Magazine article American Cinematographer

Cinema Workshop

Magazine article American Cinematographer

Cinema Workshop

Article excerpt

IMAGE SHARPNESS

Feminine mystique appears to be a popular phrase these days, but among cinematographers there seems to be more mystique associated with lenses. There are so many different parameters that must be considered when designing a lens and an equally staggering number of variables must be considered when determining the performance of a lens. When evaluating a lens, one must consider chromatic aberration, spherical aberration, astigmatism, acuteness, resolution, contrast and that all-encompassing question: "Yeah, but is it sharp?"

Rather than looking at lenses from the designer's point of view (what makes a lens sharp?), we'll take a look from the cameraman's angle (so how come the image is soft?).

The problem of getting a sharp image through the lens has been aggravated by the complexity of the zoom lens. When developing a fixed-focal-length lens, the designer can optimize parameters for that particular focal length. Not so with a zoom lens. The designer of a zoom lens is constantly faced with compromises and trade-offs. It is impossible to design a zoom lens that will compensate all aberrations equally throughout the zoom range. It is true that every zoom lens will exhibit varying degrees of aberration correction as the focal length is changed.

Actually, a zoom lens will display its best compensation at only two or three specific focal lengths. At all other values during the zoom, correction will be plus or minus these optimum parameters. So, if a zoom lens appears to be sharper at certain focal lengths, one may assume it is a problem with the original lens design - right? Not quite.

Rarely can a soft image be blamed on this phenomenon of zoom lens design. Technology has reached the level where these deviations from optimum correction are kept to a minimum and only in rare and exacting circumstances will these deviations be visually apparent. In most cases a less than perfect image is the result of damage to the lens, improper lens seating or improper use of the lens.

In many cases it is within the power of the cameramen to improve the image through the lens. Every lens has an optimum aperture. This is usually between two and three stops down from wide open. For an f/1.8 lens, the optimum aperture would be about f/4. True, there will be a greater depth-offield at smaller apertures, but the image will not be sharper. As the lens is stopped down, a phenomenon known as diffraction begins to degrade the image.

Diffraction occurs around the edge of the iris blades and can best be described as a bending of the light rays. …

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