What the Photo Exposure Reciprocity Calculator does
The Photo Exposure Reciprocity Calculator estimates a shutter time that preserves the same exposure when you change the aperture. Enter a known exposure time and its aperture, then enter the aperture you want to use. The result is an estimated exposure time in seconds.
This tool applies a straightforward aperture relationship. It assumes the scene brightness and ISO remain unchanged, and that the camera's shutter time is the setting adjusted to compensate for the aperture change. For example, closing the aperture lets in less light, so the shutter must stay open longer to maintain the same exposure. Opening the aperture has the opposite effect.
The term “reciprocity” here describes this basic exchange between aperture and exposure time. It does not model film reciprocity failure, which can affect long exposures with some film stocks. For those exposures, consult the film manufacturer's guidance and account for the particular film and conditions.
How to use the Photo Exposure Reciprocity Calculator
Start with an exposure that you know is appropriate for the scene. Record its shutter time in seconds and the aperture used. Then enter the new aperture you plan to use. Keep the f-number format consistent, such as f/8 entered as 8.
- Base exposure time: enter the shutter duration in seconds. A fraction such as 1/125 second is about 0.008 seconds.
- Original aperture: enter the f-number associated with that base exposure, such as 8 for f/8.
- New aperture: enter the f-number you intend to set, such as 16 for f/16.
The result is shown in seconds. With the defaults, a one-second exposure at f/8 converted to f/11.3 produces about 2.01 seconds. This is close to the one-stop change from f/8 to f/11, with slight variation because 11.3 is a rounded aperture value. If your camera offers standard shutter speeds, choose the nearest practical setting and review the image or meter reading.
How the Photo Exposure Reciprocity Calculator formula works
The formula is base_seconds × (new_aperture ÷ base_aperture)2. Aperture f-numbers describe the ratio between focal length and the effective opening diameter. The area of a circular opening changes with the square of its diameter, which is why the aperture ratio is squared in this calculation.
Worked example: suppose a scene is correctly exposed at 1/125 second and f/4, and you want to photograph it at f/8. Convert the starting shutter time to seconds, so 1/125 is 0.008 seconds. Then calculate 0.008 × (8 ÷ 4)2 = 0.008 × 4 = 0.032 seconds. That is approximately 1/31 second. Because f/8 admits one quarter as much light as f/4, the exposure duration becomes four times longer under the stated assumptions.
The equation works for a wider or narrower aperture. If the new f-number is smaller than the original, the ratio is below one and the calculated time decreases. If the new f-number is larger, the time increases. Equal aperture values return the original exposure time.
Use cases for the Photo Exposure Reciprocity Calculator
A photographer may use this conversion when changing depth of field while trying to preserve overall exposure. A landscape image made at a relatively wide aperture could be recalculated for a smaller aperture to increase the depth of field. The longer time may require a tripod, image stabilization, or a different ISO, depending on the scene and camera.
The calculation can also help with a controlled studio setup. If lighting and ISO remain fixed, it shows how shutter duration changes as aperture is adjusted. This can be useful when comparing exposure settings during a still-life or product photography session.
For motion photography, the result helps reveal the tradeoff. A longer shutter time can blur moving subjects, while a shorter time can freeze more motion. The calculator only computes the exposure relationship. It does not determine whether the resulting shutter speed suits the subject or the photographer's intended effect.
Other factors to consider when calculating exposure
Exposure depends on more than aperture and shutter time. ISO, illumination, metering, lens transmission, filters, and subject movement can all affect a practical photograph. The calculation isolates one relationship and assumes the other relevant conditions stay fixed. If you change ISO or add a neutral-density filter, include those changes separately using a reliable exposure method.
F-number entries represent ideal aperture values. Real lenses may transmit slightly different amounts of light at the same marked f-number, and camera shutter speeds are available in discrete increments. The calculator therefore gives a mathematical estimate rather than a guarantee of a specific image brightness.
Long exposures on film need extra care. Some films lose sensitivity during extended exposures, so multiplying the shutter time by the aperture ratio alone may not be sufficient. Reciprocity corrections vary by film stock and exposure duration. Check the film maker's current technical information, and bracket exposures where appropriate for the work.
Very long calculated times can also introduce practical issues such as movement, changing light, or noise in digital images. The tool accepts times from 0.001 to 3600 seconds and f-numbers from 1 to 64. Values within those bounds make the arithmetic available, but do not mean a particular setting is supported by every camera or lens.
Photo Exposure Reciprocity Calculator FAQ
Does this account for film reciprocity failure?
No. It calculates the basic aperture and shutter-time exchange while assuming exposure time responds linearly. Use the film manufacturer's reciprocity guidance for long exposures on film.
Why is the aperture ratio squared?
The area of the aperture opening changes with the square of its diameter. The f-number ratio captures the change in opening size, and squaring it expresses the corresponding change in light through the opening.
How do I enter a fractional shutter speed?
Convert the fraction to decimal seconds. For example, 1/250 second is 0.004 seconds. Keep the value within the calculator's stated range.
Should ISO stay the same?
For this calculation to preserve the exposure through shutter time alone, treat ISO and scene brightness as unchanged. If either changes, the displayed result does not include that adjustment.
Can the result be used as an exact camera setting?
It is an estimate in seconds. Cameras typically provide a limited set of shutter speeds, so select the nearest available value and verify exposure with your meter, histogram, or test image.