Formulaic Conundrum


The focal length of a lens is calculated as the distance between the lens and the image plane, when focused at infinity. If that is the formula, why is the length of my 200 mm lens only 136 mm?                   

Dhruv Solanki, via e-mail                                                                                                                                                                                                                   

What you mentioned stands true for simple lens designs using a single lens element. Here the barrel literally has to be almost the length of the focal length for focus to be achieved on the film or sensor plane, with the lens focused at infinity.

The highest quality lenses in the world are constructed this way, with single focal lengths and relatively simple optics. However, most lenses today have complex optical constructions. They use refraction and multiple lens elements to bend the light, to reduce the length of the lens.

To reduce the elongation of the barrel needed to focus at closer distances, different lens groups within the lens move in relation to each other. The resultant aberrations caused by complex designs are then taken care of with special lens coatings, aspherical elements, and glass made of materials with low refractive indexes. An example here would be lenses that move inwards towards the middle of the zoom range and out again at the extreme ends of the zoom range, as opposed to the push/pull zoom lenses of old. While the formula holds true for simple lenses, there are many factors that affect calculating precise parameters for complex lenses.

Tags: better photography, formula, formulaic conundrum, Lenses, May 2015, Question of the Month