What is “luminous exposure”?
In photography, luminous exposure is the volume of light film receives during the exposure (“shutter-open”) time. It is measured in lux-seconds, which quantify that volume as light intensity (lux) over time (seconds).
Luminous exposure uses a log base 10 scale, which means +1 lux-second constitutes a 10x light volume increase.
Film manufacturers label their film characteristic curves with lux-seconds to indicate the amount of light the film was exposed to. Lux-second is a scientific unit of measurement in photometry.
✪ Note: I spell luminous exposure units as lux-seconds (with a dash), though they are commonly spelled without one (lux seconds).
What are “stops of light” in photography?
Stop (n.) is a logarithmic unit for measuring relative light transmission, emission, and sensitivity in photography.
Stops (see further explanation and examples) are used for exposure compensation with camera settings and during film development/printing. They are dimensionless units on log base 2 scale. This means that each +1 stop doubles the amount of light reaching film/paper/sensor or its sensitivity, yielding a brighter image.
Stops are used with lens apertures (f-stops), shutter speeds (seconds), and film/paper/sensor sensitivity (ISO)*. In each of those cases, +/-1 stop indicates the same change in exposure. However, 𝒇1, 1s, and ISO 1 all mean different things.
✱ — Filter factors (ND), flash guide numbers (GN), and other tools that affect photography exposure also use stops.
What are “exposure values” (EV) in photography?
Exposure value (EV) is a number with a corresponding set of equivalent shutter speeds and aperture settings that transmit a specific portion of light onto film.
Exposure values are incremented in stops with EV 0 corresponding to an aperture of 𝒇1 with a 1s “shutter-open” time. Examples of equivalent shutter speed and aperture settings for EV 0: 𝒇1.4 + 2s and 𝒇2.0 + 4s. For EV 15: 𝒇16 + 1/125s and 𝒇8 + 1/500s.
Exposure values were used on mechanical film cameras to simplify matching the light meter readings with corresponding camera settings.
(However, film comes in variable light sensitivities, which must be included in the calculations to produce an optimal image¹.)
EV₁₀₀/LV is a set of equivalent camera settings that produce an optimal image¹ on an ISO 100 film from exposure to a specific light intensity reaching the lens² (lux).
EV₁₀₀ 0 corresponds to 𝒇1 + 1s with 2.5lux (pretty dark, but you can still see with 2.5lux). EV₁₀₀ 15 corresponds to 𝒇1 + 1s (or 𝒇16 + 1/125s) with 81,920lux (full sun, “Sunny 16”).
EV₁₀₀ is used to describe the amount of light coming from the scene. LV and EV₁₀₀ are the same thing. Adding to the confusion, many modern manufacturers omit the subscript “100” when printing EV₁₀₀ output, so you’ll often have to have the context to understand what is being measured whenever someone mentions “EV.”
¹ — The manufacturer defines optimal image qualities by the ISO standard; not the same as “perfect exposure.”
² — Note that EV₁₀₀ is tied to incoming/scene light intensity (lux) — a light meter’s output — and is not luminous exposure on film (lux-second). Remember, luminous exposure is a portion of ambient light (lux) transmitted onto the sensor via the lens aperture for a given time (shutter speed).
How to convert lux-seconds to stops.
Dynamic range is an important property that describes the scope of light intensities your film can register. You can use this information to match your scene to your film and avoid blown highlights and crushed shadows (especially with slide film and other expensive emulsions).
You can figure out your scene’s dynamic range by using a spot meter, recording its readings in the deepest shadows and brightest highlights, and calculating the range between the two in stops. Ex. 1, if your highlights read EV 15 and your shadows read EV 5, your scene’s dynamic range is 10 stops. Ex. 2, if your light meter prints 𝒇16 when you point it at the highlights and 𝒇2.8 in the shadows, your scene’s dynamic range is 5 stops.
If you know that your film has a dynamic range of 5 stops, you can safely assume that it will be able to capture your scene in Ex. 2 entirely with all the shadows and highlights intact. However, it will not capture the scene in Ex. 1, leaving you with either (or both) shadows and highlights missing information.
But film manufacturers do not print their film’s dynamic range on the box. To figure it out, you’ll need to look for your film’s datasheet, which will almost always include values written in lux-seconds under its film characteristic curves (use this guide to extrapolate dynamic range from those graphs).
You will then need to convert 𝚫lux-seconds from the datasheet to stops so that you can compare them to your light meter readings for your scene’s dynamic range. To do this, you will need to use this equation:
2ˢᵗᵒᵖˢ = 10ˡᵘˣ ˢᵉᶜ
Both sides of this equation represent a single arbitrary unit or step increment expressed in different ways. On the left, a single step doubles the power (2¹ = 2); on the right, a single step is 10x the power (10¹ = 10). Thus an increase in 1 stop should equal:
stops = log₂(10ˡᵘˣ ˢᵉᶜ)
The calculator above will do all this work for you.
Now that you know your film’s dynamic range, you also know how well it can capture your scene.
In practice, you can just look for dynamic range and other film properties or just expose guide in reviews. But not all film reviews (here or elsewhere) list this info.
How to convert lux to EV.
Your light meter may sometimes output lux (scene illuminance) value, which will need to be converted to EV₁₀₀ to be used with a camera. To do this, you need to consider this equation, which can be used to convert EV₁₀₀ to lux:
0 = log₂(lux × 100 / 250)
The simplified and balanced formula to compute EV₁₀₀ from lux is:
EV₁₀₀ = log₂(lux × 0.4)
Again, the calculator above will do all this work for you (both ways).