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One filter, two lenses

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Have you heard of the split-field diopter filter? You have probably seen its effects in a few scenes in the cinema, as it has seduced many American directors, from John Ford to Quentin Tarantino. It offers sharpness in both the foreground and the background.

The focus is on the background. As you move closer to the saxophone player, he becomes sharp when 50 cm away, thanks to the 2 dioptre close-up lens. The cap visor and the saxophone mouthpiece, located at the cut-off point of the filter, are blurred but give the effect of movement. Better Focus Split Field filter with +2 dioptres. Nikon D850, 24-70 mm at 34 mm, 1/800 s at f/8, ISO 400.

Focusing

Taking a photograph means deciding how to frame and focus it. Sharpness is achieved on a precise plane. The aperture of the diaphragm controls the foreground and background blur. The tighter the aperture, the greater the depth of field. However, no depth of field allows you to be sharp at 1 m and infinity. The cinema has solved this problem with a special filter: a split field diopter. Tiffen or Schneider makes excellent and costly cinema models. For example, they are more affordable at Tide Optics at around €50.

Split-field close-up lenses are manufactured in a limited range of diameters. Large-diameter filters are more expensive but more versatile. Filter adapter rings, which are inexpensive, can be used on lenses with smaller diameters.

In cinema

A close-up lens is a converging lens fixed to the front of the lens, cut in half to produce the optical effect on only one half of the image. It produces a sharp image of an object placed close to the camera while maintaining optimum sharpness on objects further away. The filter pivots according to the desired zone of sharpness. John Ford used it for the first time in The Long Voyage Home (1940), followed by Orson Wells for Citizen Kane (1941). Brian De Palma often uses it in his films, notably Carrie and Blow Out, as does Quentin Tarantino in Reservoir Dogs. The professional organisation American Cinematographer published an article on the subject.

Close-up lenses and diopters

A close-up lens is a converging lens mounted on the front of the lens. It is used to get closer to the subject. It reduces the focal length of the optical system without transforming it into a wide-angle lens. Less powerful than a macro lens, it reduces the lens’s resolving power and generates optical aberrations. Close the diaphragm to reduce them. The focal length of a close-up lens is expressed in dioptres, like that of hand-held magnifiers or reading glasses. A converging lens is a positive lens: a + precedes its power, hence the +1, +2 or +3 mentioned on magnifying glasses or close-up lenses. Irving Penn sometimes used close-up lenses with his Rolleiflex for his famous portraits.

When a close-up lens is cut in half, the hollowed-out part of the filter does not affect the image. The lens only acts on half the field, which here is 2 dioptres. The filter can be rotated, like a polarising filter. The foreground is sharp depending on its orientation. Here, it is in the vertical position to obtain a sharp image on the left and right parts of the image.
The half-lens is positioned on the lower half of the lens for a view framed horizontally or a scene shot vertically. If you are looking for sharpness from the foreground to the background, set the lens focus to infinity and move the camera until the sharp foreground corresponds to the desired composition.
The lens is focused on the bridge in the distance. The camera is brought closer to the padlocks until they are in focus. The water between the bars, in the lens’s field of view, is blurred, but it adds relief to the image. Varying the focus on the balustrade would change the sharpness of the background.

Diopter and focal length

A converging lens has a focal length. The relationship between diopter (D) and focal length (f) is the equation D = 1/f. If the focal length is expressed in mm, we have D = 1000/f and f = 1000/D. A diopter of +2 has a focal length of 500 mm (i.e. f = 1000/2). When a close-up lens is attached to the front of the lens with the focus set to infinity, the image is sharp at the focal length of the close-up lens. A +2 diopter lens attached to a 50mm lens with the focus set to infinity will bring any object 500mm from the lens into focus. The focal length of the lens on which the close-up lens is mounted is irrelevant: when the lens is set to infinity, the focusing distance corresponds to the focal length of the diopter: 1 m for +1 diopter, 50 cm for +2 diopters, 33.33 cm for +3 diopters, etc.

A close-up lens imposes a focusing distance relative to its focal length. When the lens on which the close-up lens is mounted is set to infinity, a close-up lens with 2 diopters will focus at 50 cm from the subject. It does not matter what the focal length of the lens is. You can get closer to the subject when the lens focus is below infinity.

Close-up and infinity: where is the focus?

The dioptric power of a lens can be calculated. Divide 1000 by its focal length. The result of a 50 mm lenst is 1000/50, or 20 dioptres. The focal length of the combination of windscreen and lens can be roughly calculated as follows. A 50 mm lens corresponds to 20 dioptres, so we add the dioptric power of the close-up lens, for example, +2. This gives a total of 22 (20+2). Divide 1000 by 22 and you get the overall focal length: 45.45 mm. That said, as the lens is positioned at a certain distance in front of the objective, simply adding up the dioptres gives only a rough idea of the focal length of the lens + close-up lens combination. Irving Penn’s 75mm Rolleiflex, combined with a close-up lens, captured the famous portrait of Picasso with a hat using a Rolleinar close-up lens. The Rolleinar 1 (+1 diopter) focuses on infinity at a distance of 1 metre and 1 metre at 50 cm. The Rolleinar 2 (+2 dioptres) brings infinity back into focus at 50 cm and 1 m at 33 cm. Finally, Rolleinar 3 (+3 dioptres) brings infinity down to 33 cm and 1 m to 25 cm.

For math geeks

When a close-up lens is mounted, the focusing distance for a distance less than infinity is calculated as follows. The distance between the close-up lens and the object is Db, the distance between the object and the focusing ring is Do, and the diopter is D. We have Db=1/(1/Do+D) and Do=1/(1/Db-D). Values are in metres. If you focus on the background at 3 meters, without a close-up lens, the area affected by the close-up lens will be sharp at 43 cm. For a foreground in focus at 40 cm using the close-up lens, the background will be in focus at 2 metres.

The camera focuses on the background first. This makes it easier to check the sharpness of the image. We then move towards the foreground to bring it into focus. With a 2-dioptre converging lens, the panel at 4 m implies that the parasol is in focus if it is 44 cm from the lens. The right-hand side of the parasol is blurred because the filter lens does not cover it completely.

Discover the photography courses at Spéos

Spéos offers various training courses ranging from simple one-week photography workshops (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

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