The magnifying glass is the most used optical instrument. It is not the prerogative of only Sherlock Holmes-like detectives or lepidopterists. It is commonly used for precision work or to decipher details too small to be seen with the naked eye. Placed in the field of a camera, it provides surprising effects.
The inventor of the magnifying glass is unknown. Archaeologists have found transparent, rounded and polished stones, made more than three thousand years ago. From a scientific point of view, the magnifying glass is a simple optical instrument composed of a biconvex lens, whose two faces are curved. The light rays coming from a distant object pass through the lens. They converge in a point called focus.
The distance between the optical center of the lens and its focus defines its focal length. When a magnifying glass is used, it is always placed near the object to be magnified. The object is then between the focus and the lens. The image appears right side up. If, in place of our eye, we install a camera equipped with a lens (standard or macro, it doesn’t matter), the photographed image is magnified, too, without inversion.
When the object is beyond the focus, at a distance greater than the focal length, the image is reversed. If a camera lens were replaced by a magnifying glass, the image would be projected onto the film or sensor in reverse after focusing, just as a camera lens would. But a biconvex lens delivers a lower quality image because of its chromatic and geometric aberrations.
At The Front Of The Lens
The upside-down image created by a magnifying glass can be photographed with a standard or macro lens mounted on the camera, by moving the magnifying glass away from the object and towards the lens. The most spectacular effects are obtained by photographing the image of distant objects observed with the magnifying glass placed in the field of the lens: everything is blurred apart from the inverted image produced by the magnifying glass.
From a practical point of view, the larger the diameter of the magnifying glass, the more it will cover the observed field. That said, interesting effects can be obtained with all kinds of diameters. Note that the colored fringe of chromatic aberrations produced by biconvex lenses provides an aesthetic touch that is undeniable.
Diopter And Focal Length
The power of a lens is technically expressed in diopters (the unit of measurement of vergence of optical systems). Spectacles for presbyopes are referenced in diopters. Commercial hand-held magnifiers indicate their magnification: 2x (4 diopters), 3x (8 diopters), 4x (12 diopters), etc. The focal length of the magnifier is determined by dividing the diopter by 1000. The result is given in mm. 8 diopters/1000 = 125 mm.
Regardless of these considerations, we see that the more the lens is curved, the more it magnifies the image. And generally, the more curved the lens, the smaller its diameter and the narrower its field of view. Magnifiers with a large diameter, about ten centimeters, with a magnification of 3x to 4x are very well suited to photographing with a magnifying glass.
Discover the photography courses at Spéos
Spéos offers various training courses ranging from simple one-week photography courses (initiation and advanced level) to 3-year courses. The long courses to become professional photographers allow you not only to master all the photographic techniques and its vocabulary (blurs, hyperfocus, sharpness zone, depth of field, backlighting, focal length, shutter release, autofocus, wide-angle, rule of thirds, etc.), but also all the stages of shooting and image processing.
Visiting the school allows you to discover the premises, the studios and the equipment, and is undoubtedly the best way to familiarize yourself with your future way of working. This is why, in addition to the open days, Spéos offers throughout the year personalized visits by appointment to come and discover the school with a member of the team.
Text and photos: Philippe Bachelier, teacher of Printing techniques at Spéos