LENS KNOWLEDGE (101)

Still in the mood of NAB 2011 i would like to share with you some basic knowledge i culled up from one of my resources about lens. 

●Angle of view and focal lengthThe angle of view represents the subject range that can be imaged with the image sensor, and is expressed in degrees. The shorter the focal length of the lens, the greater the angle of view (i.e. the lens is wide-angle), and the longer the focal length, the smaller the angle of view (i.e. the lens is telephoto). The angle of view is determined by the lens' focal length and the image sensor's diagonal length, and usually expressed with the following formula.

With the Four Thirds system, the diagonal length of the image sensor is about 1/2 that of 35mm film. This facilitates the conversion of the focal length required for a given angle, which is about 1/2 of the focal length of the 35mm film camera lenses. Users who are accustomed to the traditional 35mm film camera can easily estimate the angle of view of a Four Thirds camera (for example, a 14-50mm lens for Four Thirds camera is equivalent to a 28-100mm lens for 35mm film camera). 

●F-numberF-number represents the brightness of the lens. A bright lens has a low F-number and a darker lens has a higher F-number. The F-number is expressed with the formula shown above. In general, the advantages of a bright lens include; 1) smooth defocusing; 2) quality imaging at high shutter speeds, in addition to a bright and easy-to-see viewfinder image.

●Depth of fieldDepth of field is the distance in front of and behind the subject which appears to be in focus. The depth of field in front of the subject is not the same as that behind the subject. Usually, the former is smaller than the latter. The depth of field also increases when the focal length of the lens is short (wide-angle), the shooting distance is long or the lens aperture is stopped down. Therefore, enhancing these factors make it possible to increase the depth of field and attenuating them makes it possible to decrease the depth of field.

●PerspectivePerspective refers to the appearance of distance between a nearby subject and faraway subject (the visual effect expressing the distance between them). With a wider-angle lens, the image sizes of nearer subjects increase and the distance of the background is exaggerated. A telephoto lens, on the other hand, compresses distance so the distance between the subject and background is lost.

●Image magnificationMagnification is the value representing the size of the image of a subject captured by the image sensor compared to the actual size of the subject. The magnification at which the subject image is largest is referred to as the maximum image magnification. For example, an image sensor that can capture the image of a subject in life size has 1x magnification, while an image sensor that can capture the image only at half size has 0.5x magnification. Image capturing with a high magnification is usually possible with a macro lens, but it is also possible to increase the magnification of an existing lens with a simplified method using an extension tube or teleconverter.

(Note)
The maximum image magnification of a Four Thirds lens can be converted into the maximum image magnification of a 35mm film camera lens by multiplying "the indicated maximum image magnification x 2". 

●Difference between focusing distance and working distanceThe focusing distance is the length between the focused position of the subject and the image sensor, and the working distance is the length between the focused position of the subject and the front of the lens. This means that, when the focusing distance is identical, the working distance is smaller when a lens with a larger overall length is used.

●MTFThe MTF (Modulation Transfer Function) curve is a lens performance indicator that represents how faithfully a lens can reproduce the contrast of the subject. The MTF curve is drawn by taking the contrast reproduction rate (%) on the vertical axis and the distance from the image center (mm) on the horizontal axis. Evaluation is performed with a set of two curves, including the S (Sagittal) curve in the radial direction and the M (Meridional) cover in the concentric circle direction. The MTF curve chart shown here shows how the contrasts of an image containing 20 sine waves per mm and an image containing 60 sine waves per mm can be reproduced. In general, the curve for lower frequency approaches 100% when the lens has a high contrast, and that for higher frequency is high when the lens has high resolution. The frequencies used for this evaluation vary depending on the design policies of manufacturers.