Understanding Camera Numbers and their Origin.
Stop Calculator for f/stop, Shutter Speed, ISO

The f/stop numbers are explained on the previous page. The calculator below computes difference in individual values, in all three modes, f/stop, shutter speed, and ISO. There is also another Exposure calculator to compare two "total" exposures determined by including all three f/stop, shutter speed, and ISO parameters combined.

How Many Stops?

It may not be obvious that the difference between f/2 to f/4 is 2 stops, and f/4 to f/5 is 2/3 stop, while f/8 to f/9 is 1/3 stop, so the calculator purpose is to help with the math, for f/stops, shutter speed or ISO. The calculator options below provide finding differences of the possible settings of your camera.

Other syntax used here about the camera settings:

There’s not a great numerical difference, but the Nominal numbers are just what we’ve always been used to, and still assume will be seen, but the Precise numbers are the plan now. Users are OK with the Nominal values (because the camera design will handle the precision), but design and math and calculation need the precise values.

Calculator to find stops difference
in f/stops, shutter speed, or ISO

Compute Stops Difference

f/stop values
Shutter Speed values
ISO values

Show nearest nominal Thirds   Halves

1. Precise results of nominal values
from A: to B:

2. Difference in any two numeric values
from A: to B:

3. Add ± EV Exposure to a value
Add EV to
(Add formats like ± 1, 2.333, 2/3, 1 1/2 EV)

4. Compare metered Tenths and nominals
  from A: Full Stop plus EV
  compared to B: Nominals

Half stops are marked with *½ (to aid avoiding them, because your modern camera surely is set to use third stops, which are more precise exposures).

The Range of the Nominals here is large, but not quite infinite. The “nearest nominal” shown has limits, but precise calculations can go further.

Range here just meaning, it computes further, but the suggested nearest third nominals hold at those limits.

Option 1 selects the marked camera nominal numbers, but it computes with the corresponding precise values that the camera actually uses (better accuracy than the nominal marked numbers). The next page has a chart of the actual precise values that the camera uses, and this calculator matches that internally here.

Option 2 can be any numbers, assumed to be exact or precise numbers (But NOT nominal, so use Option 1 for nominal numbers, it understands nominals). Comparing results of calculated values would use this Option 2, however not sure there is much use for this option, since the only numbers we can select on the camera are the marked nominal numbers (computed in Option 1). But for the reasons stated, in Option 2, the numbers of nominal f/8 to f/11, or from 1/125 to 1/60 second, does Not compute exactly 1.00 EV. In Option 1, it does. And again, the camera always knows how to do it right.

Option 3 assumes that adding corrective +EV exposure compensation adds more shutter time, or causes a lower and wider f/stop number, or a higher ISO number (increasing exposure like the camera compensation does).
The EV increment entered can be any numeric value, like 1.5 EV, but 1/3 stop is 0.3333 EV, 2/3 stop is 0.6667 EV, and 1/2 stop is 0.5 EV (or you can enter easier formats 1/3, 1/2, or 2/3 there, or 1/6 or 1/10, etc, which are calculated precisely).

There are confusion factors here about exposure numbers. As Shutter speed reaches longer time duration and as ISO reaches higher speed sensitivity numbers, exposure increases. But as f/stop reach higher numbers, exposure decreases.

There are also differences in light brightness and exposure. In terms of exposure or compensation, we think of metered f/4 as being more exposure than f/5.6. And it is, when actually metering each light, metering f/5.6 saw a brighter light that needs stopping down to f/5.6 (less exposure), as compared to a dimmer light that meters f/4 (requiring that additional exposure). So (arguably) to reduce any confusion with the signs, the A > B comparisons here try to refer to exposure of the setting values.

Tenth Stop Usage

Cameras today can use third stops. If using a modern camera, I cannot imagine any reason not to choose third stops. Greater precision of adjustment can only be a good thing. When the light meter reads a certain value, the camera can be set to the nearest third stop. That difference is ± 1/6 stop (half a third), which is closer to what was metered. So the calculators above try to flag the 1/2 stop values as possibly unintended, but all the numbers work.

But handheld light meters typically can also be set to read tenth stops (advantages for metering multiple flash). If you set your light meter to read in tenth stops, the format of the result value we see is (for example):

f/8 plus 7/10 stop

But this is NOT f/8.7. It is 7/10 of the way between f/8 and f/11, about f/10, and read as "f/8 plus 7/10 stop".

The equivalent value of f/8 plus 7/10 stop rounds to simply two third-clicks past f/8, or one third-click below f/11 (easy to set). The camera dial will indicate f/10 there, but we can instead meter and work in tenth-stop differences from full stops.

Fractions: 1/10 EV is 0.1 stop. The fraction 1/3 stop is 0.333 stops, and 2/3 stop is 0.667 stops, so a reading of 3/10 is around one third stop, and one of 7/10 is about two third stops. The camera can only be set to third stops, so just pick the nearest third stop: 0, 1/3, 2/3, or 1 stop.

There would not seem much point of 1/10 stop meter readings for daylight (IMO), since we can only set the camera to the nearest third stop. Maybe metering in tenths could give us an indication that the 1/3 stop exposure we set was actually a bit more or less than metered.

However there are two very good reasons to use tenth stops for metering multiple flash. One is for greater precision in adjusting the power level of individual flash units — the actual difference between two lights could be controlled more closely.

But the overwhelming advantage of metering flash in tenth stops is when pondering fill level for that lighting ratio of two lights. If one light is f/10, how much is one and a third stop less than f/10? It is about f/6.3, but who knows that? That math is more difficult. But if we read these two values as f/5.6 plus 3/10 stop vs. f/8 plus 6/10 stop, then we easily see ratio is 1.3 stops difference, immediately, in our head.

That's really a big deal to know and use, tenths are a really fast and easy and convenient and precise way to set the lighting ratio, precisely, all in our head (see more about actual use of tenths).

The tenths chart below shows the precise calculated values. Light meters reading tenths don't show that calculated number, they just show values of full stops plus tenths, as for example f/4 plus 2/10 EV. And in addition to tenth stops, it was convenient to also show precise third values below too). Half stops are the +0.5 EV value.

f/stop chart of Tenth Stops, 1/10 EV

Change precision shown below

Notes:   f/stop = √2 (stop number + fraction)   (√2 is 1.4142)

e.g., 2/10 stops past f/11 (stop number 7) is √2 7.2 = f/12.126
Or 1/3 stop past f/11 is √2 7.3333 = f/12.699

If interested, here is a one page printable file of this tenths f/stop table.

Nominal and precise values of all (full and third and half and sixth and tenth) stops for shutter speed, f/stop and ISO are on the next page.

Note that "Stop Number" and "f/stop number" are different things. Stop Number is the exponent of √2 that computes the f/stop number. Example: Stop Number 6 is f/8, this way: √2 6 = f/8.

Continued, Nominal and Precise Camera Settings, and charts.

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