ISO stops difference calculator

Enter a starting ISO and a target ISO to evaluate the base-two logarithm of their ratio. A positive answer means the target setting is more sensitive, a negative answer means it is less sensitive, and zero means the values match. The result works for standard full-stop numbers, intermediate settings, and custom measured values, making it useful for exposure planning, camera tests, lighting notes, and repeatable photography workflows.

What a stop difference between ISO values means

In photography, one full stop represents a factor of two in exposure-related quantity. Doubling ISO from 100 to 200 is therefore an increase of one stop, while moving from 200 to 800 is an increase of two stops because the sensitivity ratio is four. This calculator extends that familiar rule to any two positive ISO values, including third-stop settings and values that do not appear on a camera dial. It treats iso_from as the reference and iso_to as the destination. The signed result describes the move from the first value to the second: positive is an increase, negative is a decrease, and zero means no change. The accompanying ratio gives another way to read the comparison. A ratio of eight corresponds to three stops; a ratio of one quarter corresponds to negative two stops. ISO itself does not add light to a scene, and changing it is not physically identical to opening an aperture or lengthening a shutter time. The stop language is still valuable because it expresses the proportional sensitivity change on the same base-two scale used throughout exposure calculations.

How the logarithmic calculation works

The calculation is stops_difference = log2(iso_to / iso_from). First, the target ISO is divided by the starting ISO. The base-two logarithm then answers how many doublings, or fractional doublings, produce that ratio. If the target is ISO 800 and the start is ISO 100, the ratio is eight and log2(8) is three. Reversing the inputs produces a ratio of one eighth and a result of negative three. Values do not have to form an exact power of two: moving from ISO 100 to ISO 125 is about 0.321928 stops, which is close to a third-stop increment but preserves the mathematical value rather than forcing it to a camera setting. Both inputs must be finite and greater than zero because zero and negative sensitivity values cannot form a meaningful logarithmic ratio. Results are rounded to twelve decimal places for stable output while retaining more precision than ordinary exposure decisions require. The direction field summarizes the sign, but the numeric stop result should be used when compensating another exposure variable or comparing measurements.

Using the result in practical exposure work

Use the signed stop difference whenever you need to record or compensate for an ISO change. If the result is positive three stops, maintaining a comparable image brightness under a simple manual-exposure model would require removing three stops elsewhere, such as using a shutter time one eighth as long, closing the aperture by an equivalent amount, or combining adjustments. Real cameras complicate that comparison: ISO changes amplification and processing, available ISO markings may be rounded, dynamic range and noise can change, and automatic modes may alter more than one setting. For that reason, treat this calculator as an exact ratio converter rather than a promise that two photographs will look identical. It is especially helpful in automated test reports because the output includes the original values, their ratio, the signed difference, and an explicit direction. The algorithm is deterministic and sends no request to another service. Interactive browser use keeps the calculation local, while a successful API calculation costs $0.002 per item. For reproducible notes, preserve the input order and state whether the result is being used as a sensitivity comparison, an exposure compensation step, or a description of a camera test.