Tables for F/stop, Shutter Speed, ISO, and EV

Lens f/stop in photography is the ratio of lens focal length divided by the effective lens aperture. Aperture is not the obvious diameter (shown by black vertical lines below), but instead is the apparent "working" diameter as seen through the magnification of the front lens element.

That is math and physics, but still, the very useful purpose of f/stop numbers is the grand concept that f/8 will the same exposure in any lens, regardless of focal length, or physical size of construction. (OK, yes, there can be slight minor variations, in the old days in lenses without modern coatings, and even today in fancy lenses containing many glass elements - slight variations, but important in professional movie cameras, see T-stops.)

And while we are momentarily distracted, the marked Focal Length is when focused at infinity. The focal length is measured to the rear Principle Point . The Principle Point is the designer's apparent plane where the image appears to be. Curved lens elements can move this point, and this point is in fact often outside the actual lens, either in front or behind. In telephoto lenses, this point is slightly in front of the front lens element, and the actual definition of "telephoto" is that the lens is physically shorter than its focal length. Wide angle lenses are often retro-focus, which means the rear node is well behind the rear element. This allows the short lens to be mounted well forward, leaving space for the SLR mirror to be raised. Otherwise, an 18 mm lens could not possibly allow a 24mm mirror to raise.

Is f/stop written f/stop or f-stop? We see both, but I write f/stop, because we write f/8, to be remindful of the division defining it:

f/stop = focal length / aperture diameter.

So, f/8 implies (focal length / 8), which represents the effective diameter of the lens.

The tables below are the computed f/stop numbers of your camera aperture. The first table is the fractional tenth stop steps.
These charts show the numbers, and the relationships, and one purpose is to aid determining span in stops between two values.

F/stop Table, in Tenth-stops

Notes: f/stop = sqrt(2)^(stop number + fraction)

If you set your flash meter to read in tenth stops, it may read for example: f/8 plus 7/10 stop.

This is NOT f/8.7. It is 7/10 of the way to f/11 - about f/10.

If you set the flash meter to read in third stops, equivalent will instead read f/10.

By definition, both of these two equivalent values are 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 need not care (we are instead working in third stop differences from full stops).

One reason to use tenth stops is for greater precision in adjusting the power level of individual flash units, but the big advantage is when pondering fill level for lighting ratio - how much is 1.3 stop less than f/10? It is about f/6.3, but who knows that? But if we read this as f/5.6 plus 3/10 stop vs. f/8 plus 6/10 stop, then we know 1.3 stops in our heads, immediately. Or, the chart above can show it too.

Rounding: We round off to say it as f/11, but f/11.3 is the correct calculated value.
Guide Number calculations for speedlights ought to use 11.3 instead of 11.
To make that fact be obvious, note that progressions arranged into rows of every-other doubled aperture values are also defined as:

Some shutter speed numbers are also rounded. For example, the standard shutter speed chart below shows 1/20 second and 1/10 second (and 10 and 20 seconds) to be both third stop values and half stop values. It cannot be both, and the camera computes the actual value closer (half stop 20 seconds will be 22 seconds), but sometimes it shows us easy numbers we humans can handle.

Why the Values are Numbered Like This (slight rounding exists)

The area of a circle is defined as Pi r² (aperture is circular.) Sqrt(2) is 1.414, and Pi (1.414 x r)² = 2 Pi r², giving double area of circle r. Double area is one stop, twice the light.

f/stop numbers are in 1.414x multiples. From ANY aperture value number, 1.414x the f/number is one stop increase (or /1.414 is a one stop decrease... or 1/1.414 = x0.707).

In all cases, double any numeric aperture value is two stops greater (f/6.2 is two stops above f/3.1). Every second stop is the doubled f/number.

Shutter speed is of course the time duration when the shutter is open, exposing the sensor or film to light. On many cameras, numerical values for shutter speed are marked on the camera using two methods with different meanings - for example, as 4 or 4". Just the number alone, like 4, is an implied fraction (1 over the number), meaning 1/4 second. The same number written 4" means four whole seconds, not a fraction. A slow shutter is a longer duration, and a fast shutter is a shorter duration.

These values for shutter speed and ISO are linear scales, meaning that 2x the number is 2x different, and 2x is one stop. But half and third stop numbers are not simply 1.5x or 1.33x, that's wrong. Because - two half stops computed 1.5 x 1.5 comes out 2.25, not 2.0. And three third stops computed 1.33 x 1.33 x 1.33 comes out 2.37, not 2.0.

The next full stop is 2x greater than previous full stop value.

The next half stop is square root of 2 (1.414) greater than previous half stop.

The next third stop is cube root of 2 (1.26) greater than previous third stop.