Focal Length Comparison Tool
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 lensesoffering 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) sensorsoften 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.