Exposure & Metering — 60 Terms Defined

F-number Exposure & Metering Dimensionless ratio The ratio of a lens's focal length to the diameter of its entrance pupil, written f/2.8, f/8 and so on, and the standard way of stating aperture. N = f / D

Filter Factor Exposure & Metering stops (EV) The exposure increase, in stops or as a multiplier, needed to compensate for the light a filter absorbs. Stops = log₂(factor)

Flicker Reduction Exposure & Metering Hz A camera function that times the shutter release to the peak of a pulsing artificial light source, preventing exposure and colour shifts between frames. Cycle = 2 × mains frequency

Grey Card Exposure & Metering % reflectance A physical card of known neutral 18 per cent reflectance, held in the subject's light to give both a reliable exposure reading and a white balance reference.

Handheld Light Meter Exposure & Metering EV, f-stops A separate instrument for measuring incident, reflected or flash light independently of the camera.

Highlight Alert Exposure & Metering A playback warning, often called blinkies, that flashes any area of the image where the brightest tones have clipped.

Highlight-weighted Metering Exposure & Metering A metering pattern that biases exposure toward protecting the brightest area of the frame from clipping.

Histogram Exposure & Metering A graph of how many pixels in an image fall at each brightness level, from black on the left to white on the right.

Illuminance Exposure & Metering lux (lx) The amount of light arriving on a surface per unit area, measured in lux, independent of what that surface does with it. E = Φ / A

Incident-light Metering Exposure & Metering EV Measuring the light falling onto a subject rather than the light reflecting off it, using a handheld meter with a white diffusing dome.

ISO Exposure & Metering ISO (arithmetic scale) The standardised measure of how strongly a camera amplifies the signal it captures, determining image brightness for a given exposure. ISO ∝ 1 / H_required

Looney 11 Rule Exposure & Metering stops (EV) The lunar counterpart to Sunny 16: a full moon is correctly exposed at f/11 with a shutter speed of one over the ISO. t = 1/ISO at f/11

Luminance Exposure & Metering cd/m² (nits) The brightness of a surface as seen from a given direction, measured in candela per square metre. L = I / A

Manual Mode Exposure & Metering A mode, marked M, in which the photographer sets aperture and shutter speed directly and the camera only advises.

Meter Calibration Constant Exposure & Metering cd·s/m² The factor K in the exposure equation that ties a measured luminance to a recommended camera setting. N² / t = L × S / K

Metering Mode Exposure & Metering The pattern a camera uses to sample scene brightness when calculating an automatic exposure.

Middle Grey Exposure & Metering % reflectance The reference tone, reflecting about 18 per cent of the light that strikes it, that every reflected-light meter is calibrated to reproduce. L* ≈ 50

Native ISO Exposure & Metering ISO Any ISO value produced by real analogue gain in the sensor's readout circuitry, as opposed to numbers created by brightening the file in software.

Overexposure Exposure & Metering stops (EV) An exposure that records more light than the sensor can hold, driving the brightest tones to pure white with no recoverable detail.

Partial Metering Exposure & Metering A metering pattern reading roughly the central eight to fifteen per cent of the frame — broader than spot, narrower than centre-weighted.

Program Mode Exposure & Metering An automatic mode, marked P, in which the camera selects both aperture and shutter speed while leaving other controls to the photographer.

Reciprocal Rule Exposure & Metering seconds A handholding guideline stating that the slowest safe shutter speed is roughly one over the focal length in use. t ≤ 1 / (focal length × crop factor)

Reciprocity Law Exposure & Metering lux-seconds (lx·s) The principle that exposure depends only on the product of light intensity and time, so halving one and doubling the other leaves the result unchanged. H = E × t

Reflected-light Metering Exposure & Metering EV Measuring the light bouncing off a subject toward the camera, which is what every built-in camera meter does.