Since it is difficult to estimate the error, most estimates being much less than the actual value, the error should be measured by bringing the top and bottom lines into coincidence so that the error can be measured at the bottom. Since frame lines are not perfectly centered between perforations, the frame line must not be used for this purpose.

    Blend line positions are controlled in printing with the same target material in the routine laboratory checks and by strips of this target material in the leader of every roll. It is controlled so that the blend is best when the lines are coincident in the center of the frame.

    It is worth noting the slight distortion of the vertical line of the target. Fig. 10 This represents the combined complex distortions of the camera and projector lenses with a slight additional pincushion contribution from the fact that the screen itself bows toward the audience along the matchline rather than hanging perfectly straight. This represents the kind of mismatching that is always present in the picture.

    The blend line however is put on the film in a perfectly straight line and therefore only the projector lens distortion and the screen shape determine the uniformity of the blend from top to center to bottom of the picture. The camera lens does not contribute its distortion in this case depending on how well the projector lens distortion cancels the distortion arising from the bowing of the screen which is variable depending on screen size, material, tension, age, lens focal length, and projector layout, it is possible to have the matchline go light or dark at top or bottom when the center is correctly adjusted. Fig. 10


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