shutter speed fraction → decimal seconds Converter
Convert fractional camera shutter speeds (such as 1/250, 1/1000, or 1/60) into exact decimal seconds, milliseconds, microseconds, and true binary ($2^{-n}$) exposure durations. Switch seamlessly between fraction-to-decimal and decimal-to-fraction modes with standard 1/3-stop camera dial references and ND filter stop multipliers.
Standard 1/3-Stop Shutter Speed Conversion Table
Click any row below to load that shutter speed directly into the converter above.
| Camera Dial Fraction | Exact Decimal Seconds (s) | Milliseconds (ms) | Microseconds (µs) | 180° Shutter FPS |
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How to Convert Shutter Speed Fractions to Decimal Seconds
Cameras display fast exposure durations as unit fractions of a second (such as 1/60, 1/250, or 1/1000) so photographers can read them at a glance on a viewfinder or top LCD. When programming camera intervalometers, calculating flash pulse durations, configuring motion-control rigs, or entering exposure metadata into scientific logs, you must convert those fractions into decimal seconds or milliseconds.
Core Conversion Formulas
- Fraction to Decimal Seconds: Divide the numerator ($N$) by the denominator ($D$):
t(s) = N ÷ D - Decimal Seconds to Milliseconds: Multiply decimal seconds by 1,000:
t(ms) = t(s) × 1,000 - Decimal Seconds to Microseconds: Multiply decimal seconds by 1,000,000:
t(μs) = t(s) × 1,000,000 - Decimal Seconds to Reciprocal Shutter Fraction: Divide 1 by the decimal exposure time:
Denominator = 1 ÷ t(s)(so0.004 sbecomes1 / (1 ÷ 0.004) = 1/250 s).
Nominal Camera Dial Markings vs. True Binary ($2^{-n}$) Exposure
Each full stop in photography halves or doubles the amount of light reaching the sensor. Mathematically, halving a 1-second exposure repeatedly produces the binary power-of-two progression: 1/2 s, 1/4 s, 1/8 s, 1/16 s, 1/32 s, 1/64 s, 1/128 s, 1/256 s, 1/512 s, and 1/1024 s. To keep camera dials clean and human-readable, manufacturers round those markings to convenient numbers (1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000). Internally, however, most modern digital cameras time their electronic and mechanical curtains along the true $2^{-n}$ binary progression (per the ISO 516 standard) so every full stop equals an exact 1.00 EV change.
Worked Examples
Divide 1 ÷ 250 = 0.004 seconds. Multiply by 1,000 to get 4.0 milliseconds (ms), or 4,000 microseconds (μs). Its true binary $2^{-8}$ camera timing is 1/256 s = 0.00390625 s.
At 24 frames per second with a 180° rotary shutter angle, exposure time is 1 ÷ (24 × 2) = 1/48 s. Dividing 1 ÷ 48 yields 0.020833 seconds (20.83 ms).
Take the reciprocal of 0.00125 s: 1 ÷ 0.00125 = 800. Therefore, 0.00125 seconds equals an exact camera shutter speed of 1/800 s (1.25 ms).
Frequently Asked Questions
How do I convert 1/250 shutter speed to decimal seconds?
Divide 1 by 250: 1 ÷ 250 = 0.004 seconds. In milliseconds, multiply 0.004 by 1,000 to get 4 milliseconds (ms).
What is 1/60 of a second in decimal and milliseconds?
1/60 of a second equals 0.016667 seconds (repeating) or 16.67 milliseconds (ms). It is the standard 180-degree shutter speed when filming at 30 frames per second.
Why do cameras jump from 1/8 to 1/15 and from 1/60 to 1/125?
While true halving of exposure time follows powers of two (1/8, 1/16, 1/32, 1/64, 1/128, 1/256), camera dials use rounded human-friendly numbers (1/15, 1/30, 1/60, 1/125, 1/250). The maximum rounding difference is only about 2.4% (0.034 EV stops), which is visually imperceptible, and most cameras internally trigger at the exact binary step anyway.
How do I convert a decimal exposure time like 0.4" into a shutter fraction?
Find the reciprocal by dividing 1 by 0.4: 1 ÷ 0.4 = 2.5, which corresponds to 1/2.5 s (or the reduced integer fraction 2/5 s). Many camera viewfinders display 1/2.5 s as 0"4 or 0.4".
How does an ND filter affect decimal shutter speed?
An $N$-stop neutral density (ND) filter multiplies your decimal exposure time by $2^N$. For example, starting at 1/250 s (0.004 s) and attaching a 10-stop ND1000 filter ($2^{10} = 1,024$) increases the required exposure to 0.004 × 1,024 = 4.096 seconds (approximately 4" on your camera dial).