TUTORIAL – 2
ALL ABOUT LENS AND FOCAL LENGTH SIMPLIFIED
PART I : FOCAL LENGTH & LENS
FOCAL LENGTH: Whenever we talk or think about lens certain numbers like 50mm, 80mm, 100mm etc., comes in our mind. These numbers refer to ‘Focal Length’ of a lens in millimeters. Normally Focal Length(FL) is not the actual length of a lens but a measurement of the distance between the optical center of a lens and focal point at film plane/image sensor of a camera when focused at infinity. Optical Center is a point within a lens at which light rays coming from the object outside converge to form sharp image on sensor. The lenses having shorter focal length provide wide field of view with less magnification and lenses of longer focal length give narrow field of view but larger magnification. ( But in microscopy in which object is closer to lens, the shorter FL presents greater magnification – WKP).
See diagram given below.
CROP FACTOR: When any full frame lens ( EF, FX ) is used onto a crop sensor body of camera (DX Format) we have cropping effect which is approximately 1.6. This means that a photo taken at 35mm on DX DSLR (having crop sensor) will be closer to 50mm image. The size of full frame sensor is approximately 36mm(horizontal) X24mm(vertical) equal to 35mm camera frame. The size of standard crop sensor APS-C used in DX cameras is approximately 24x16mm.
LENS: A camera lens is comprised of numbers of elements and groups. Elements are the individual glass lens elements within the lens itself while groups are either separate elements or two or more elements cemented together. Suppose two elements are fixed together in a lens with six elements then it is a lens with six elements and five groups. Glass is the most common material used to construct lens elements but plastics like acrylic is also used in manufacturing of strongly aspherical lens elements which are difficult to manufacture in glass. But plastics are not used for outermost elements. Each of these elements directs the path of light rays, coming from the object outside, in such a way to form the image as accurately as possible on the digital sensor. The purpose of elements is to minimize aberrations. Now a days most lenses are multi-coated to minimize lens flare other unwanted effects. Normally a high quality lens has more number of elements and groups. For example Canon 70-200mm f/4L IS with 20 elements and 15 group is considered the best lens of it’s kind for it’s sharpness, fast focusing and fine image quality by professionals. Similarly Nikon 14-24mm with 14 elements and 11 groups is known as excellent wide angle zoom lens for it’s sharpness and minimal softness along the edges. And also Sigma 50mm f/1.4 DG with 13 elements in 8 groups prime lens known for it’s super sharpness and fast auto-focus. But it is not always true as Canon EF 55-200mm f/4.5-5.6 II USM (12 elements and 10 groups) and Nikon 24-120mm f/3.5-5.6 AF VR ( 15 elements in 13 groups) are poor in sharpness and produce dark corners (vignetting).
To be Continued in Part – II
About the Author:
A.FIAP, Hon FIP, Hon.ECPA
M.A.Philosophy (Gold Medalist), Ph.D., Retd.professor & Head, Dept Of Philosophy. Photography is his passion. Got more than 200 awards including Kodak, Nikon & 10 Gold medals. 20 slide shows at Singapore, MIT U.S.A., Mumbai. Ahmadabad, Varanasi,Indore etc. Published pics in ‘Popular Photography’ U.S.A., Practical Photography. Amateur Photographer England, Invited as a member of jury by Lalit Kala Academy, Lucknow and Ahmadabad and about 60 International salons held in India.