar as they are symmetric about the center with the transparency decreasing with an increase in distance in the radial direction.

Then as for achieving this absorbing property, such means are cited as follows:

A.sub.1. adhering photo-absortive material to the surface of a plane-parallel transparent plate or a lens in the optical system by means of vacuum evaporation, said plate being used as a density filter,

A.sub.2. coating photosensitive material over the surface of the plate or the lens and exposing the material to light having an intensity so varied with a portion of the surface on which the light impinges that the degree of blackening of the material may be made variable with such portion.

Accordingly, it is possible to obtain various desirable properties of transparency distribution, although they may not be suited for manufacturing production. Then, if such properties are allowable for the transparency distribution is as follows:

T (r). Apprxeq. exp (.alpha. r.sup.2)

T (): transparency

.alpha. : constant with a negative value

r : distance from the center

The property can be achieved by disposing an optical concave lens in the optical system with the proper degree of absorption which is uniform throughout the material. Such may be made readily available for manufacturing.

FIG. 5 shows a triplet type lens system as one embodiment in which the concave lens is made of the optical material as mentioned above.

FIG. 6 shows a density filter consisted of a planoconcave lens and a planoconvex lens cemented to one another along their curved surface to produce a doublet lens.

It should be noted that the lens and the filter which has the proper transparency distribution as mentioned above are generally called "optical elements" hereinafter.

FIG. 7 shows the location element of a filter as the optical element disposed in a triplet lens system in which 1 is a first biconvex lens, 2 is a biconcave lens, 3 is a biconvex lens, and 5 designates an image plane. Various setting positions are available for the filter such as in front, intermediate and to the rear of the lens system. When the filter is disposed in front of the lens system as shown at 4 in FIG. 7, the first biconvex lens 1 serves as a pupil for the triplet lens for light beams M parallel with the optical axis so that light beams limited by reference numerals 41 to 42 through filter 4 are transmitted through the triplet lens to the image plane 5. On the other hand, for light beams N impinging obliquely to the lens system, there is a vignetting effect produced by the second biconvex lens 3, although the light beams which transmit the triplet lens system between 43 and 44 can reach the image plane 5. In other words, in FIG. 8 which shows the filter 4 seen from the left in FIG. 7, the light beams M through the lens system are limited by the circular arc between 41 and 42 and the light beams N through the lens system are limited by the arc from 43 to 44. Since the distribution of illuminance in the circle of confusion and the form of the circle are similar to those of transparency in a portion of the filter through which light rays for forming the circle of confusion passes, the circle of confusion formed by said oblique light beam N is distorted and the distribution of illumination in the circle is not symmetric. Consequently, defocused images are disturbed. For avoiding these drawbacks and fully achieving an improvement of the defocused image according to the present invention, means are employed so as to restrict the area in the filter through which light beams may pass, namely, the aperture of the filter. In other words, the transmission distribution must be given within the pupil of the optical system and the optical element must be in the pupil. Such means comprises a light limiting member as shown by 45 in FIG. 9 consisting of an aperture in an opaque body and which is disposed closely to the filter. Here, the radius of said aperture should be less than the distance 40 to 43 in the filter so that vignetting may not occur for the light beam to be most oblique that can be focused on the area within a given frame on the image plane with its outermost ray passing through point 43. (The outermost ray is defined as a ray incident to the first surface of the system heading for outermost point P.sub.o on image plane 5 for the light beams N.) Accordingly, a favorable circle of confusion can be achieved with rotational symmetric distribution of illuminance, with the quantity of light reaching the image plane being reduced by the limiting member 45.

When the filter 4 is disposed within the system at a location shown by dotted line 6 in FIG. 7, i.e., through the point where the principal ray of oblique light beam N crosses the optical axis, vignetting does not occur even for the oblique light beam without said light limiting member. In this case, if the diameter of the filter is equal to the pupil of the system, the most quantity of light led to the lens system will be able to reach the image plane.

As for a lens system of a photographic camera, a diaphragm for controlling light incident to a film is used and is disposed as an optical stop, for example, in the position as mentioned above. Consequently, if the optical element is adopted for such a lens system of the photographic camera, the optical element and the diaphragm should be disposed closely to one another. However, in this case, the feature of transparency distribution on the optical element changes in response to diaphragm aperture. Therefore, when it is desired that the light incident to the film is controlled without disturbing the feature of the transparency distribution, a neutral density filter or a pair of polarizing filters may be used instead of the diaphragm. Otherwise, a plurality of optical elements which are different in sizes of their effective apertures but are similar in each property of transparency distribution may be prepared for the lens system, and light controlling is carried out by selecting a suitable one from them.

Furthermore, it is also possible to adopt such a means for light controlling so as to exchange optical elements wherein every one of which has different property of transparent distribution from another as shown, for example, in FIGS. 10 to 12 respectively. It is also possible for the light control to adopt a plurality of the above-mentioned means by combining it.

It should be noted that the invention has been described mainly for the triplet type lens system although the application of the invention need not be limited to such a specific lens type. It is therefore possible to apply the invention to various types of lens systems such as the Gauss type, the telephoto type and so on for improvement of the property for the defocused image.

According to the present invention, since the original features of objects are retained in the defocused images without unreasonable modification, it is possible to ascertain the outline of original features from the defocused images which are converged in front of or behind the image plane. Accordingly, the present invention is available not only for a photographic lens system but also for a microscope objective lens, a lens system of a projector, and the like, for observation and/or measurement of optical images.

[...][/quote]

Das folgende Patent geht ebenfalls auf die Verwendung eines Apodisationsfilters zur Bokeh-Optimierung ein, beschreibt darüber hinaus aber auch spezielle Filter, mit denen andere Effekte möglich wären. Besonders interessant finde ich den Schnitt durch ein Objektiv (Fig. 21), der unnötig detailliert für eine Patentschrift erscheint und vermuten läßt, daß es sich nicht nur um eine reine Fiktion zur Erläuterung des Prinzips handelt. Mir ist aber kein Objektiv bekannt, das dieser Abbildung entsprechen würde. Kennt Ihr eines?

US-Patent 4013347, angemeldet von Minolta am 1975-05-02, erteilt am 1977-03-22:

http://www.google.com/patents?id=W4wsAAAAEBAJ
ZITATMovable filter optical system for providing improved optical transmission characteristics

Abstract

An image forming optical system, such as a photographic objective lens assembly is provided with a movable optical element. The movable optical element is designed and mounted so that it can optically interact with the transmission rays of the optical system or permit the rays to pass unaffected. The optical element can be a filter capable of improving the image forming characteristics of target object image to provide an advantageous soft focus, for example, as a portrait lens system or to provide a soft defocussed image. The optical filter element can have a central region capable of passing the converging or diverging transmitted rays through the optical system substantially free of refraction and a peripheral region capable of decreasing the transmitted image point intensity of rays transmitted therethrough. The optical filter element can be movable along the optical axis within a predetermined range of movement. This range of movement includes one position wherein the central region of the optical filter element will transmit all the incident rays. Various types of optical filters can be used such as filters with a peripheral region shaded to vary transmission intensity or given a refractive power.

Inventors: Nakamura; Akiyoshi (Sakai, JA)
Assignee: Minolta Camera Kabushiki Kaisha (Osaka, JA)
Appl. No.: 05/573,917
Filed: May 2, 1975

[...]

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to an image forming optical system and more particularly to means for improving the image forming characteristics with a unique filter structure and mounting.

2. Description of the Prior Art

The desirability of improving the imaging characteristics of various lens systems are well known in the prior art. For example, it is known to use a soft focus filter to produce a faithful but soft tone portrait in the field of photography. Frequently, optical elements are permanently mounted in the lens barrel, traversely removable from the lens barrel or attached to the end of the lens barrel.

Also, it is highly desirable to improve the images that are not conjugate to the image or film plane. For example, the background objects behind a principal target object must supplement or complement the photograph of the principal image while still retaining a general outline that is softly blended.

An image can be defined by a spatial distribution of illumination existing in a determined plane which is the plane of the image. It has been known to determine the image properties of an optical system not only by the phase of the pupil function but also by its amplitude transmittance. For any optical system, a family of curves may be established which express the transmission factor as a function of the spatial frequency or wavelength. By the use of Fourier transformation the imaging properties of an optical system can be measured by the optical transfer function. Any optical system and therefore any optical instrument, may thus be considered as a transmitter of signals corresponding to sinusoidal functions in which the function is the luminance of a point and the variable is the position of the point. It is known that by decreasing the transmitted image point intensity of a defocussed image the photographic characteristics of the image can be improved. This improvement can be realized in other optical systems than cameras, such as microscopes and telescopes.

The prior art has suggested various ways of improving the optical transfer function of a defocussed optical system such as the Mino et al U.S. Pat. No. 3,843,235 wherein a filter is provided having a characteristic of providing an optical transfer function of a monotonously decreasing function in accordance with an increase in spatial frequency. Further the optical transfer function must remain essentially positive.

Various optical systems have been suggested for providing a filter to provide a desired image such as the Tsujiuchi U.S. Pat. No. 3,045,530, Marechal et al U.S. Pat. No. 3,090,281, Sayanagi U.S. Pat. No. 2,959,105, Nomarski U.S. Pat. No. 3,476,457, and Land U.S. Pat. No. 3,397,023.

As noted above, a photographic lens assembly is generally evaluated in accordance with the sharpness of a focussed image on a film plane. However, the commercial acceptability of a photographic picture frequently requires images of photographic objects located both frontwards and rearwards of a sharply defined target photographic object to have a pleasing natural appearing blurred image. In some cases it is even desired to provide a blur or soft defocussed image for the target photographic object itself. The Mino et al patent mentioned above, is representative of one such apodization optical system.

The combining however, of such a filter element in a commercial lens system has caused problems, for example, in focussing and setting the diaphragm aperture. Generally, the optical filters have been directly attached to the exterior lens or lens barrel or affixed at one position in the interlens space of the optical system. The prior art has not provided a versatile commercially acceptable lens in a compact mode which permits a selective utilization of a filter optical element.

SUMMARY OF THE INVENTION

The present invention provides an improvement in an optical system for forming images of objects. In particular, a movable optical element is movably mounted along the optical axis so that it can either actively alter the transmission properties of the optical system or remain passive and permit the transmission rays to pass unaltered. The present invention is highly useful in photographic objective lens assemblies for the improvement of defocussed images. The optical lens assembly of the present invention can incorporate an optical filter element having a central region capable of passing the transmitted rays substantially free of refraction and a peripheral region having a filter characteristic of improving the defocussed image by decreasing the transmitted point intensity of the defocussed image. The present invention can also provide a soft focus image for the target photographic object. Mounting means are provided for moving the optical filter element relative to the lens along the optical axis for a predetermined range of movement. This range of movement advantageously includes one position wherein the central region encompasses the entire bundle of rays or flux transmitted through the objective lens assembly to permit the formation of substantially unfiltered images. At other positions along the range of movement, the peripheral region can intersect and decrease the transmitted image point intensity or distribution, for example, proportional to the radial direction off the optical axis. Various forms of optical filter elements can be utilized such as those having an absorbing coating or refractive power.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The present invention, both as to its organization and manner of operation, together with further objects and advantages thereof, may best be understood by reference to the following description, taken in conjunction with the accompanying drawings.

[...][/quote]
Erstaunlich finde ich, daß diese Patente bereits 1972 bis 1975 angemeldet wurden, das erste Objektiv, das einen solchen konkaven Graufilter benutzt, aber erst 1998 zum 70-jährigen Firmenjubiläum von Minolta vorgestellt wurde. Inzwischen sind weitere zehn Jahre ins Land gegangen, ohne daß dieses Prinzip sich in anderen Designs manifestiert hätte. Allerdings besaß die 1999 vorgestellte Minolta Dynax 7 eine spezielle STF-Funktion, die unter bestimmten Voraussetzungen mit jedem Objektiv einen Smooth Trans Focus-Effekt erzeugen konnte. Wenigstens hat Sony das STF-Objektiv nicht aus dem Programm genommen.

In jedem Fall sind die beiden Patente, wie ich finde, ein eindrucksvolles Zeugnis dafür, daß Minolta sich schon vor etlichen Jahrzehnten sehr intensiv mit dem Thema Bokeh beschäftigt hat.

Viele Grüße,

Matthias

PS. Siehe auch: http://www.dyxum.com/dforum/forum_posts.as...D=443220#443220