
Originally Posted by
Henry Peach
Twin lens reflex: A camera design that uses 2 lenses. One for the viewfinder and one to make the photo. A mirror is used to angle the projected lens upwards to the viewfinder which is on top of the camera.
Single lens reflex: A camera design that uses a mirror and prism to allow viewing and photographing through the same lens. The mirror retracts out of the way of the sensor or film when the exposure is made.
When the term "reflex" is used in a camera description it refers to the use of mirror to redirect the projection of the lens to a viewfinder.
Four Thirds and Micro Four Thirds: Standards in format size and lens mount to be used by multiple brands. The Four Thirds system was designed for DSLRs. The Micro Four Thirds system is designed for smaller cameras without a mirror or prism. The sensor format is still 4/3, but the lens mount differs.
Format size: The size of the frame of film or sensor. Some modern formats are 4/3, APS-C, 35mm, 6x4.5cm, 6x6cm, 6x7cm, and 4x5 inch.
Medium format: Commonly 6x4.5cm, 6x6cm, and 6x7cm, but usually refers to any format shot on 120 or 220 film (same width, a roll of 220 is twice as long): 4x4cm (Super Slides), 6x8cm, 6x9cm, 6x12cm, 6x17cm.... For digital it's any size sensor between 35mm and 4x5 (at least until some manufacturer starts referring to their smaller than 4x5 sensor as large format).
Large format: 4x5 inch and larger.
Full frame: This used to mean printed from the entire neg without cropping. Today it is more commonly used to refer to 35mm format.
Depth of field (DOF): The area of the photo that is in acceptable focus. DOF is influenced by aperture size, focusing distance, focal length, and format size. In most cameras the DOF (plane of focus) is always parallel to the film/sensor plane. View cameras and tilt-shift lenses allow the plane of focus to be adjusted.
Focal length: The distance between the rear nodal point and the film/sensor plane when the lens is focused on infinity. Focal length determines magnification in the camera.
F/stop: Focal length divided by the diameter of the entrance pupil (effective aperture size). You don't really need to know the math as long as you understand that f/2 is the same intensity of light on all lenses. The math explains why a small f/# is a large aperture, and vice versa.
Stop: A doubling or halving of the amount of light or exposure.
Reciprocity: There are 3 controls for exposure. ISO determines how much exposure is necessary. A doubling or halving of the ISO is one stop. ISO 200 is one stop more sensitive (needs 1/2 the light) than ISO 100, and one stop less sensitive (needs twice the light) than ISO 400. Shutter controls the length of the exposure. A doubling or halving of the shutter speed is one stop. 1/30th of a second is twice as much exposure as 1/60th. 1/60th is twice the exposure of 1/125th. Aperture controls light intensity, and is designated with a f-number or f/stop. Each doubling or halving of the f/# is two stops. F/4 is a quarter (2 stops means 2x2=4) of the exposure of f/2 and four times as much exposure as f/8.
ISO in 1 stop increments: ... 25 50 100 200 400 800 1600 3200 6400 12800 25600 ...
shutter speed in 1 stop increments: ... 1sec, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000, 1/4000 ...
aperture in 1 stop increments: ... f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45, f/64 ...
Reciprocity just means that as you adjust one control to get a desired visual effect (DOF, freeze or blur motion, fine or coarse grain) you can change one or both others to even out the exposure. All of the following settings are the same exposure. As one change increases or decreases the amount of exposure the others are adjusted to even it out.
1/125th @ f/4 @ ISO 400
1/250th @ f/4 @ ISO 800
1/500th @ f/4 @ ISO 1600
1/125th @ f/5.6 @ ISO 800
1/60th @ f/4 @ ISO 200
1/30th @ f/5.6 @ ISO 200
Reciprocity failure: When using film shutter speed reciprocity starts breaking down at very long (>1 sec) and very short (