science

Light value

Light value is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Light value rather than just read about it. In short: In photography, light value has been used to refer to a "light level" for either incident or reflected light, often on a base-2 logarithmic scale. The term does not derive from a published standard, and has had several different meanings: An arbitrary value indicated by an exposure meter such as the Weston Master V, discussed in Adams (1948, 14–18).

Key takeaways

Light value belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Light value to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Light value from memory before moving on to harder problems.

Reference excerpt

In photography, light value has been used to refer to a "light level" for either incident or reflected light, often on a base-2 logarithmic scale. The term does not derive from a published standard, and has had several different meanings:

An arbitrary value indicated by an exposure meter such as the Weston Master V, discussed in Adams (1948, 14–18). This may have been the origin of the term. The indicated light value was transferred to the meter's exposure calculator, which then was used to determine camera settings. Ray (2000) uses the term, with the acronym 'LV', in this sense. The Honeywell/Pentax 1°/21° spot meter indicated in "light level" ("LL"), with LL essentially exposure value (EV) for ISO 100 film speed. The later Pentax Spotmeter V and Digital Spotmeter indicated directly in EV for ISO 100, but they made no mention of "light level", "light value", or LV. A synonym for incident light value, from the Additive system of Photographic EXposure (APEX). Zakia and Stroebel (1993) and Stroebel, Compton, Current, and Zakia (2000) used the term in this sense. They used the APEX symbol B v {\displaystyle B_{v}} , normally used for luminance value. An apparent synonym for luminance value, from APEX. Stroebel, Compton, Current, and Zakia (2000) referred to "scene illuminance" in the text of the article, but the example used units of luminance. The defining equation used the symbol L v {\displaystyle L_{v}} . The table at the end of the article used units of illuminance and the symbol B v {\displaystyle B_{v}} , as noted above. A synonym for exposure value (EV) (e.g., Kyoritsu calibrated light sources, for which the luminance ranges are specified in terms of "LV at ISO 100". Kyoritsu specify the luminance ranges of their multi-function camera testers in EV, presumably at ISO 100). A synonym for "EV at ISO 100 film speed". This usage appears on many web pages, usually without attribution of an authoritative source. Eads (2000) proposed a revised APEX in which luminance value was equal to EV for ISO 100 speed, but he did not use the term light value. Literally speaking, the English term light value could be translated as "Lichtwert" in German language, however, this is not what the term Lichtwert, as it was introduced by the German shutter manufacturer Friedrich Deckel in 1954 and defined as Belichtungswert in DIN 19010, is used for in German-speaking countries. Instead, the established term Lichtwert (abbreviated as "LW") describes what became known as exposure value (EV) elsewhere. In APEX, luminance value and incident light value are numerically equal, so that for a given "light value", either meaning 2 or 3 would result in the same camera settings.

Notes

See also APEX system Exposure value Sunny 16 rule

References Adams, Ansel. 1948. The Negative: Exposure and Development. Boston: New York Graphic Society. Eads, Andrew C. 2000. "Simplifying and Understanding Photographic Exposure with the APEx System", PHOTO Techniques, Jan/Feb 2000, 54–59. Ray, Sidney F. 2000. "Camera Exposure Determination". In The Manual of Photography, 9th ed. Oxford: Focal Press. Stroebel, Leslie, John Compton, Ira Current, and Richard D. Zakia. 2000. Basic Photographic Materials and Processes, 2nd ed. Boston: Focal Press. Zakia, Richard D. and Leslie Stroebel, eds. 1993. Focal Encyclopedia of Photography, 3rd ed. Boston: Focal Press.

Illustrations

Worked examples

Example 1 — a first encounter with Light value

Start with the simplest possible case. Write down what Light value claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Light value before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Light value ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Light value

In research: Light value appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Light value in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Light value is common in secondary-school and first-year university syllabi. It links to neighbouring topics Science of photography, so understanding it makes those chapters shorter.
In everyday life: Look for Light value outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Light value in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Light value means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Light value out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Light value in simple terms?

Why does Light value matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Light value?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Light value.