www.scantips.com

Equivalent Exposure Calculator
to Compare Any Two Camera Exposures

How many stops difference? What are the EV numbers? Find an Equivalent Exposure?
Want to know the value of f/6.3 + 1.67 EV of exposure?

There are three modes here:

The fields in this calculator work in nearest third or half stops, like the camera. If wanting to compare individual relationships (differences of two fstops, or shutter speeds, or ISO), then see another calculator.

Exposure Calculator

1. Difference AB
Shutter Speed
f/stop
Settings EV
ISO
The Exposure is
Light Value
EV at ISO 100
Choose 1 of the 3 choices for Options 2 or 3 below:
The shutter speed changes (aperture preferred)
The f/stop changes (shutter preferred)
The ISO changes (manual settings)

2. Find an Equivalent Exposure B to be equal to A

3. Add EV to a value in B
      (accepts EV formats like 2.3333 or -2 1/3)

Option 1 recomputes if a field changes. Option 2 & 3
recompute when the Compute button is clicked

The selections above provide the possible camera settings. Half stops are marked with *½.

The calculator range of Nominals is large, but not quite infinite.

The f/stop range is f/0.5 to f/512 (20 EV range).
Shutter speed range is 2048 seconds (34 minutes) to 1/32000 second (26 EV range).
ISO is from ISO 1.56 to 3276800 (21 EV range). Seems adequate for photography.
The wide range prevents some equivalents of unsuitable settings from simply going out of range.

In the calculator:

The camera settings EV term might be misunderstood, because it is computed from only the shutter speed and f/stop values (a calculator is on next page). This Settings EV is seemingly independent of ISO, except the EV number is computed from the camera settings already chosen for whatever appropriate ISO is in use, which makes ISO in fact be all important. Numerical EV was developed when light meters were to be added into film cameras (late 1950s) to aid computing exposure. Then film speed (called ASA then, ISO today) was a temporary constant determined by the roll of film in the camera. This ISO number was identified to the camera, so the metering knew it and could compute how much the camera settings needed to change to match exposure to this film speed. The settings combination of f/16 at 1/125 second is numerically EV 15 for any ISO, but those settings are chosen and only applicable for the one specific ISO currently being used. So maybe the formula to calculate EV does not include the ISO number, but the choice of the proper camera settings used depends on ISO. The camera settings are not necessarily a correct exposure unless ISO does match them to the scene. More on next page (EV Chart).

The calculator default numbers represent full bright direct sunlight. The way the exposure is said is 1/2000 second at f/11 (EV 18) at ISO 800 (is EV 15100). An equivalent is also 1/125 second at f/16 (EV 15) at ISO 100 (is EV 15100), or other Equivalents, like 1/250 at f/11 at ISO 100 (also is EV 15100). All of these are the same exposure. However in practice, the EV number alone is not meaningful without specifying the corresponding ISO number.

That settings EV is that Exposure Value that a light meter would read when set to that ISO. 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. The one EV chart is for ANY AND EVERY ISO value, meaning specifically whichever chosen one is in use. If we change ISO, it changes the metered EV and the settings we would select.

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 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 that one row of the EV chart.

Light Value converts the ISO to an arbitrary but standard ISO 100 value for the purpose to compare two actual exposures at the same ISO 100 values. The number 100 is NOT a special ISO number at all, Light Value could compare by converting both readings to any same ISO number, but 100 is merely a popular convenient number. Light Value represents the corresponding relative "brightness level", so to speak, which then would require the appropriate proper corresponding exposure. More about Light Value on next page, and the formulas are on a math page. These are still the same two exposure levels (EV 18 at ISO 800 and EV 15 at ISO 100), but when both EV are converted to same ISO 100 for easy comparison, then here, both Light Values are EV 15100, or again, same light levels require Equivalent Exposures (at the same ISO 100). The Settings numbers may not have been Equivalent, but the resulting brightness level (adjusted for ISO) of the two Light Values of the exposures are the same, and Equivalent.

The calculator's initial default values of 1/2000 second and f/11 is EV 18 at any ISO. It is a correct exposure at only one ISO. Here, the exposure is specified as EV 18 settings at ISO 800. Set a light meter to ISO 800, and it would meter EV 18 here, and the EV Chart row (next page) for EV 18 will reflect the settings necessary for ISO 800. All columns of that row EV 18 are Equivalent Exposures.

The second 1/125 second at f/16 is EV 15 at any ISO. Here exposure is the EV 15 settings at ISO 100. Set a light meter to ISO 100, and it would meter EV 15 here, and the EV Chart for row EV 15 will reflect the settings necessary for ISO 100.

Which are Equivalent Exposures, because 1/2000 f/11 to 1/125 f/16 second is +3 EV more exposure, but ISO 800 to ISO 100 is -3 EV less exposure. This is a simple case, but the numbers can get more complicated.

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 faster 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, must match for any reasonable use.

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

A: 1/2000 second, f/11, for EV 18, but at ISO 800, it is Light Value EV 15.
B: 1/125 second,   f/16 for EV 15, but at ISO 100 it is Light Value EV 15.

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 1/2000 f/11 is less exposure of brighter light at higher ISO.

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 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 © 2014-2021 by Wayne Fulton - All rights are reserved.

Previous Menu Next