How this instrument works
This calculator solves the standard exposure value formula for shutter speed instead of EV: given an aperture (f-number) and a target exposure value -- read off an EV chart, a light meter, or matched to another exposure you want to replicate -- it returns the exact shutter speed in seconds needed to hit that EV. It's the direct algebraic inverse of this site's exposure calculator, which instead solves for EV given aperture and shutter speed.
Exposure value (EV) is a base-2 logarithmic figure from the APEX exposure system that combines aperture and shutter speed into one number describing scene brightness at ISO 100. Because it's logarithmic, each whole EV step corresponds to exactly one photographic 'stop' -- doubling or halving the light -- so solving backward for shutter speed at a fixed aperture involves a power-of-two calculation rather than simple division.
This is useful whenever you know the light level as an EV value and your desired aperture, but need the matching shutter speed -- for instance, working from a standard EV chart ('EV 15, bright sun'; 'EV 13, hazy sun') to set manual exposure without a light meter, or reproducing a known-good exposure at a different aperture than it was originally shot at.
- Enter f-number (f/N) -- the aperture you want to shoot at.
- Enter Target exposure value (EV) -- from an EV chart, a light meter reading, or another exposure you're matching.
- Read Shutter speed (seconds) beneath the inputs -- the exact exposure time needed to hit that EV at your chosen aperture.
- To convert the decimal-seconds result into a familiar shutter-speed fraction, take 1 divided by the result -- 0.0078125 seconds becomes 1/128s.
Worked example -- f/8 at EV 13
Enter an f-number of 8 and a target exposure value of 13 -- a standard EV-chart entry for bright, hazy sun. Shutter speed comes out to 8^2 / 2^13 = 64 / 8192 = 0.0078125 seconds, exactly 1/128s -- the shutter speed that pairs with f/8 to hit EV 13.
Questions
How is this different from the exposure calculator on this site?
The exposure calculator takes aperture and shutter speed as inputs and solves for EV -- useful for checking what EV a given exposure setting produces. This calculator runs the identical formula in reverse: it takes aperture and a target EV as inputs and solves for the shutter speed needed to reach it. Both use the same underlying EV = log2(N^2/t) relationship; only which variable you're solving for changes.
Where do I find a target EV value to enter?
Standard exposure value charts, widely published in photography references and camera manuals, list typical EV figures for common lighting conditions -- EV 15 for bright sun on snow or sand, EV 13 for hazy sun with soft shadows, EV 11 for open shade or an overcast sky, and so on down to negative EV values for night scenes. A handheld light meter or your camera's built-in meter can also give a direct EV reading of the scene in front of you.
Why does shutter speed change so much for a small change in EV?
Because EV is a base-2 logarithmic scale, so shutter speed depends on 2 raised to the EV power in the denominator -- each single-stop increase in EV halves the required shutter speed at a fixed aperture, and the effect compounds quickly. Moving from EV 13 to EV 15, for example, cuts the required shutter speed to a quarter of its original value, since two full stops means halving twice.
How do I convert the decimal shutter speed into a fraction like 1/125?
Take 1 and divide it by the calculator's decimal result -- a result of 0.0078125 seconds gives 1 / 0.0078125 = 128, so the shutter speed is 1/128s. Most cameras display shutter speeds as these familiar fractions rather than decimals, but the underlying exposure math, including this calculator's formula, always works in decimal seconds.
What aperture and shutter speed combination should I use for a given EV?
There's no single correct answer -- any aperture works, as long as you solve for the matching shutter speed at that EV, and different combinations trade off differently for depth of field and motion blur. A wider aperture at the same EV needs a faster shutter speed, useful for freezing motion or shallow depth of field; a narrower aperture needs a slower shutter speed, useful for greater depth of field or intentional motion blur.