VIEWFINDER APERTURE CORRECTION

Abstract: In a viewfinder aperture correction, contours in a viewfinder signal are enhanced (R5, R9) to obtain stressed contours when said viewfinder signal is correctly focused. To accentuate their visibility, and thus to better indicate correct focussing, even when viewed from a distance of about 0.5 m, the stressed contours are modulated (SS, T4) at a given frequency to accentuate their visibility.

Description: Viewfinder aperture correction. The invention relates to a method and a device for carrying out a viewfinder aperture correction, and to a camera having such a device. Cameras which do not have an auto-focus feature, often use an aperture correction for the viewfinder signal in such a manner, that when the picture is correctly focused, contours appear exaggerated. This indicates a correct focusing to the cameraman. The method of US-A-3,657,559 or that described in SMPTE Journal, vol. 78, nr. 4, April 1969, pp. 249-255, could be used to obtain such an aperture correction. Nowadays, portable cameras are sometimes held at hip-height, which means that the viewfinder is at a larger distance from the cameraman, so that he can no longer easily judge the correctness of the focusing by means of the contour-enhanced viewfinder signal. It is, / ' «ter alia, an object of the invention to mitigate this problem. To this end, a first aspect of the invention provides a method as defined in claim 1 and a device as claimed in claim 4. A second aspect of the invention provides a camera as defined in claim 5. Advantageous embodiments are defined in the dependent claims. A primary aspect of the invention provides a viewfinder aperture correction in which contours in a viewfinder signal are enhanced to obtain stressed contours when said viewfinder signal is correctly focused. To accentuate their visibility, and thus to better indicate correct focusing, even when viewed from a distance of about 0.5 m, the stressed contours are modulated at a given frequency to accentuate their visibility. A preferred embodiment is based on the following recognitions. The eye sensitivity for, inter alia, line flicker can advantageously be used to simplify focusing. This property of the eye is used by amplitude-modulating horizontal contours in a picture at a low frequency well visible to the eye. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. The drawing shows an embodiment of a camera having a viewfinder aperture correction in accordance with the invention. In the drawing, a camera signal obtained by a sensor S is preprocessed by a preprocessor PP and applied to a viewfinder aperture correction device through a transformer T. A resistor chain R1-R4 is connected to the output side of the transformer T. A capacitor Cl is connected in parallel to the resistor R4. Contrast is controlled by means of a potentiometer R5 connected in parallel to the resistors R1-R3. The slider of the potentiometer R5 is connected to a base of a PNP transistor TI through a capacitor C2. A current source I is connected between the base of the transistor TI and ground. The emitter of the transistor TI is connected to a base of a PNP transistor T2, and to a 9 V supply voltage through a resistor R6. The collectors of the transistors TI and T2 are connected to ground. The emitter of the transistor T2 is connected to the 9 V supply voltage through a resistor R7, and to ground through a series connection of resistors R8, RIO, a capacitor C5, a resistor R12, and a diode D. An emitter of an NPN transistor T3 is connected to a connection point of the capacitor C5 and the resistor R12. A collector of the transistor T3 is connected to the 9 V supply voltage through a resistor Rl l and to a viewfinder VF. A base of the transistor T3 connected to a slider of a potentiometer R9 which provides for a crispening control. A first main terminal of the potentiometer R9 is connected to a connection point of the resistors R8, RIO. A second main terminal of the potentiometer R9 is connected to ground through a series connection of capacitors C3, C4. An input of a sync separator SS is connected to an output of the transformer T. So far the viewfinder aperture correction device corresponds to the prior art. In accordance with the present invention, a field identification output signal of the sync separator SS, which has a frequency of 25 Hz (PAL) or 30 Hz (NTSC), is applied to a gate of a transistor T4 through a resistor R13. Main terminals of the transistor T4 are connected to a connection point of the capacitors C3 and C4, and to ground, respectively. A switch S is connected between the gate of the transistor T4 and a 5V supply voltage. If the switch SW is closed, the operation of the viewfinder aperture correction device is like that of the prior art device. However, if the switch SW is open, horizontal contours in the image flash clearly visible at the frame rate when the camera has been correctly focused, thus making correct focusing far better visible than in the prior art, even at eye-hip distance when the portable camera is held at hip height. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. Application of the invention is not limited to horizontal contours only. The given frequency may be any clearly visible frequency; the field frequency would also do. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The invention can be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means can be embodied by one and the same item of hardware.

Claims: CLAIMS: 1. A method of viewfinder aperture correction, comprising the steps of: providing (T) a viewfinder signal; stressing (R5, R9) contours in said viewfinder signal to obtain stressed contours when said viewfinder signal is correctly focused; and characterized by modulating (SS, T4) said stressed contours at a given frequency to accentuate their visibility. 2. A method as claimed in claim 1, wherein said modulating step affects the horizontal contours. 3. A method as claimed in claim 1, wherein said given frequency equals a frame frequency of said viewfinder signal. 4. A device for carrying out a viewfinder aperture correction, said device comprising: means (T) for providing a viewfinder signal; means (R5, R9) for stressing contours in said viewfinder signal to obtain stressed contours when said viewfinder signal is correctly focused; and characterized by means (SS, T4) for modulating said stressed contours at a given frequency to accentuate their visibility. 5. A camera, comprising: a device (T, R5, R9, SS, T4) for carrying out a viewfinder aperture correction as claimed in claim 4; a viewfinder (VF) for displaying said viewfinder signal with modulated stressed contours.

Inventors: VAN DEN HERIK FLORUS BERNARDUS, VAN DEN HERIK, FLORUS, BERNARDUS
Applicants: KONINKL PHILIPS ELECTRONICS NV, PHILIPS SVENSKA AB, KONINKLIJKE PHILIPS ELECTRONICS N.V., PHILIPS AB
Application year: 1998
Main classification: H04N5
IPC codes: H04N5