Exposure value
In photography, exposure value (EV) is a standardized numerical index that combines a camera's shutter speed and aperture (f-number) to represent the overall exposure setting, independent of ISO sensitivity, allowing photographers to identify equivalent combinations that yield the same light intensity on the image sensor or film.[1][2] This metric simplifies exposure calculations by quantifying how these two variables interact to control the amount of light reaching the recording medium, with each increment of 1 EV corresponding to a one-stop change—doubling the exposure when increased or halving it when decreased.[3][2] The formal calculation of EV at ISO 100 is given by the formula EV = log₂(N² / t), where N is the f-number and t is the shutter speed in seconds; for other ISO values, the effective EV adjusts by the logarithmic difference in sensitivity (e.g., doubling ISO from 100 to 200 increases the usable EV by 1).[1][2] EV values typically range from -3 (dark night scenes) to 15 (bright sunlight), with common daylight scenes around EV 13–15 at ISO 100; indoor candlelight is typically EV 2–5.[1][2] Examples include EV 15 equating to f/16 at 1/125 second or f/8 at 1/500 second, both providing identical exposure under the same lighting.[2] This system originated in the mid-20th century as part of the Additive Picture Exposure eXchange (APEX) framework to standardize exposure metering across devices.[3] EV is distinct from light value (LV), which measures the absolute luminance of a scene independent of camera settings or ISO, typically ranging from -15 (starlight) to 18 (bright snow reflections); EV equals LV at ISO 100, but diverges with ISO adjustments to maintain proper exposure.[3] In modern digital cameras, EV compensation allows photographers to deliberately over- or underexpose by whole or fractional stops (e.g., +1 EV for brighter images), aiding creative control in auto modes or when metering challenging scenes like high-contrast landscapes.[1] EV charts and calculators remain essential tools for manual exposure decisions, ensuring consistency across varying lighting conditions without relying solely on in-camera automation.[2]Fundamentals of Exposure Value
Formal Definition
Exposure value (EV), denoted as EV, is a dimensionless quantity in photography that represents combinations of camera aperture and shutter speed yielding equivalent exposure for a given film or sensor sensitivity. Formally, it is defined by the equation EV = \log_2 \left( \frac{N^2}{t} \right), where N is the f-number (relative aperture) and t is the exposure time in seconds.[4] This formulation arises from the Additive System of Photographic Exposure (APEX), where EV combines the aperture value AV = \log_2 (N^2) and time value TV = -\log_2 (t), such that EV = AV + TV.[4] The EV scale is logarithmic base-2, meaning an increase of 1 EV unit corresponds to doubling the amount of light reaching the sensor (or equivalently, halving the exposure time for the same aperture), representing one "stop" of exposure change.[4] This standardization simplifies exposure computations by allowing photographers to interchange aperture and shutter speed settings while maintaining consistent exposure levels. The concept of EV originated in the 1950s, when camera and shutter manufacturers developed systems to express exposure through a single numerical value, later formalized by the International Organization for Standardization (ISO) to streamline photographic calculations.[5] Under the standard assumption of ISO 100 sensitivity, EV 0 corresponds to an aperture of f/1 and exposure time of 1 second; for other ISO values, EV adjusts via the relation EV_S = EV_{100} + \log_2 (S / 100), where S is the ISO arithmetic speed.[4] EV approximates the physical luminous exposure on the image plane but serves primarily as a practical metric for camera settings rather than a direct measure of light energy.[5]Relation to Luminous Exposure
Luminous exposure, denoted as H, quantifies the total amount of light energy incident on a surface per unit area in photography. It is formally defined as the product of illuminance E (measured in lux) and exposure time t (measured in seconds), so H = E \times t, with units of lux-seconds.[6] This physical measure directly represents the cumulative light flux reaching the image sensor or film, independent of camera settings.[7] Exposure value (EV) derived from camera settings serves as an approximation of luminous exposure H, but it simplifies the relationship by assuming ideal conditions such as perfect lens transmission and uniform sensor efficiency.[3] In practice, the actual H on the sensor is influenced by scene luminance and real-world factors like lens light transmission losses, which EV does not account for directly.[2] This approximation holds for standard photographic scenarios but breaks down under non-ideal optics or sensor variations.[8] To link camera settings to physical exposure, the required EV for correct exposure of a scene can be expressed in terms of its luminance. For a given ISO speed S, the equation is: \text{EV} = \log_2 \left( \frac{L \times S}{K} \right) where L is the scene luminance in candela per square meter (cd/m²) and K is a constant specific to the metering mode (e.g., K = 12.5 for reflected metering at 18% gray).[2] This formula derives the EV needed to achieve an H that produces proper density on the medium, bridging luminance-based light measurement to adjustable camera parameters.[3] In film photography, reciprocity failure introduces deviations from EV predictions at extreme exposures, such as very long shutter speeds or intense illuminance, where the film's chemical response no longer scales linearly with H.[9] This non-linear behavior requires empirical adjustments beyond standard EV calculations to maintain accurate exposure.[10] Digital sensors, by contrast, exhibit minimal reciprocity issues due to their electronic nature.[11]Representing Camera Settings with EV
EV from Aperture and Shutter Speed
The exposure value (EV) quantifies the combined effect of a camera's aperture and shutter speed on the amount of light reaching the sensor or film, assuming a standard ISO sensitivity of 100. It is derived from the luminous exposure, which is proportional to the exposure time (shutter speed t in seconds) divided by the square of the f-number N (since aperture area is inversely proportional to N^2). To normalize these into a logarithmic scale where each unit represents a doubling or halving of exposure (one stop), EV is defined as: \text{EV} = \log_2 \left( \frac{N^2}{t} \right) This equation arises because the intensity of light exposure scales with N^2 / t, and the base-2 logarithm converts doublings into additive units, facilitating easy adjustments in camera settings.[12][13] To compute EV, first determine N^2 for the given aperture, then divide by t, and take the base-2 logarithm. For instance, with an aperture of f/2.8 (N = 2.8, so N^2 = 7.84) and shutter speed of 1/60 s (t = 1/60 \approx 0.0167 s), N^2 / t \approx 470, and \log_2(470) \approx 8.9, corresponding to EV 9. Similarly, f/4 (N = 4, N^2 = 16) at 1/60 s yields $16 / 0.0167 \approx 960, \log_2(960) \approx 9.9 or EV 10 exactly in rounded systems. These calculations allow photographers to verify if a combination delivers the desired exposure level for given lighting.[13][14] In aperture-priority mode, where the photographer selects the f-number and the camera adjusts shutter speed, changing the aperture by one stop shifts EV by 1 unit. One stop corresponds to multiplying or dividing N by \sqrt{2} \approx 1.414, which doubles or halves N^2 and thus EV, since \log_2(2 \cdot N^2 / t) = \log_2(N^2 / t) + 1. For example, switching from f/4 to f/5.6 (smaller aperture, less light) increases EV by 1, so the camera compensates by slowing the shutter (e.g., from 1/60 s to 1/30 s) to restore balance and maintain the same overall exposure. This reciprocal relationship ensures consistent results across equivalent settings.[2][12] Conversely, in shutter-priority mode, the photographer sets the shutter speed while the camera chooses the aperture. Halving the shutter speed (e.g., from 1/125 s to 1/250 s) doubles $1/t, increasing EV by 1 unit (reducing exposure). The camera compensates by selecting a one-stop wider aperture (e.g., from f/5.6 to f/4) to maintain constant exposure. Doubling the shutter speed has the opposite effect, requiring a narrower aperture for the same EV. These adjustments highlight EV's utility in balancing creative choices like depth of field or motion freeze with proper exposure.[2][14] A practical benchmark for EV using aperture and shutter speed is the Sunny 16 rule, which estimates settings for bright sunlight at ISO 100: set aperture to f/16 and shutter speed to approximately 1/100 s, yielding EV ≈14.6, often rounded to EV 15 (precisely f/16 at 1/125 s). This rule, derived from typical outdoor illuminance of around 100,000 lux, enables quick manual exposure without metering and serves as a reference for adjusting to other conditions by shifting EV units.[15]Incorporating ISO Sensitivity
In photography, the exposure value (EV) scale, originally defined for ISO 100 sensitivity, is adjusted to account for variations in ISO speed, which represents the sensitivity of film or the image sensor to light. The adjusted exposure value, denoted as EV_s, incorporates ISO through the formula: \mathrm{EV_s} = \mathrm{EV} + \log_2 \left( \frac{\mathrm{ISO}}{100} \right) where EV is the base exposure value at ISO 100, and EV_s is the effective value at the given ISO setting.[4] This logarithmic adjustment reflects the doubling of sensitivity with each +1 EV shift; for instance, ISO 400, being four times more sensitive than ISO 100, shifts the EV by +2, allowing the same exposure with settings that admit half the light (higher EV).[4] Higher ISO values enable proper exposure in lower light conditions by effectively increasing the EV for the scene, meaning less light is required to achieve the same image density or brightness. However, in digital sensors, elevating ISO amplifies both the signal and inherent noise sources, such as read noise from the analog-to-digital conversion process, resulting in visible graininess or reduced image quality, particularly in shadows.[16] This trade-off is more pronounced in digital systems than in film, where higher ISO primarily affects grain without the same electronic noise amplification.[17] The standardization of ISO sensitivity for digital cameras is governed by ISO 12232:2019, which defines methods like Standard Output Sensitivity (SOS) based on the light level producing a specified output signal saturation, and Recommended Exposure Index (REI) for user guidance on noise performance. This contrasts with older film standards, such as the arithmetic ASA (now part of ISO 6) and logarithmic DIN systems (ISO 544), which measured sensitivity via the exposure required to achieve a specific density on negative film without digital processing considerations. ISO 12232 thus adapts the ISO arithmetic scale to digital contexts, allowing consistent labeling across devices while accounting for sensor-specific responses.[18] For example, the combination of f/8 aperture and 1/125-second shutter speed yields EV 13 at ISO 100. At ISO 400, the effective EV_s becomes 15, permitting the same exposure with reduced light input, such as by using f/16 at 1/125 second instead.[4]EV in Photographic Practice
EV for Lighting Conditions
Exposure value (EV) serves as a metric for quantifying the brightness of a photographic scene, particularly for a middle-gray subject reflecting 18% of incident light, independent of specific camera settings. This scene EV is calculated using the formula \text{EV} = \log_2 \left( \frac{L}{K} \right) + \log_2 (S), where L is the luminance of the subject in candela per square meter (cd/m²), K is the reflectance calibration constant (typically 12.5 for an 18% gray card), and S is the ISO sensitivity.[19][20] This formulation derives from the APEX system standardized in ISO 2720, which defines meter calibration for reflected light measurements assuming a middle-gray reflectance to ensure proper exposure.[4] Typical EV values at ISO 100 provide a practical scale for common lighting scenarios, aiding photographers in anticipating exposure needs. For instance, bright sunlight on a clear day yields an EV of 15, representing strong, direct illumination suitable for outdoor portraits or landscapes. In open shade, EV is around 13, while in heavy overcast conditions, it is around 12 (or 10 for dark overcast), indicating softer, diffused light that reduces contrast but requires longer exposures or wider apertures. Under clear starlight, EV is approximately -12 (ranging from -10 to -15 depending on sky conditions), a very low-light environment where faint celestial objects dominate, demanding high ISO or extended shutter times for visibility. These ranges highlight EV's logarithmic nature, where each unit change doubles or halves the light intensity, offering a quick conceptual gauge for scene brightness.[3][14] Unlike illuminance units such as lux or candela, which measure absolute light intensity without regard to photographic outcomes, EV normalizes brightness in a way that directly correlates to camera exposure settings like aperture and shutter speed. This photographic relevance allows users to match scene EV to equivalent camera EV values for balanced exposures, simplifying decisions in varied conditions without converting between disparate units.[2] For example, a scene at EV 15 can be immediately compared to f/16 at 1/125 second on ISO 100 film, embodying the Sunny 16 rule. However, EV calculations assume uniform, diffuse lighting on a middle-gray subject, which may not hold for scenes with specular highlights, deep shadows, or high-contrast zones. In such cases, the metric can overestimate or underestimate exposure for non-average reflectances, necessitating zonal metering techniques to evaluate specific areas rather than overall scene luminance.[21]Exposure Value Tables
Exposure value tables offer a practical way to visualize the combinations of aperture and shutter speed that yield specific EV values at ISO 100, enabling photographers to quickly identify equivalent exposures without calculations. These tables are constructed based on the EV formula, where each cell at the intersection of an aperture (f-number) and shutter speed indicates the corresponding EV for proper exposure in a given light level.[14] The following sample table illustrates common combinations, with rows representing shutter speeds from 30 seconds to 1/1000 second and columns for apertures from f/1 to f/22. Values are for ISO 100; negative EVs indicate low-light scenarios requiring longer exposures or wider apertures.| Shutter Speed | f/1.0 | f/1.4 | f/2.0 | f/2.8 | f/4.0 | f/5.6 | f/8.0 | f/11 | f/16 | f/22 |
|---|---|---|---|---|---|---|---|---|---|---|
| 30 s | -5 | -4 | -3 | -2 | -1 | 0 | 1 | 2 | 3 | 4 |
| 15 s | -4 | -3 | -2 | -1 | 0 | 1 | 2 | 3 | 4 | 5 |
| 8 s | -3 | -2 | -1 | 0 | 1 | 2 | 3 | 4 | 5 | 6 |
| 4 s | -2 | -1 | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
| 2 s | -1 | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
| 1 s | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
| 1/2 s | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
| 1/4 s | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 |
| 1/8 s | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 |
| 1/15 s | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 |
| 1/30 s | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 |
| 1/60 s | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 |
| 1/125 s | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 |
| 1/250 s | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 |
| 1/500 s | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 |
| 1/1000 s | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 |