Photography Exposure Time Calculator
Enter your baseline settings and target settings to calculate the new shutter time. Great for ND filters, long exposure landscapes, and quick exposure adjustments in the field.
What this exposure time calculator does
This tool helps you quickly compute equivalent shutter speed when you change aperture, ISO, and/or add an ND filter. In photography, exposure is a balance between three variables: shutter speed, aperture, and ISO. If one changes, another has to compensate to keep brightness consistent.
Instead of doing stop math in your head, this calculator applies the exact conversion in one click. It is especially useful when your final shutter value becomes long enough that camera dials are inconvenient (for example, 20 seconds, 2 minutes, or longer).
How the math works
Core formula
The calculator uses this relationship:
newTime = baseTime × (newAperture / oldAperture)^2 × (oldISO / newISO) × 2^(ND stops)
- Aperture change: going from f/8 to f/11 lets in less light, so shutter time must increase.
- ISO change: going from ISO 100 to ISO 200 makes the sensor more sensitive, so shutter time can decrease.
- ND filter: each stop doubles required shutter time.
Why this matters in real shooting
If you meter without a filter and then attach a 10-stop ND, your exposure can jump from fractions of a second into multi-second territory. This is where errors are easy. A reliable calculator saves missed shots and speeds up setup.
Step-by-step usage
- Meter your scene and record your baseline shutter speed.
- Enter your current and desired aperture values.
- Enter your current and desired ISO values.
- Select ND filter strength in stops.
- Click Calculate Exposure Time to get the new shutter value and stop breakdown.
Practical examples
Example 1: Landscape with a 10-stop ND
Suppose your metered exposure is 1/125 s at f/8, ISO 100. You switch to f/11 and add a 10-stop ND. Your new shutter speed becomes dramatically longer, giving you smooth water and streaked clouds.
Example 2: Night city scene with ISO reduction
If you lower ISO to reduce noise (say, 800 to 200), shutter time must become longer to keep brightness. The calculator handles this directly so you can focus on composition and timing.
Common mistakes to avoid
- Mixing up aperture direction: a higher f-number means less light.
- Using nominal ND values only: real filters may transmit slightly more or less light than labeled.
- Forgetting reciprocity or sensor behavior: very long exposures may need testing and adjustment.
- Skipping stability: use a tripod, remote trigger, and turn off stabilization on tripod-mounted shots when appropriate.
Field tips for long exposure photography
Before pressing the shutter
- Compose and focus before attaching very dark ND filters.
- Use manual focus to prevent refocus errors.
- Cover optical viewfinders on DSLRs during long exposures to avoid light leaks.
During the exposure
- Use a remote release or timer to avoid vibration.
- Enable mirror lockup on compatible DSLRs for maximum sharpness.
- For exposures above 30s, use bulb mode and a timer.
FAQ
Does this calculator work for video?
It is primarily designed for still photography. Video shutter decisions are often constrained by frame rate and motion blur goals (e.g., the 180° shutter guideline).
Can I trust the result exactly?
Use it as a precise baseline. Real-world conditions (filter tolerance, changing light, camera metering behavior) may still require a test shot and minor correction.
What if my calculated time is very long?
If you enter multi-minute territory, watch for sensor heating, moving light conditions, and battery drain. Consider multiple shorter exposures if the scene allows.