Glossary of Photographic Terms (J to N)

Karat cassette	Agfa's answer to Kodak's 135 film in the (now ubiquitous) cassette. Introduced in 1936, it was almost identical to the Agfa Rapid cassette.
Leader	The first part of the film in a 35 mm cassette. It is trimmed to about half the width of the film.
leaf shutter	A 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 coating	a 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 cap	A 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 hood	a 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 node	the 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 meter	a device to measure the amount of light so a good exposure can be calculated – most include a calculator. See exposure meter
LTM	Leica Thread Mount. 39 mm diameter and 1 mm pitch. See below.
M39	The 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
M42	A 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.
macro	Strictly 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 focussing	Where the photographer must adjust the focus of the lens instead of relying on the camera to do so.
Match-needle	a 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.
mercury cells	A 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-prisms	a 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.
Mirror	The 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.
Monochrome	a picture in one colour – usually black and white but necessarily so.
motor-drive	A powered device to wind on the film and take the next picture.
M synch	This 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.
negative	The 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 finder	A crude viewfinder either with no lens of with a simple magnifying lens