APO: 
Stands for ‘apochromatically corrected’. In normal lens designs, blue and green light converges on one focal plane, while the (longer) wavelength of red light is refracted to a slightly different plane of focus. This effect (known as ‘chromatic aberration’, or ‘colour fringing’) is more pronounced in longer focal lengths – making it a particular problem in long telephoto lenses. In APO lenses, on the other hand, the construction of the lens elements and the use of low-dispersion glass enables all colours to converge at the same point of focus – resulting in a sharper image without colour fringing.

ASPH.: 
Stands for ‘aspherical design’. Normal lenses have a spherical design, meaning that the radius of curvature is constant across the entire lens. Spherical lens elements are manufactured in a straightforward production process, whereby the glass is ground while being rotated. However, with this type of lens design, the scope for optical corrections (which enable the renditions to look as realistic as possible) is severely limited. ASPH. lenses, on the other hand, are extremely elaborate to produce, and include at least one lens element with a non-constant radius of curvature. This allows the manufacturer to integrate optical corrections into compact lens designs that would otherwise be impossible to implement. In practice, the increased correction of the pathway of light leads to an improved lens performance – resulting in sharper renditions, both at the centre and all the way into the corners of the image. At the same time, aspherical lenses are not significantly larger than their spherical counterparts.

Bokeh: 
Refers to the aesthetic quality of the out-of-focus parts of a photographic image, as rendered by a particular lens. While the perception of bokeh is essentially subjective, there are pronounced differences – especially with regards to the smoothness of the out-of-focus areas, and the rendition of out-of-focus points of light. The term is most likely derived from the Japanese word ‘bo-ke’, meaning ‘blur’.

Central shutter: 
The Leica S system offers 5 lenses with a central shutter in its portfolio. The S camera offers the option of switching between the camera's internal focal plane shutter and the lens' central shutter. With the central shutter, users gain additional freedom in light composition with professional studio flash units to suppress ambient light and can use flashes in bright environments with an open aperture. A central shutter is also integrated in the cameras of the Leica Q family instead of the focal plane shutter.

Close focus distance: 
The close focusing distance or working range of a lens indicates how close you can get to an object in order to bring it into focus (measured from the sensor plane). For example, with a closest focusing distance of 30cm, you can get as close as 30cm to the object and still be able to bring it into focus.

Compur: 
Refers to the leaf shutters produced by the Munich-based company F. Deckel from 1912 onwards. They were featured in the lenses of the Leica I Compur (1926–1941) and in the Leica Summicron Compur 50 f/2.0 (1959).

Distortion: 
Some lenses have a distortion caused by their construction. This means that mostly straight lines close to the edge of the image (like edges of a building) are bent in a barrel or pincushion shape. In most cases, the camera software corrects the distortion automatically.

ELCAN: 
Stands for Ernst Leitz Canada. The acronym was used to describe a number of lenses manufactured at the Ernst Leitz subsidiary in Midland, Ontario, for military purposes during the 1960s and early 70s.

Elmar: 
Indicates a maximum lens aperture of f/3.5. The Elmar 50 f/3.5 was launched in 1925 as the replacement for the Elmax 50 f/3.5. The ‘Elmar’ classification was initially applied to Leica screw-mount and M lenses. Later, it was also used for Leica R lenses and, eventually, for lenses in the S and SL-Systems.

Elmarit:
Indicates a maximum lens aperture of f/2.8. The name is a derivation of the earlier (and slower) Elmar. The ‘Elmarit’ classification was initially applied to Leica screw-mount and M lenses. Later, it was also used for Leica R lenses and, eventually, for lenses in the S and SL-Systems.

Elmax: 
Acronym for Ernst Leitz and Max Berek. Ernst Leitz was the founder of Ernst Leitz Werkstätten in Wetzlar. His employee, Prof. Dr. Max Berek, designed the first ever Leica lens for Oskar Barnack’s prototype, the ‘Ur-Leica’ of 1913. The resulting 50 f/3.5 lens was initially called Anastigmat; however, the name was later changed to Elmax.

Filter Mount: 
The filter mount indicates how large the diameter (in mm) of a photo filter must be in order to fit the respective lens. For example, a filter thread E67 means that a matching filter has a diameter of 67mm.

Focus breathing: 
With lenses, it can happen that the field of view/image section becomes larger (or smaller) when the focal distance is adjusted. This is a particularly unattractive distortion when you want to zoom while filming for example. Lenses that are optimised for video are corrected for focus breathing. In photography, this "image distortion" plays a subordinate role.

Hektor: 
Indicates a maximum lens aperture of (usually) f/2.5. Fun fact: ‘Hektor’ was the name of Prof. Max Berek's dog.

Leica: 
Acronym of LEITZ CAMERA.

Mandler design: 
Describes the design of certain vintage Leica lenses. The term refers to Dr. Walter Mandler, a legendary designer of Leica lenses.

Noctilux: 
Indicates the maximum aperture of the M-System’s fastest 50mm (and currently its fastest 75mm) lens. The ‘Noctilux’ classification was initially applied to a 50mm lens with a maximum aperture of f/1.2, then f/1.0, and now f/0.95. The name is an amalgamation of ‘nocturnal’ (‘nocturnus’ = Latin for ‘of the night’, ‘nox, noctis’ = ‘night’) and ‘lux’, the Latin word for ‘light’.

Reproduction ratio: 
The reproduction ratio of a lens indicates how large the object is reproduced on the sensor (independent of the sensor size). For example, a lens with a reproduction ratio of 1:2 will reproduce an object of 2cm with a size of 1cm on the sensor.

Summar: 
Since 1933 – starting with the Summar 50 f/2 – the term ‘Summar’ has been used to indicate a maximum lens aperture of f/2. It also served as the basis for a series of subsequent lens names, such as the Summitar 50 f/2 of 1939, or the Summarex 85 f/1.5 of 1948. The term is most likely a play on ‘summit’, alluding to the highest attainable level of achievement.

Summaron: 
Derived from the afore-mentioned Summar, the name ‘Summaron’ applies to three wide-angle lenses with different focal lengths (28 mm and 35 mm) and apertures (f/2.8, f/3.5, and f/5.6).

Summarit: 
Derived from the Leica Summitar, the term ‘Summarit’ originally indicated a maximum lens aperture of f/1.5. It was subsequently used as the name for the 40 mm f/2.4 lens of the Leica Minilux. In 2007, the term re-appeared in the form of entry-level M lenses (35 mm, 50 mm, 75 mm, and 90 mm) and, later, in lenses for Leica’s medium-format S-System.

Summicron: 
Indicates a maximum lens aperture of f/2.0. The term was originally assigned to the most popular focal lengths in Leica’s M-System: 28 mm, 35 mm, 50 mm, 75 mm, and 90 mm. Later, it was also used for R lenses and, subsequently, SL lenses with a maximum aperture of f/2. The first Summicron was an evolution of the original Summar 50 f/2 of 1933. The term ‘Summicron’ is widely believed to be a combination of ‘summit’ (alluding to the highest point of achievement) and a contraction of ‘chroma’ (Greek for ‘colour’). Other sources suggest that ‘cron’ was, in fact, a reference to the English company ‘Crown’, which supplied the glass for the earliest Summicron models. Indeed, the first batch of Summicron lenses was called ‘Summikron’ with a ‘k’ – possibly in allusion to the German word ‘Krone’, meaning ‘crown’.

Summilux: 
Indicates a maximum lens aperture of f/1.4. The combination of ‘summi’ (meaning ‘highest point’) and ‘lux’ (Latin for ‘light’) results in the intimation of ‘maximum light’ – referring to the enhanced light-gathering abilities of these lenses. The name is applied to several f/1.4 standard and wide-angle lenses within the M-System, as well as the Summilux-M 90 f/1.5 ASPH. In the SL-System, we find it in the form of the Summilux-SL 50 f/1.4 ASPH; furthermore, the name is used to describe the 28mm f/1.7 fixed-mounted lens featured on the Leica Q and Q2.

Telyt: 
Short for ‘telephoto lens’. The term – which is derived from the Greek adjective ‘tēle’, meaning ‘far away’ – is used to describe optical instruments designed to work across long distances.

Thambar: 
The Leitz Thambar 90 f/2.2 is a classic portrait lens, whose legendary soft focus can be further enhanced by a centre spot filter. This screw-in glass filter with a central black spot is designed to block the rays close to the optical axis (which generate the sharpest rendition) – thereby intensifying the Thambar’s inherent soft-focus effect. The exact origins of the name are unknown, but may be rooted in the Ancient Greek term ‘thambo’, which translates as ‘blurred’. Its figurative meaning is ‘to be blinded by beauty’ (me thambose me teen omorfia tis).

Vignetting: 
The corners of the image field can show a drop in brightness compared to the rest of the image due to a lens with high vignetting. Automatic vignetting correction is usually applied by the camera's software.