Exposure Value Calculator

TL;DR: To calculate Exposure Value (EV), use the formula EV = log₂(N²/t), where N is your aperture f-number and t is your shutter speed in seconds; this calculator computes the result instantly when you enter those two values.

What Is the Exposure Value Calculator?

The Exposure Value (EV) Calculator is a free online tool that determines the exposure value of a photographic setting based on two fundamental camera controls: aperture and shutter speed. Exposure value is a single number that represents a combination of these settings, standardized across all cameras and lenses. An EV of 0 corresponds to an aperture of f/1.0 and a shutter speed of 1 second, and each increment of 1 EV represents a doubling or halving of light reaching the sensor.

This calculator is essential for photographers who want to understand the theoretical brightness of a scene, compare different camera settings, or work with techniques like the Sunny 16 rule. It is also valuable for cinematographers, videographers, and students of photography who need to reason about exposure mathematically. By entering your aperture and shutter speed, you get an immediate EV value that tells you whether you are working in bright daylight (EV 15-16), overcast conditions (EV 10-12), or low light (EV 5 and below).

Unlike light meters that measure scene luminance, this calculator focuses purely on the camera-side settings. This makes it a deterministic tool: given the same inputs, you always get the same output. It removes guesswork when you are planning a shoot, comparing lenses, or troubleshooting why a particular image came out underexposed or overexposed relative to your intent.

How to Use the Calculator

Using the Exposure Value Calculator requires only two inputs. Follow these steps for an accurate result:

1. Locate the aperture input field. Enter your lens aperture as an f-number (e.g., 2.8, 4, 5.6, 8, 11). The calculator expects the numeric f-stop value, not a fraction or letter designation like "f/2.8" — just type "2.8".
2. Locate the shutter speed input field. Enter your shutter speed in seconds. Use standard decimal notation: 1 second is "1", 1/250 second is "0.004", and 1/1000 second is "0.001". The calculator does not accept fractions like "1/250" directly, so you must convert to a decimal.
3. Verify both values. Double-check that you have not accidentally entered a shutter speed in milliseconds or an aperture as a T-stop (which differs from f-stops due to light transmission losses).
4. Click the calculate button. The calculator applies the formula and displays the Exposure Value, typically rounded to one decimal place (e.g., EV 12.3).
5. Read the result. The output tells you the combined effect of your settings. A higher EV indicates a brighter scene or larger exposure; a lower EV indicates dimmer conditions.

There are no hidden fields or advanced options. The calculator expects only these two numeric inputs and ignores any other text or formatting. If you enter an invalid number (like a negative shutter speed), the calculator will return an error or "undefined" because the logarithm of a negative number is not defined in the real number system.

Formula and Calculation Method

The Exposure Value formula is a logarithmic expression that ties together aperture (N) and shutter speed (t). The equation is:

EV = log₂(N² / t)

Where:

• N is the relative aperture (f-number), e.g., 2.8, 4, 5.6.
• t is the exposure time in seconds, e.g., 0.004 for 1/250 s.
• log₂ is the base-2 logarithm.

This formula assumes ISO 100 as the baseline. If you are using a different ISO, you must adjust: EV_ISO = EV_100 + log₂(ISO/100). For example, at ISO 200, the effective EV is one stop higher than at ISO 100 for the same aperture and shutter.

The method works in three steps. First, square the aperture value (N²). Second, divide that squared value by the shutter speed (t). Third, take the base-2 logarithm of the resulting quotient. The base-2 logarithm is used because each whole number change in EV corresponds to a doubling or halving of light, which is exactly how aperture and shutter stops behave.

Worked example: Suppose you set your aperture to f/4 and your shutter speed to 1/125 second (0.008 s). Step 1: Square the aperture: 4² = 16. Step 2: Divide by shutter speed: 16 / 0.008 = 2000. Step 3: Take log₂(2000). Since 2¹⁰ = 1024 and 2¹¹ = 2048, log₂(2000) ≈ 10.97. Therefore, EV ≈ 10.97, which rounds to EV 11.0. This is a typical setting for a bright, overcast day or open shade.

Another example: f/2.8 at 1/60 second (0.0167 s). Step 1: 2.8² = 7.84. Step 2: 7.84 / 0.0167 = 469.46. Step 3: log₂(469.46) ≈ 8.87. So EV ≈ 8.9. This is a common indoor setting with moderate ambient light.

Practical Examples

Here are three realistic scenarios that show how the calculator behaves with different inputs and what the EV result means in practice.

Notice that the EV value does not depend on the specific combination, only on the ratio of N² to t. For instance, f/8 at 1/500 s gives EV ≈ 14.0, and f/4 at 1/125 s also gives EV ≈ 14.0, because both let the same amount of light reach the sensor — they are equivalent exposures. This is the core utility of EV: it collapses infinite possible aperture/shutter combinations into one comparable number.

Tips for Accurate Results

To get reliable EV calculations, pay close attention to the following details:

• Always convert shutter speeds to seconds. A common mistake is entering "250" for 1/250 second. That is mathematically wrong because the formula expects seconds, not a fraction denominator. Enter 0.004, not 250.
• Do not round intermediate results. When you square the aperture or divide by the shutter speed, keep as many decimal places as possible. Rounding N² or t early can shift your EV by several tenths, which might matter in precise work.
• Check the validity range. The formula requires t > 0 and N > 0. A shutter speed of 0 seconds (which is impossible physically) or a negative aperture will produce an invalid logarithm. If the calculator shows "undefined" or "error", re-check your inputs.
• Remember ISO is not included. The calculator assumes ISO 100. If you are shooting at ISO 400, your actual exposure value for the scene is EV_calculated + 2. Adjust manually if you need an ISO-specific EV.
• Use standard f-stop values for consistency. If you enter f/4.3 (a non-standard aperture), the calculator still works, but the result will be an intermediate EV that may not correspond to a whole stop. This is fine for comparison, but know that real cameras typically use full, half, or third-stop increments.

The most frequent error users make is entering a fraction like "1/100" in the shutter speed field. The calculator does not parse fractions. You must compute 1/100 = 0.01 seconds and enter that decimal. Likewise, avoid scientific notation if your device does not handle it well — type "0.001" rather than "1e-3".

Frequently Asked Questions

What does an EV of 0 mean in photography?

An Exposure Value of 0 corresponds to an aperture of f/1.0 and a shutter speed of 1 second at ISO 100. This combination lets in a very small amount of light compared to typical daylight scenes. EV 0 is often used as a reference point in photography and in the design of light meters. It is an extremely dim setting, roughly equivalent to a candle-lit room. In practical terms, you would only encounter EV 0 with very slow shutter speeds (several seconds) and very wide apertures (f/1.2 or wider) in near-darkness. Most modern cameras can handle EV 0 with high ISO settings, but it requires a tripod for sharp results.

How do I convert EV to Lux or foot-candles?

You can approximate scene luminance from EV using the relation: Lux ≈ 2.5 × 2^EV. For example, EV 15 (bright sunlight) gives approximately 2.5 × 32768 = 81,920 lux. At EV 10 (overcast), you get about 2.5 × 1024 = 2,560 lux. However, these conversions assume a subject with 18% reflectance (the standard gray card value). If your subject is darker or brighter than middle gray, the lux reading will differ. For precise light measurement, you should use a dedicated light meter rather than relying on EV-to-lux conversion formulas, as lens transmission and sensor efficiency also affect the actual light hitting the sensor.

Can I use this calculator for video exposure?

Yes, with one caveat. The EV formula applies equally to video because aperture and shutter speed (or shutter angle) function the same in cinematography. However, video typically uses a 180-degree shutter rule, meaning the shutter speed is approximately twice the frame rate. For example, at 24 fps, your shutter speed is about 1/48 second. In that case, you can enter t = 0.0208 seconds. The resulting EV will tell you whether your lens and lighting are sufficient for that frame rate. Be aware that video cameras often use T-stops instead of f-stops, which account for light loss in the lens elements. If your lens gives T-stops, the EV calculation will be slightly different because T-stops are photometrically accurate, whereas f-stops are geometric. For most practical purposes, using f-stops is acceptable for EV estimation in video.