Glossary J to N

Karat cassetteAgfa’s answer to Kodak’s 135 film in the (now ubiquitous) cassette. Introduced in 1936, it was almost identical to the Agfa Rapid cassette.
LeaderThe first part of the film in a 35 mm cassette. It is trimmed to about half the width of the film.
LeatheretteA plasticised cloth used as a covering for camera bodies and for folding camera bellows
leaf shutterA shutter either between the glass elements of the lens, or just behind them that consists of a number of thin metal plates that move to allow light into the camera
lens coatinga very thin coating applied to the surface of lenses to increase contrast and reduce flare. On early lenses (from 1930) this was just on the front surface of the front element a but later was applied to all glass surfaces. This became normal from around 1950.
Lens capA cap that fits over the front of the lens to protect it when the camera is not in use. A rear lens cap is also available to protect the rear of the lens while it is detached from the camera.
Lens doorThe hinged ‘door’ of a folding camera that holds the lens and shutter in place.
lens hooda shade for the front of a lens to prevent oblique light from entering the lens. This is more important with older, uncoated lenses as they will produce flare if used pointing towards a light source.
lens nodethe effective centre of a lens. For a 50 mm lens this will be 50 mm in front of the film. Sometimes the node is actually outside the physical lens
lens standard The board or frame that holds the lens in place.
light metera device to measure the amount of light so a good exposure can be calculated – most include a calculator. See exposure meter
Light sealCameras are required to be light tight apart from the lens. When the camera has an opening back, this needs to be rendered light tight. German (and other) camera makers used deep interlocking flanges to achieve this. The Japanese used strips of foam around the edges of the door.
LTMLeica Thread Mount. 39 mm diameter and 1 mm pitch. See below.
M39The standard thread for fitting a lens to a camera introduced by Leitz for their Leica cameras in the 1920s. Also used by many other manufacturers.  Also known as LTM
M42A standard thread for fitting a lens to a camera introduced by East German Zeiss Ikon in the late 1940s. Used for most 35mm SLR cameras until the 1980s and became known as the Pentax thread.
macroStrictly used to indicate that the image on the negative is life size but is used by lens manufacturers to indicate the lens can be used for close-ups.
manual focussingWhere the photographer must adjust the focus of the lens instead of relying on the camera to do so.
Match-needlea system used in light (exposure) meters where the user turns a dial until the meter needle and the dial needle are in the same place. At this point, the required exposure can be read off a scale.
Meniscus lens  A simple lens consisting of one piece of glass where both glass surfaces curve in the same direction.  
mercury cellsA form of battery now banned throughout the world. Usually a modern battery can be used it it place, but there will be a voltage difference to take into account.
Micro-prismsa focussing aid that keeps the image out of focus until it is correctly focussed. Usually found in Japanese SLRs of the 1960s, 70s and 80s.
MirrorThe mirror in reflex cameras redirects the light coming in the lens to a focusing screen, enabling the image to be seen. If there is just a mirror, the image will be reversed left to right. If a pentaprism is included, the image will be the right way around.
Monochromea picture in one colour – usually black and white but necessarily so.
motor-driveA powered device to wind on the film and take the next picture.
M synchThis is a method of synchronising the firing of the flash bulb with the opening of the shutter. M synch flash bulbs are fairly slow so the delay between the firing of the bulb and the opening of the shutter is longer than with F synch.
negativeThe picture formed in the camera on the piece of film. It is called a negative because the dark parts of the scene will be light and the light parts of the scene will be dark.
Newtonian finderA crude viewfinder either with no lens of with a simple magnifying lens
Normal lensFor a given film (or sensor) size, this is a lens that has a focal length equal to the diagonal of the film (sensor). This should have a very similar angle of view to which the human eye has when looking at a hand-held object.