The present invention relates to the field of photography and, more specifically, to automatic exposure control systems and methods usable in cameras for regulating the transmission of image-forming light rays from a scene to the camera film plane and configured for both ambient mode operation for making exposures when the scene to be photographed is illuminated by constant intensity natural available light, and a flash and fill flash mode of operation when scene illumination comprises varying proportional mixtures of ambient light and the transient light output from a combustible filament photoflash lamp.
Of particular interest are those exposure control systems that employ a dynamic aperture or scanning shutter operated under the control of a photoelectric circuit that measures and integrates scene brightness level and provides a trigger signal at a predetermined percentage of optimum exposure value to initiate the shutter closing phase of the film exposure cycle.
Typically, such dynamic aperture shutters include a multi blade mechanism that is displaceable between a light blocking first arrangement where the exposure aperture is closed and a second arrangement where the blades cooperate to define a maximum available exposure aperture. As the blade open, the area of the exposure aperture progressively increases until a peak aperture is reached and then the direction of blade displacement is reversed causing the exposure aperture area to progressively decrease until it closes to terminate the exposure interval. The peak aperture is the largest aperture opening that is achieved during the course of an exposure interval and it may be smaller than the maximum available aperture. For ambient mode operation, the peak aperture generally is correlated to scene brightness level. For flash mode operation, peak aperture typically is selected in accordance with both camera-to-scene distance and brightness inputs.
The operation of the blade mechanism may be graphically depicted by plotting an aperture size (area) versus time (exposure interval) trajectory curve which provides both quantitative and qualitative information about the nature of the film exposure. When the scene brightness level is known, the integrated area under the trajectory curve is indicative of the total amount of light (exposure value) that reaches the film plane. The general shape of the curve characterizes the exposure in terms of depth of field and motion stopping ability. For example, one trajectory curve may have a shutter opening portion that rises quickly to a relatively large peak aperture followed by a similar shutter closing portion. The exposure interval will be relatively short, providing good motion stopping ability, but depth of field will suffer somewhat because of the relatively large effective or average exposure aperture. For the same scene lighting conditions and exposure value, the shutter may be operated in a different manner so that the trajectory rises more slowly to a lower peak aperture, for good depth of field, but the exposure interval will have to be increased accordingly so motion stopping ability will suffer. The shape of a trajectory suitable for general picture taking situations will fall somewhere between these two exremes in an attempt to effect a balance between the depth of field and motion stopping ability parameters.
During ambient mode operation, the light detection and integration circuit monitors scene brightness level as the blades open and provides a signal at a predetermined percentage of optimum exposure value to trigger the blade closing phase. Because of inertial characteristics of the blade mechanism and its associated drive system, it is impossible to achieve instant blade closure so there will be some finite time between the provision of the trigger signal and movement of the blade mechanism to its fully closed position during which the exposure will continue. If the blade mechanism has not reached the maximum exposure aperture defining position where the blades are at rest when the trigger signal is provided, it will also take some time for blade deceleration and reversal of the drive direction. In other words, there will be some overshoot time which also must be accounted for to achieve total control over the exposure process.
If the operating characteristics of the shutter system are well defined and are repeatable, the general shape and area bounded by the portion of the trajectory curve that occurs after the provision of the trigger signal may be accurately predicated or anticipated. Because the scene brightness level is essentially constant over the entire course of the exposure interval for ambient exposures, the additional exposure that will occur after the trigger signal may be accurately predicated. Therefore, for ambient mode operation, there is no problem in providing the trigger signal at a predetermined percentage of optimum exposure value in anticipation that the remaining exposure will take place during the deceleration, blade reversal and shutter closing phase.
However, during flash mode operation, the scene is illuminated with a mixture of constant level ambient light and a transient burst or pulse of light provided by an artificial source of illumination such as an electronic strobe unit or photoflash lamp. The light output characteristics of a strobe unit generally are highly predictable and quenchable type units can be controlled with great accuracy. Also the light pulse is of very short duration compared to the total exposure interval and it can be fired at any selected point along the trajectory curve so that its occurrence coincides with a particular aperture or small range of apertures which are correlated to camera-to-scene distance. Therefore, this very predictable and controllable source of light presents no great difficulty in anticipating its effect on the exposure even when it occurs or persists into the anticipatory portion of the exposure that takes place after the provision of the trigger signal.
On the other hand, a combustible filament photoflash lamp provides a transitory light output over a relatively long time period and its output cannot be controlled or quenched after it has been fired. The light output characteristics of such photoflash lamps are specified by time-intensity performance or burn curves provided by the lamp manufacturers. Following the provision of an electrical flash fire signal, a fine filament is heated to ignite a primer which in turn ignites a wire or foil filament that produces the light output. Following foil ignition, the light intensity builds up to a peak level and then decays.
Tests have shown that the photoflash lamp performance curves are at best a guide to its actual light output characteristics and there is substantial difference in the output characteristics of lamps manufactured to a given specification. Some lamps tend to burn "hot", rapidly rising to peak intensity and then decaying in a shorter time than specified. Other lamps tend to burn "cool" providing its light output over a longer time than specified. In both cases there also may be a substantial deviation in the total amount of light emitted from the lamp during the course of its burn time. For the purposes of this disclosure the terms "hot" and "cool" indicate that the output characteristics deviate from specification and encompass lamps that vary in burn rate and/or total light output.
In the flash mode of operation, the photoflash lamp is employed as the primary source of scene illumination when the ambient light is very low. As the ambient light level increases, the photoflash lamp is used in a fill flash capacity to soften shadows and/or provide additional illumination to dark areas of back or side lighted scenes. Thus, depending on scene lighting conditions, there will be a varying proportional mixture of ambient and flash contribution to the total exposure.
When the ambient light is very low, regulation of the amount of flash light that reaches the film plane is controlled by correlating the taking aperture to subject distance employing well known follow focus techniques. Gradually, the shutter is opened to a subject distance related peak aperture and is held there while the flash is fired to provide the primary source of scene illumination. Because the lamp output generally is decaying when the trigger signal is provided and the ambient light is very low there is very little additional exposure during the shutter closing phase so the "anticipation" effect is minimal. However, in the fill flash mode, the light output from the flash lamp may be quite intense during the shutter closing and, coupled with the high ambient contribution, the exposure that occurs during that period between the trigger signal and the termination of exposure by the shutter reaching its fully closed position will represent a significant portion of the total exposure.
There have been many attempts in the prior art to deal with the anticipation problem when a photoflash lamp is used for flash mode operation. For example, see commonly-assigned U.S. Pat. No. 4,008,481 wherein the trigger level to initiate shutter closing is set lower for the flash mode than the ambient mode in anticipation of the transient addition of scene light intensity caused by the firing of the photoflash lamp. Commonly-assigned U.S. Pat. Nos.
4,023,187 and 4,188,103 describe systems that employ multilevel trigger circuits and scene distance inputs for providing the shutter closing signal at different points in the exposure cycle depending on whether the ambient or flash light is the dominant constituent of scene illumination. Also, see commonly-assigned U.S. Pat. No.4,047,191 for a system that fires the flash lamp in leading relation to the trajectory curve as another way of anticipating the effect of the flash unit on total exposure.
The anticipation problem becomes even more critical for exposure control systems that are adapted for use with the new higher speed color film units (i.e., 400-600 ASA) that have recently become commercially available for use by amateur photographers. With the higher film speed, it is possible to reduce the total exposure interval for given scene lighting conditions, but this means that the anticipated exposure that occurs after the provision of the trigger signal may represent a larger portion of the total exposure and therefore the shutter closing characteristics will have to be very consistent for anticipation purposes.
The above-noted prior art systems tend to be successful in anticipating the flash effect on exposure over the limited range of camera-to-scene distances and ambient light levels provided that the photoflash lamp output follows or is very close to the specified performance curve. However, if the flash lamp output substantially deviates from the specification or the subject to be photographed is at the close or far end of the effective distance range of the system and/or the ambient light level is quite high, the limits to which the anticipatory exposure can be predicted will be exceeded, thereby resulting in either over or underexposed pictures.
Commonly-assigned copending application U.S. Ser. No. 222,562 (now abandoned and replaced by continuing application U.S. Ser. No. 343,160 filled Jan. 27, 1982), filed Jan. 5, 1981, suggests that the exposure appearing after the provision of the trigger signal can be better anticipated if the photoflash lamp is redesigned to alter its output characteristics to provide the same total output but with a lower peak intensity and an extended burn time. While some improvement in exposure control will result, this system still depends on the lamp having a predictable output characteristic and it is not configured to compensate for deviations if the lamp burn is "hot" or "cool".
A common characteristic of the prior art systems is that there is but a single decision point in the exposure process, i.e., the triggering of the shutter closing phase. Once the trigger signal is provided, a commitment has been made to follow a predetermined trajectory curve to exposure termination. If the scene lighting conditions vary from expectation because of variations in the photoflash lamp output, then the anticipated exposure component will be more or less than necessary to reach optimum exposure value.
Because the output from a photoflash lamp is (a) not controllable and (b) not subject to precise prediction, it is apparent that better control over flash exposures can be achieved by reducing the anticipatory portion of the exposure to a minimum.
One approach is to design the shutter blade mechanism and its associated drive system so that it closes as rapidly as possible thereby allowing the trigger signal to be delayed until a relatively high percentage (80-90%) of optimum exposure value has been achieved so that the anticipatory exposure is but a small percentage of total exposure. This approach tailors or optimizes the trajectory curve for photoflash lamp mode operation. However, this type of trajectory will not necessarily be the best type for ambient mode exposures in that it will not necessarily provide the desired balance between motion stopping ability and depth of field.
The above-noted prior art systems have blade drive subsystems that limit the choice of trajectory shape to a single general shape. The blades are driven open by a spring member at a predetermined rate and are closed at a predetermined rate in response to the retraction of a plunger in a solenid that is energized when the trigger signal is provided.
Other prior art systems employ a stepper motor that is driven at a constant rate for moving the blade mechanism at a predetermined velocity between its closed and open positions. For example, see commonly- assigned U.S. Pat. Nos. 3,950,766 and 3,977,012. Again, because the drive rate is unalterable during the course of the exposure process, the system is limited to providing one general trajectory shape.
Thus, the systems have to make a compromise in trajectory choice. If the trajectory is optimized for ambient mode operation, flash mode operation will suffer somewhat in that the shutter closing phase will not be as fast as possible to minimize the anticipatory portion of the total exposure. Conversely, if the trajectory is designed to favor the flash mode performance, the ambient mode performance will not be optimized.
Commonly-assigned copending applications U.S. Ser. No. 216,831 (now U.S. Pat. No. 4,325,614), filed Dec. 16, 1980, and U.S. Ser. No. 275,718 (now U.S. Pat. No. 4,354,748), filed June 22, 1981, disclose an automatic exposure control system wherein the shutter blade mechanism is driven by a stepper motor that is operated under the control of a microcomputer. The microcomputer includes a trajectory data base which it utilizes to develop a plurality of different trajectory signal programs for the stepper motor to define corresponding different trajectory shapes that are tailored for any number of different desired exposure effects. The first-mentioned application discloses the ambient mode of operation and the second discloses a quench strode flash mode which utilizes an expanded data base that includes strobe flash mode trajectory data along with strobe firing and quenching data. The operation of this system in both the ambient and strobe flash modes is amenable to exposure anticipation because the scene lighting conditions are constant for ambient mode operation and highly controllable for strobe flash mode operation.
However, there is a need for an automatic exposure control system that is versatile enough to provide a wide variety of trajectories that are tailored for particular desired exposure effects including optimizing the flash and fill flash mode of operation when a combustible filament photoflash lamp is used as the source of artificial illumination.
Therefore it is an object of the present invention to provide an automatice exposure control system that is operative to reduce the amount of anticipatory exposure content in a flash/fill flash mode when a photoflash lamp is used as the source of the artificial illumination.
It is another object of the invention to provide such a system that has provisions for automatically adjusting the trajectory curve characterizing the operation of its dynamic aperture shutter in response to the actual light output characteristics of the photoflash lamp.
It is yet another object to provide such system which minimizes the anticipatory portion of total exposure thereby allowing the effective distance and ambient light level range over which good flash performance can be expected to be expanded.
Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter.