Exposure Calculator
to Compare Any Two Camera Exposures

How many stops difference? What are the EV numbers? Find an Equivalent Exposure?

There are two modes here:

If just wanting to compare individual relationships (f-stops, shutter speed, or ISO), then see the individual calculators.

Exposure Calculator

1. Difference AB
Shutter Speed
Settings EV
Working ISO

Light Value
EV at ISO 100
Find an Equivalent Exposure B, to be equal to A
2. Aperture Preferred (the shutter changes)
3. Shutter Preferred (the f/stop changes)
4. Manual settings (the ISO changes)

The selections above provide the possible settings (of full, third, half stops).
Full stops are Green. Third stops are Blue. Half stops are Red, also flagged with .

The calculator range is large, but not quite infinite.
The f/stop range is f/0.5 to f/512 (20 EV).
Shutter speed range is 2048 seconds (34 minutes) to 1/32000 second (26 EV).
ISO is from ISO 1.56 to 1638400 (20 EV). Seems adequate.

The camera settings EV term includes only shutter speed and f/stop (does Not specifically include ISO). EV is from film days, when ISO was a constant determined by the roll of film in the camera. The formula to calculate EV does not include the ISO number, but of course the choice of proper camera settings used definitely does depend on ISO. It works for the ISO you are using. But the camera settings are not necessarily a correct exposure unless ISO does match them to the scene.

That settings EV is that Exposure Value that a light meter would read when set to that ISO (the calculator default numbers represent full bright sunlight). It is how the EV Chart would be used, we would use the settings on that indicated EV chart row, for the proper metered exposure at that ISO (example next page). EV is technically computed from only the camera settings (only shutter speed and f/stop are computed), but those choices course depend on the ISO value. If we change ISO, it changes the metered EV and the settings we would select.

About the ± sign on EV regarding Exposure difference:

Comparisons here refer to Exposure, not to numerical EV or settings. Plus is a greater Exposure. A greater EV number implies a brighter light, so less Exposure is needed, which is minus. A greater EV number is a lower row in the EV chart with faster shutter speeds, which is less Exposure. EV number is about the necessary camera settings, and EV number increases in the opposite direction from the Exposure required (one EV is same amount as one stop, both are a 2x change in exposure).

ISO: That settings EV is the row in the EV chart where that combination of equivalent settings is found, and might appear to be independent of ISO in that way. However, when the light meter is set to an ISO and then it meters EV X, then the EV chart row X is the right settings for a correct exposure at that same ISO. ISO is of course very important. A different ISO would meter different settings on a different EV row. So the settings (shutter speed and f/stop) are selected with attention to the ISO (cause and effect), and certainly we do get a different EV with a different ISO.

Basically, the EV number is the "name" of the set of Equivalent Exposure combinations of f/stop and shutter speed found on one row of the EV chart.

Exposure Value and the EV Chart is covered on the next page. But either of the initial default settings above give the same equivalent metered exposure in typical bright sunlight. Both A and B are also the Sunny 16 Rule (except Sunny16 is often 1/3 stop more exposure than the EV system meters). The actual meter doing this is shown on next page. This is two actual real world meterings of two equivalent exposures.

That concept is, for the two ISO values, the light meter reads EV 15 or 18. Look up either EV 15 or EV 18 in the EV chart, and it will show these settings appropriate for that ISO. That's what the chart's about. Since ISO 800 exposes 3 EV brighter, it offsets the 1/1000 second shutter speed, so these two are Equivalent Exposures. The purpose of this calculator is to similarly compare any two exposures.

The calculators top "EV (settings)" value is computed from the settings entered, and (assuming a correct exposure) is what a light meter will read at the working ISO, and this is the EV to look up in a EV chart for use at that ISO value. The actual camera setting choices will have already been affected by that ISO, so this EV is not further affected by the ISO that we enter now.

The entered ISO values (the working ISO actually used by the setting exposure) are used for the "EV at ISO 100" values (often called Light Value), which simply converts both exposures from these values used, to be the same ISO 100, for purposes to compare the difference in the exposures for equality (ISO 100 is not a magic number, any SAME ISO value allows exposure comparison). The box above is organized its way to indicate that this ISO entry does not affect the top EV value again, but this ISO (now known) does affect converting to EV at ISO 100. The camera setting ISO used, and the calculator ISO entered, really must of course match for any reasonable use.

Specific example: The calculator defaults when this page loads are:

A: 1/1000 second, f/16, for EV 18 at ISO 800
B: 1/125 second,   f/16, for EV 15 at ISO 100.

Settings EV (without effect of ISO), then EV of settings A is +3 EV more than settings B (EV is the combination of shutter speed and f/stop settings). EV of A is greater (EV 18) because the 1/1000 shutter speed is faster, which is for less exposure of brighter light.

However Exposure Difference, if including ISO, then these two exposures are seen as Equivalent Exposures, because the higher ISO 800 of A compensates for its shutter speed. The EV 18 camera settings require +3 EV more ISO to be equal exposure of the EV 15 camera settings. Which of course it has that ISO here with ISO 800, so the exposures are seen Equal when adjusted to the same ISO 100 (EV100).

The Bottom Line at ISO100 shows 0 EV exposure difference (two equal exposures in this example).

There are more details about EV and LV on the EV chart next page.

Copyright © 2014-2018 by Wayne Fulton - All rights are reserved.

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