Exposure Value and Reciprocity


A photograph’s exposure is determined by a combination of three factors: f/stop, shutter speed, and ISO.

The amount of light that reaches the camera’s sensor when taking a photograph determines how bright or dark the image will seem. Getting the right exposure is essential for creating a well-balanced and visually appealing photo.

The exposure triangle in photography comprises aperture, shutter speed, and ISO, determining overall image exposure. Exposure compensation adjusts meter suggestions when lighting is misinterpreted, preventing overexposed or underexposed images. To achieve the desired exposure, prioritize the exposure triangle components based on creative objectives and the current situation.

Law of Reciprocity

The interaction of light intensity and duration affects the sensitive surface, resulting in exposure. Reciprocity governs the relationship between shutter and aperture. A one-stop aperture increase equals doubling the shutter duration (halving it), both leading to a one-stop brightness increase.

The relationship between light intensity and exposure duration is inversely proportional, ensuring the correct exposure in photography. ISO serves as an extra control parameter.

For instance, when cooking chapati with regular intensity, it takes time for it to cook thoroughly. To expedite the cooking process and have the chapati ready in half the previous time, what measures can be taken?

In the law of reciprocity, the correct use of the relationship between shutter and aperture leads to the birth of a good image. Photography – Abin Alex | Camera: Canon EOS 5D Mark IV, Focal length: 35mm, Aperture: f/14, Shutter speed: 1/200 sec. ISO: 100

The oven’s speed should be decreased to quicken the cooking of the chapati by enhancing the flame’s combustion. This concept resembles the operation of aperture and shutter speed in a camera, where various combinations exist to achieve a quality image. The decision of which combination to select rests with the artist.

To ensure consistent image quality, it’s important to modify the duration in relation to the intensity. When the intensity rises due to a wider aperture, the duration should be shortened. On the other hand, if the aperture widens while decreasing the intensity, the duration needs to be extended to uphold the identical level of exposure.

The interplay between aperture and shutter speed plays a pivotal role in grasping the concept of exposure. This form of connection is referred to as the reciprocity law.

Exposure = intensity x time

For instance:

An exposure of 1/30 second at f/5.6 is equivalent to 1/60 second at f/4.
Likewise, an exposure of 1/125 second at f/4 corresponds to 1/250 second at f/2.8.

Exposure Value (EV)

Raising the shutter speed by one stop while simultaneously lowering the aperture by one stop yields an unchanged exposure. While this adjustment can influence factors like depth of field and other image attributes, the interconnectedness of these adjustments ensures a consistent level of exposure. When dealing with three interrelated yet independent variables (aperture, shutter speed, ISO), all quantified in stops, it can become perplexing to compare distinct exposures. This is where EV (Exposure Value) and LV (Light Value) prove to be useful tools. EV and LV provide a standardized scale that simplifies exposure assessment and adjustment, streamlining the process of achieving desired photographic outcomes.

Renowned landscape photographer Ansel Adams introduced the concept of light value numbers (LV) in a 1948 article on the zone system. Meanwhile, EV, which stands for “exposure value,” serves as a widely used measurement in light meters. Equal combinations of shutter speed and aperture yield identical EV numbers, provided ISO remains constant.

Starting with a one-second exposure at f/1.0, an arbitrary EV 0 designation is established. Subsequent EV numbers can be calculated by incrementing stops. For instance, at f/2.0, a one-second exposure corresponds to EV 1, while at f/2.8, it becomes EV 3, and so forth. Combinations of settings resulting in equal exposure share identical EV numbers. Therefore, an exposure of 1/30 second at f/5.6 yields an EV of 10, whereas exposures of 1/60 second at f/4.0 and 1/15 second at f/8.0 differ in EV values.

Picture taken by EV 13. Photography – Abin Alex | Camera: Canon EOS 5D Mark IV, Focal Length: 35mm, Aperture: f/14, Shutter Speed: 1/200 sec. ISO: 100

 ev cHART

Shadows of images taken between EV 13 and 15 will be harder. Photography – Abin Alex | Camera: Canon EOS 5D Mark IV, Focal Length: 100mm, Aperture: f/8, Shutter Speed: 1/160 sec. ISO: 100

While referring to LV or “light values” while discussing brightness levels, the EV numbers correlate perfectly to the LV numbers when recording photographs at the basic ISO 100. The normal LV range observed in natural situations is from LV 1 to LV 17.When the ISO is elevated to 200, maintaining an equivalent level of light necessitates a reduction in either the shutter speed or aperture by one stop. As a result, when operating with ISO values other than 100, the equality between EV and LV is disrupted.

sunny 16 rule

The Sunny 16 rule is a technique for achieving accurate exposure during daylight hours without relying on the camera’s meter. Under optimal sunny conditions, when the aperture is set to f/16, the shutter speed will be matched to the chosen ISO value. For instance, with an ISO of 200 and aperture at f/16, the corresponding shutter speed would be 1/200 second.When utilizing an ISO of 100, the shutter speed will be set at 1/100 second. To maintain a consistent exposure, an adjustment in one of the other variables is required. If one aspect is increased by one stop, the opposite element must be decreased by one stop, and vice versa.

On a bright sunny day f/16 or f/22 is best according to the 16 rule. Photography -Abin Alex | Camera: Canon EOS 5D Mark IV, Focal Length: 35mm, Aperture: f/22, Shutter Speed: 1/125 sec. ISO: 100

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