Exposure Calculator

The exposure triangle is the relationship between the three camera settings that control exposure: aperture (f-number), shutter speed, and ISO. Changing one by a full "stop" doubles or halves the light, so any one of them can be traded against the other two while keeping the same exposure. The Equivalent Exposure tab above lets you lock any two values and solve for the third — for example, if f/8 at 1/125 s and ISO 100 gives the right exposure, then f/5.6 at 1/250 s and ISO 100 gives exactly the same exposure (one stop more aperture, one stop less time).

Exposure value is calculated with the formula EV = log2(N^2 / t) + log2(100 / ISO), where N is the f-number, t is the shutter time in seconds, and ISO is the sensor sensitivity. For example, f/8 at 1/125 s and ISO 100 gives EV = log2(64 / 0.008) = log2(8000) = 12.97. Each increase of 1 EV means half the light. You can invert the formula to solve for a missing setting: shutter t = N^2 / (2^EV x ISO/100). So at EV 13, f/8 and ISO 100, the shutter is 64 / 8192 = 1/128 s. The calculator above does this math for you in the Equivalent Exposure tab.

An equivalent exposure keeps the same EV while changing the settings. Enter your known-good aperture, shutter speed and ISO in the Equivalent Exposure tab to get its EV, then change one setting (for example open the aperture from f/8 to f/4, a 2-stop change) and lock the other two — the solver computes the new shutter speed (1/500 s in this example, 2 stops faster) that preserves the exposure. The tab also lists a full table of equivalent aperture/shutter/ISO combinations at that EV, and shows the nearest standard full-stop and 1/3-stop values so you can actually dial them in on your camera.

The Sunny 16 rule says: on a bright sunny day, set the aperture to f/16 and the shutter speed to 1/ISO for a correct exposure — for example f/16 at 1/100 s with ISO 100. That combination equals about EV 15, and the rule extends to other light: f/11 for hazy sun, f/8 for cloudy-bright, f/5.6 for heavy overcast, f/4 for open shade. The Scene EV tab above lists these conditions with their EV values and converts any scene to real settings at your chosen aperture and ISO, which is the modern version of the same idea.

Using the standard incident-light meter convention, EV at ISO 100 = log2(lux / 2.5), so lux = 2.5 x 2^EV. Bright sunlight at 100,000 lux is therefore EV 15.3, and a dim 100-lux interior is EV 5.3. To convert at another ISO, add log2(ISO/100) stops — for example EV 15.3 at ISO 100 becomes EV 17.3 at ISO 400. The Light Meter Converter in the Scene EV tab converts lux, foot-candles (1 fc = 10.764 lux) and EV in both directions, and maps the result to a scene description and camera settings.

The 500 rule gives the longest shutter speed you can use before stars start to trail: shutter (seconds) = 500 / (focal length x crop factor). With a 24 mm lens on a full-frame camera that is 500/24 = 20.8 s; with a 50 mm lens on an APS-C camera (1.5x crop) it is 500/75 = 6.7 s. It is a conservative rule of thumb — modern high-resolution sensors show trailing sooner. The Astro tab computes it alongside the stricter NPF rule so you can compare both.

The NPF rule is a more precise version of the 500 rule that accounts for your aperture and your sensor's pixel size: shutter = (35 x aperture + 30 x pixel pitch in micrometers) / focal length in mm. For example, a 24 mm f/2.8 lens on a camera with 5.93 um pixels gives (98 + 177.9) / 24 = 11.5 s — noticeably shorter than the 500 rule's 20.8 s, because it models how far a star actually drifts across real pixels. Pick your camera from the built-in database (pixel pitch derived from manufacturer sensor specs) in the Astro tab, or enter a custom pixel pitch, to get both numbers side by side.

Use the Looney 11 rule: for a properly exposed moon, set the aperture to f/11 and the shutter speed to 1/ISO — for example f/11 at 1/100 s with ISO 100 (about EV 13.6). The moon is sunlit, so it is far brighter than the night sky around it; metering the whole frame will usually overexpose it. The Astro tab includes the Looney 11 preset plus eclipse-phase exposure guidance, and the Star Trails planner estimates total shoot time from frame count and interval.

An ND (neutral density) filter reduces the light by a fixed number of stops, and the shutter speed must be multiplied by 2^stops to compensate. A 10-stop ND1000 filter turns 1/60 s into (1/60) x 1024 = 17.1 s. Filter numbers convert to stops with log2 — ND64 is 6 stops, ND1000 is about 10 stops — and optical density converts with stops = density x 3.32, so a 3.0-density filter is about 10 stops. Enter your base shutter speed and filter in the ND & Long Exposure tab to get the new shutter time, formatted for your camera's bulb mode.

Yes — stacked ND filters simply add their stops. An ND64 (6 stops) plus an ND8 (3 stops) equals 9 stops total, so 1/250 s becomes (1/250) x 512 = 2.05 s. Watch out for vignetting and color casts when stacking, especially with wide-angle lenses. The ND & Long Exposure tab has a stacking mode where you pick two filters and it sums the stops and computes the final shutter time for you.

Shutter angle expresses video shutter speed relative to the frame rate: shutter time = angle / (360 x frame rate). The classic 180-degree rule at 24 fps gives 180 / (360 x 24) = 1/48 s, which produces natural-looking motion blur. Going the other way, 1/50 s at 25 fps equals a 180-degree angle. The Shutter Angle tab converts in both directions for common frame rates (23.976, 24, 25, 29.97, 30, 50, 59.94, 60, 120 fps) and flags whether your angle follows the 180-degree rule.

Exposure compensation (EC) shifts the exposure by a chosen number of stops from what the camera meter suggests: +1 EC doubles the light, -1 EC halves it. If the camera meters 1/125 s at f/8 and ISO 100 and you dial in +1 EC, the new shutter is (1/125) x 2 = 1/62.5 s (1/60 s nearest standard value) at the same aperture and ISO — or equivalently ISO 200 at 1/125 s. The EC & Bracketing tab applies compensation to shutter speed or ISO, and builds HDR bracketing sequences (for example 5 frames at 2-stop steps) with the exact shutter time for each frame.

Pick a base exposure and a bracket pattern such as 3 frames at 1-stop steps or 5 frames at 2-stop steps. The EC & Bracketing tab lists every frame with its EV offset and exact shutter time — for a 1/125 s base with 3 frames at +/-1 stop you get 1/250 s (-1), 1/125 s (0), and 1/62.5 s (+1, shown as 1/60 s nearest standard) at constant aperture and ISO. Use a tripod, keep aperture and ISO fixed, and vary only the shutter speed so the frames align cleanly in your HDR merge.

The rule of thumb is: use a shutter speed of at least 1 / (focal length x crop factor) to avoid camera-shake blur — for example 1/300 s or faster with a 200 mm lens on a 1.5x-crop camera. Image stabilization lets you go slower: each stop of IBIS halves the requirement, so 3 stops of stabilization turns 1/300 s into 1/38 s. The Scene EV tab's handheld calculator takes focal length, crop factor and IBIS stops and returns the minimum safe shutter speed, with the nearest standard camera value.

A pinhole's f-number is focal length divided by pinhole diameter — for example a 50 mm focal length with a 0.3 mm pinhole is f/167. Plug that f-number into the normal EV formula: at EV 15 (bright sun) and ISO 100, the shutter is 167^2 / 2^15 = 0.85 s. The Equivalent Exposure tab has a pinhole mode: enter focal length and pinhole diameter and it computes the f-number and the shutter speed for your scene's EV.

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