A filter which only passes light vibrating in a certain direction. Since the vibrational locus of the light allowed to pass through the filter is linear in nature, the filter is called a linear polarizing filter. This type of filter eliminates reflections from glass and water the same way as a circular polarizing filter, but it cannot be used effectively with most auto exposure and autofocus cameras as it will cause exposure errors in AE cameras equipped with TTL metering systems using half-mirrors, and will cause focusing errors in AF cameras incorporating AF rangefinding systems using half-mirrors.

Macro lenses are essential for shooting close-ups of flowers, insects and other small items at life-size magnification or larger. Quality optical characteristics, sharp definition and true color fidelity combine to capture the appeal of your subject with bold realism.

The distance from the front edge of the lens barrel to the film plane.

The micro USM is an advanced motor developed as a "multi-purpose miniature ultrasonic motor", and its features are as follows.

Advantages over ring-type USMs
The micro USM carries no lens diameter restrictions, so it can be incorporated into a wide variety of lens designs. The stator, rotor and output gear are integrated into a single compact unit that is roughly half the size and weight of a ring-type USM. And the cost is only about 1/3 that of the already reasonably priced ring-type USM.

Advantages over DC micro motors
Low-speed, high-torque characteristics make it possible to set the AF drive gear ratio low enough to realize quiet drive operation, (roughly 1/4 the level of a DC micro motor). The micro USM also features excellent start/stop response and control, and the large holding torque provides superior focus positioning precision.

MTF charts (short for Modulation Transfer Function) provide a graph analyzing a lens’ ability to resolve sharp details in very fine sets of parallel lines, and a lens’ contrast or ability to provide a sharp transfer between light and dark areas in sets of thicker parallel lines. Fine repeating line sets are created parallel to a diagonal line running from corner to corner of the 35mm frame, directly through the exact center of the image area. These are called sagittal lines, sometimes designated “S” on Canon’s MTF charts. At a 90° angle to these, additional sets of repeating lines are drawn, called Meridional (or “M”) line sets. Repeating extremely fine short parallel lines spaced at 30 lines per millimeter measure the lens’ ability to record fine details, or its resolution. Even more important in the eyes of many optical designers is the lens’ contrast capability, which is measured with thicker sets of parallel repeating lines drawn at 10 lines per millimeter. At first glance, it would appear that any good lens would record lines running parallel to a diagonal drawn across the film with the same accuracy as lines drawn perpendicular to them. However, in real-world testing, this is often not the case. Especially in the Meridional direction, faithful reproduction of fine line sets becomes increasingly difficult as you move away from the center of the image toward one of the corners. And it’s a fact that almost all lenses produce sharper results in general near the center of the frame than at the outer edges. MTF charts display the lens’ performance from center to corner. Running along the chart’s horizontal axis, labeled 0 to over 20, is the distance from the dead center (“0”) of a 35mm image along a diagonal line to the corner of the frame, which is about 21.5mm away. On the chart’s vertical axis is a scale representing the degree of accuracy with which the fine and coarse line sets are reproduced, in both the sagittal (parallel to the diagonal of the film format) and meridonal directions. Solid lines on the MTF charts indicate the performance of sagittal lines (parallel to the diagonal of the film), dashed lines are for the perpendicular meridional test target lines. In theory, a perfect lens would produce nothing but straight horizontal lines across the very top of an MTF chart, indicating 100% accurate reproduction from the center of the picture (toward the left of the chart) to its outermost corners (at the right side of the chart). Of course, no such thing as a perfect lens exists from any SLR manufacturer, so MTF charts typically show lines that tend to curve downward as they move left to right (tracking the lens’ performance from center to corner of the frame). Canon’s MTF charts give results at two apertures: wide-open, and stopped down to f/8, with the lens set to infinity focus. While MTF charts don’t include many factors that can be important when selecting a lens (size, cost, handling, closest focusing distances, AF speed, linear distortion, evenness of illumination, and of course features like Image Stabilization which may produce superior real-world results), they can indicate to the knowledgeable reviewer some of the optical characteristics they can expect from a particular lens.

The eye condition in which the image of an infinitely distant point is formed in front of the retina when the eye is in the accommodation rest state.

The eye condition in which the image of an infinitely distant point is formed on the retina when the eye is in the accommodation rest state.

A value used to express the brightness or resolution of a lens' optical system. The numerical aperture, usually indicated as NA, is a numerical value calculated from the formula nsin, where 2 is the angle (angular aperture) at which an object point on the optical axis enters the entrance pupil and n is the index of reflection of the medium in which the object exists. Although not often used with photographic lenses, the NA value is commonly imprinted on the objective lenses of microscopes, where it is used more as an indication of resolution than of brightness. A useful relationship to know is that the NA value is equal to half the inverse of the F number. For example, F 1.0 = NA 0.5, F 1.4 = NA 0.357, F2 = NA 0.25, and so on.

A straight line connecting the center points of the spherical surfaces on each side of a lens. In other words, the optical axis is a hypothetical center line connecting the center of curvature of each lens surface. In photographic lenses comprised of several lens elements, it is of utmost importance for the optical axis of each lens element to be perfectly aligned with the optical axes of all other lens elements. Particularly in zoom lenses, which are constructed of several lens groups that move in a complex manner, extremely precise construction is necessary to maintain proper optical axis alignment.

The entire lens optical system moves straight backward and forward when focusing is carried out. Representative examples of lenses using this type of focusing include the EF 50mm f/1.8 II and TS-E 90mm f/2.8.