THE NUMBERS, EXPLAINED
How to use this calculator.
Assumes reciprocity and equal scene brightness. Real lens transmission and sensor behaviour can vary.
Start with your inputs
Enter original exposure time (s), original f-number, new f-number, original iso, new iso. Keep amounts in the units shown on each field. The example is a starting point, not a recommended value.
| Input | Example value |
|---|---|
| Original exposure time (s) | 0.01 |
| Original f-number | 4 |
| New f-number | 5.6 |
| Original ISO | 100 |
| New ISO | 100 |
A worked example
With the example inputs above, equivalent exposure time (s) is 0.0196. Change the inputs to explore your own scenario.
Putting the example numbers into the formula
Equivalent exposure time (s)
(0.01) × ((5.6) ÷ (4))^2 × ((100) ÷ (100)) = 0.0196
| Result | Example answer |
|---|---|
| Equivalent exposure time (s) | 0.0196 |
How this is calculated
Equivalent exposure time (s)
a × (c ÷ b)^2 × (d ÷ e)
Symbols and units
- a
- Original exposure time (s)
- b
- Original f-number
- c
- New f-number
- d
- Original ISO
- e
- New ISO
Use the units shown. A list keeps the order in which you enter its values. The symbol ^ means “raised to the power of”.
Assumptions and limits
Assumes reciprocity and equal scene brightness. Real lens transmission and sensor behaviour can vary.
Calculations use the values you enter. Blank or invalid inputs show an error instead of silently substituting zero. Results are displayed to useful precision; calculations retain the underlying numeric precision.
Questions about this tool
What does the result tell me?
Equivalent exposure time (s). The result applies to the method and inputs described above. Check the units and assumptions before using it in a decision.
Why might another calculator differ?
Differences in units, rounding, timing or model definitions can produce different answers. Compare the input definitions and the method, rather than only the final number. For this tool: Assumes reciprocity and equal scene brightness. Real lens transmission and sensor behaviour can vary.
Are my inputs stored or sent to AI?
Calculations on this page happen in your browser. The separate AI question finder sends its submitted conversation to OpenAI; these calculator fields are not sent. Favourites and recent tool names are stored locally. A shared link contains the inputs you choose to include.
Review status
Software example and browser checks are recorded in our quality report. No external expert review is claimed. Created with coding assistance; calculations run as ordinary browser code.
Read the quality report →