Exposure Triangle Calculator: Aperture, Shutter, ISO

This exposure triangle calculator works out the equivalent exposure for a given set of aperture, shutter speed, and ISO settings, and shows you how to change one setting while keeping the overall exposure the same. You enter your current settings and the calculator returns the corresponding exposure value (EV) and a list of equivalent settings. It is a practical tool for photographers who want to understand the relationship between the three exposure controls without carrying a light meter.

Formula

The exposure value (EV) is a single number that represents a combination of aperture, shutter speed, and ISO. The standard formula, as defined by the ISO 2720 standard, is:

EV = log₂(N² / t) + log₂(100 / ISO)

Where:

• N is the aperture f-number (e.g., 2.8 for f/2.8)
• t is the shutter speed in seconds (e.g., 0.008 for 1/125 s)
• ISO is the film or sensor sensitivity (e.g., 400)

The first part, log₂(N² / t), is the EV at ISO 100. The second part adjusts for the actual ISO. At ISO 100, the formula simplifies to EV = log₂(N² / t).

For example, f/2.8 at 1/125 s gives N² = 7.84, t = 0.008, so N²/t = 980, and log₂(980) ≈ 9.9. At ISO 400, the adjustment is log₂(100/400) = -2, so the final EV is 7.9.

Worked Example

Using the default values from the calculator: aperture f/2.8, shutter 1/125 s (0.008 s), ISO 400.

1. Square the aperture: 2.8² = 7.84
2. Divide by shutter speed: 7.84 / 0.008 = 980
3. Take log₂ of that: log₂(980) ≈ 9.94 (using log₂(x) = ln(x)/ln(2))
4. Adjust for ISO: logâ‚‚(100/400) = logâ‚‚(0.25) = -2
5. Add: 9.94 + (-2) = 7.94, rounded to 7.9 EV

The calculator returns 7.9 EV, which matches the manual calculation.

How the Exposure Triangle Works

The exposure triangle is a model that describes how three settings control the amount of light that reaches the sensor or film:

• Aperture (f-stop) – the size of the lens opening. A smaller f-number (e.g., f/1.8) means a larger opening, letting in more light. A larger f-number (e.g., f/16) means a smaller opening, letting in less light. Aperture also controls depth of field.
• Shutter speed – the duration the sensor is exposed. A faster speed (e.g., 1/1000 s) freezes motion but lets in less light. A slower speed (e.g., 1 s) lets in more light but can blur motion.
• ISO – the sensitivity of the sensor. A lower ISO (e.g., 100) produces cleaner images but needs more light. A higher ISO (e.g., 3200) amplifies the signal, allowing shooting in low light but introducing digital noise.

Doubling the amount of light through any one setting increases exposure by one stop (EV +1). Halving it decreases by one stop (EV -1). For example, going from f/2.8 to f/2.0 doubles the light (one stop), going from 1/125 s to 1/60 s doubles the light (one stop), and going from ISO 400 to ISO 800 doubles the light (one stop).

How to Use the Calculator

To use the calculator, enter your current camera settings. The calculator will return the EV, which is a standard measure of exposure. You can then use that EV to find equivalent settings: for any given EV, there are infinitely many combinations of aperture, shutter, and ISO that produce the same exposure.

For example, if you have EV 7.9 (from f/2.8, 1/125 s, ISO 400), you could also use:

• f/4, 1/60 s, ISO 400 (same EV, because f/4 lets in half the light, but 1/60 s lets in twice the light)
• f/2.8, 1/250 s, ISO 800 (same EV, because 1/250 s halves the light, but ISO 800 doubles the sensitivity)
• f/5.6, 1/30 s, ISO 400 (same EV, two stops less aperture, two stops more time)

To find these equivalents manually, you can change one setting by a certain number of stops and compensate with another setting by the same number of stops in the opposite direction. The calculator does the math for you, but understanding the stops is essential for making creative decisions.

Common Mistakes and Misconceptions

One common mistake is confusing aperture f-numbers with stops. The f-number scale is not linear: f/2.8 is not twice as large as f/1.4 in terms of light. The light-gathering area is proportional to the square of the aperture, so a one-stop change corresponds to a factor of √2 in f-number. For example, f/2.8 to f/2.0 is one stop (2.8/2.0 ≈ 1.414).

Another mistake is mixing up shutter speed units. A shutter speed of 1/125 s is often written as “125” on the camera, but in calculations you must use the decimal (0.008 s). Using 125 instead of 0.008 would give a wildly wrong EV.

Some photographers think that EV is an absolute measure of light. In reality, EV is a relative measure: it describes a combination of settings, not the actual scene brightness. A scene with EV 12 at ISO 100 is brighter than a scene with EV 8, but the same EV can be achieved with different settings depending on ISO.

Finally, the exposure triangle ignores other factors that affect image brightness, such as neutral density filters, flash, and the reciprocity failure of film at very long exposures. For practical digital photography, the triangle is a good approximation, but it is not a complete model.

When to Use the Exposure Triangle

Use the exposure triangle whenever you need to change one setting without changing the overall exposure. Common situations include:

• Freezing motion: You want a faster shutter speed, so you open the aperture or raise ISO to compensate.
• Increasing depth of field: You want a smaller aperture (e.g., f/11) for a landscape, so you slow the shutter or raise ISO.
• Reducing noise: You want a lower ISO, so you open the aperture or slow the shutter.
• Balancing exposure: You are shooting in manual mode and need to find a combination that gives a correct exposure meter reading.

The calculator is especially useful when you are in a hurry and need to find equivalent settings without doing mental math. It is also a great learning tool for beginners who want to internalize the relationships.

Limitations of the Formula

The EV formula is based on the assumption that the camera’s light meter is calibrated to a standard (ISO 100, and the scene is 18% gray). In practice, different cameras may have slightly different meter calibrations, and the actual exposure may need adjustment based on the scene’s reflectance. The formula also does not account for lens transmission losses (T-stops vs. f-stops), which can be significant in lenses with many elements.

Moreover, the exposure triangle does not consider the dynamic range of the sensor or film. A very high EV (e.g., EV 18) might be beyond the sensor’s ability to capture highlights, while a very low EV (e.g., EV -2) might have too much noise. The calculator gives you a theoretical equivalent, but you must still judge whether the resulting settings are practical for your camera.

Finally, the formula assumes that the shutter speed is exact. In reality, cameras may have slight inaccuracies, and the ISO setting is not always exactly what it claims (some cameras have a “real” ISO that differs from the nominal). For most purposes, the differences are small enough to ignore.

Frequently Asked Questions

What is a “stop” in photography? A stop is a doubling or halving of the amount of light reaching the sensor. Increasing exposure by one stop means twice as much light. On the aperture scale, one stop is a factor of √2 (e.g., f/2.8 to f/2.0). On shutter speed, one stop is a factor of 2 (e.g., 1/125 s to 1/60 s). On ISO, one stop is a factor of 2 (e.g., 400 to 800).

Why is the aperture scale not linear? Because the f-number is the ratio of the lens focal length to the diameter of the entrance pupil. The amount of light passing through the lens is proportional to the area of the opening, which is proportional to the square of the diameter. Therefore, to double the light, you must increase the diameter by √2, which means the f-number changes by a factor of √2.

What is the difference between EV and exposure compensation? EV is an absolute measure of a combination of settings. Exposure compensation is a relative adjustment you make to the camera’s metered exposure, usually in stops (e.g., +1 EV to brighten). The calculator computes EV, but you can use it to understand compensation: adding +1 EV means you need to double the light, which you can do by opening the aperture one stop, slowing the shutter one stop, or raising ISO one stop.

Can I use this calculator for video? Yes, the same principles apply to video, but shutter speed is often constrained by the 180-degree rule (shutter speed = 1/(2 × frame rate)) to achieve natural motion blur. You would then adjust aperture and ISO to get the correct exposure.

Does the exposure triangle work for film? Yes, but film has a characteristic called reciprocity, which means that at very long exposures (typically over 1 second) the film’s sensitivity changes, and you may need to increase exposure time more than the formula suggests. For digital, reciprocity is not an issue.

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in aperture | Aperture (f-stop) | f/ | 2.8 | 1
in shutter | Shutter speed (seconds) | s | 0.008 | 0.0001
in iso | ISO | - | 400 | 50
out ev | Exposure Value (EV) | EV | round(log2((pow(aperture,2) / shutter) * (100 / iso)), 1) | 1
out equiv | Equivalent exposure (same EV) | - | ev | 1