The Tamron Focal Length Comparison Tool simulates the effect of changes in focal length. This tool is particularly useful to experience the range of versatility of various zooms lenses—offering a practical sense of the perspectives you can expect. In addition, the tool allows you to see the difference in resultant angle of view when using a given focal length on 35mm film or full-frame sensor DSLRs or DSLRs with smaller (APS-C-sized) sensors—often referred to as a crop factor or a focal length equivalent.
Focal length is the distance between a lens’ nodal point and the camera's image plane (in millimeters). Its numerical value offers a sense of a lens’ angle-of-view characteristics (broad for wide angle or narrow for telephoto.) A focal length approximating the diagonal dimension of the camera’s image plane will render an angle-of-view with negligible magnification—similar to normal human vision. Focal lengths numerically lower than normal will render negative magnification, resulting in wider angles-of-view (wide angle), while those numerically greater than normal render positive magnification, producing narrower angles-of-view (telephoto.) Zoom lens focal lengths are expressed as a range in a pair of focal length values, leading with the numerically smaller of the two values, e.g., 18-200 mm. However, depending on the format of the camera to which the lens is attached, the relationship between focal length and angle-of-view can vary.
Considering camera format. An individual 35mm film segment is approximately 36 mm x 24 mm, as is a full-frame DSLR’s sensor. Both measure approximately 43.5 mm* diagonally. Thus, lenses ranging from 45 to 50mm and attached to these camera formats are considered normal. A typical APS-C digital SLR’s sensor is approximately 25.1 mm x 16.7 mm and 30.5 mm diagonally, requiring a 30mm lens to produce a normal angle-of-view. This differential is commonly referred to as crop factor—a multiplier that can be applied to a lens’ actual focal length to help gauge the effective focal length equivalent of lens attached to a camera with an image plane smaller than 36 mm x 24 mm. For example, an APS-C DSLR with a 300 mm lens attached will render an angle of view approximating that of a 465 mm lens on a 35mm film camera or full-frame DSLR.
*Focal lengths, image plane dimensions and calculations are approximate and for example purposes only.