Understanding the Camera Numbers -
Aperture f-stop, Shutter Speed, ISO

This article is about "Understanding the camera Numbers". It is NOT a primer about using those numbers to take photos (for that, see for example).

Where the Numbers Come From

The number for lens f/stop in photography (for example, f/8) is the ratio of lens focal length divided by the effective lens aperture. Aperture is not exactly the obvious physical diameter (shown by black vertical lines below), but instead is the apparent "working" diameter D 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, especially in the old days in lenses without modern coatings (losing a lot of light), and even today in fancy lenses containing 15 or 20 glass elements (30 or 40 surfaces), losing a slight amount of light if with best coatings (lens coatings allow the light to pass through the lens, instead of reflecting it away). There are normally only slight variations, but which are important in professional movie cameras, when switching lenses on the same scene. So the professional movie lenses use T-stops, with markings which are calibrated to the actual amount of light the lens transmits, instead of the theoretical amount (a T2 lens actually transmits the light that a perfect f/2 lens should, matching the light meter). But improved modern lens coatings largely improve this today. Fancy zooms with many elements suffer a little more, which the coatings are to help it. But it is minor and relatively unimportant for still cameras, since the camera meters through the lens anyway, automatically accounting for any possible variance in the lens losses.

And while we are momentarily distracted, the marked Focal Length is when focused at infinity, but it changes as we focus closer (focal length normally becomes longer if front element is extended, but internal focus lenses probably become shorter). The actual focal length is measured to the rear Principle Point, H', as shown above. The Principle Point is the designer's apparent plane where the image appears to be. Design of curved lens elements can move this point, and this H' point is in fact often literally outside the actual lens, either in front or behind. In telephoto lenses, this H' point is always slightly in front of the front lens element, because, the actual optical technical definition of "telephoto" is that the lens is physically shorter than its focal length (which is a practical way to build the long lenses that show distant objects enlarged). Wide angle lenses are often retro-focus, which means the rear node H' is well behind the rear element. This allows the short lens to be mounted well forward, leaving space for the SLR camera mirror to be raised. Otherwise for example, an 18 mm lens would block raising a mirror 24 mm tall.   FWIW, regarding this H' distance, the ratio of subject size to image size is called Magnification, and when equal sizes (at 1:1 reproduction ratio), then also equal distances - the distance in front of the lens is necessarily equal to the distance behind the lens, at 1:1 size (similar triangles, etc.) Seems a cute fact, which aids understanding.

Is f/stop written f/stop or f-stop or fstop? The lens manufacturers properly write f/8. The internet changes things, and the term f-stop has become very popular on-line, but we also still see f/stop. I learned to write f/stop, because we also write f/8, to be remindful of the division defining it:

f/stop number = focal length / aperture diameter

f/8 is an aperture diameter, literally = focal length / 8.

Why do all lenses expose equally if all are set to the same f/stop? For two lenses at the same f/8, the lens with 3x longer focal length has an aperture diameter 3x larger. Tricky, but the 3x focal length magnifies the subject 3x, and so it sees 1/3x width and 1/3x height, which is 1/9 area, which only reflects 1/9 the light the wider lens sees. But the longer 3x lens also has aperture 3x larger, which is 9x area, and so now admits 9x more light, which before was 1/9 as much, from a 1/9 area field... so the 9x times 1/9 result is the same exposure in both f/8 lenses. Another argument is the Inverse Square Law over the 3x longer focal length is 1/9 the light, when the image reproduction reaches the sensor plane (which is just repeating the first explanation again). This is why the f/stop system is used. It's good stuff.

So, f/8 denotes (focal length / 8), which represents the aperture of the lens, and this exposure value can be compared with other lenses in this way. A series of multiple f/stop steps is designed, called "stops". Stop originally denoted the notched detent which marked the 2x area multiples. Today in photography, the word stop is used to mean any step of double or half value of exposure, also in regard to shutter speed and ISO. Each full stop towards larger f/stop numbers gives half the light exposure of the previous step (called stopping down, which also increases depth of field).

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


There are two concepts of camera numbers here. "Nominal" numbers are the numbers actually marked on the camera, which are just convenient approximations of what I call the "Precise" numbers that the camera actually uses. The camera knows to actually do it right, but the marked numbers are made easier for humans. Much more below and next page.

Nominal f/stop numerical values might be rounded, but are often truncated, or even approximated.

The cameras and light meters are marked f/11, and we say it as f/11, but f/11.31 is the necessary correct actual calculated value. This is only about 0.08 stop difference, and any difference exists only in our mind, since the camera will do it right anyway. Most other values are closer, but Guide Number calculations for speedlights can use f/11.31 instead of f/11.

To make this fact be obviously true, note that f/stop numbering is the sequence of √2 intervals, (which is 1.414) - making every other stop number be a multiple of 2. These sequence progressions, when arranged into rows of every other doubled aperture values, are:

f/   12481632
f/ 1.4142.8285.65711.31422.627

It may be handy to realize that doubling any f/stop number (for example, f/5 to f/10) is exactly two stops.

Shutter speed marking numbers are also approximated. For example, the camera nominal markings show 1/20 second and 1/10 second (and 10 and 20 seconds) to be both third stop values and half stop values. But the same value cannot be both values, and the camera does compute the actual value closer (half stop 20 seconds will be 22.6 seconds, and full stop 30 seconds will be 32 seconds, see standard shutter speed charts on next page). The camera does it right, but we humans are frequently shown easier approximated numbers.

Calculator to Compare Difference of any two f/stop values

It is not obvious that the difference between f/4 and f/5 is 2/3 stop, and f/9 to f/10 is 1/3 stop, so the calculator purpose is to help. The calculator does NOT just use camera nominal values, and then fudge by presuming to round off to thirds. The menus are nominal choices, but internally, it instead uses the same precise values the camera uses internally. Where it is a factor (Option 3), it shows both the precise result, and the precise difference between it and the precise value of the nearest nominal value it also shows.

Calculator, Stops Difference Math

1. Precise values (full, third, half stops)

  from A:    to B: 

2. Difference in any two numeric values

  from A:  f/   to B:  f/

3. Add ± EV to a f/stop value

Add EV   to  

4. Tenth stops to Nominal stop values

  from plus   to  


Tenth Stop Usage

DSLR cameras today can be set to use full stops, half stops, or third stops. I cannot imagine any reason not to choose third stops, more 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. The error is not more than 1/6 stop.

But handheld light meters typically can also be set to read tenth stops (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

This is NOT f/8.7. It is 7/10 of the way between f/8 and f/11 - or about f/10, but read as "f/8 plus 7/10 stop".

By definition, the equivalent value of f/8 plus 7/10 stop is 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 is 0.1 stop. The fraction 1/3 stop is 0.33 stops, and 2/3 stop is 0.67 stops, so a reading around 0.3 is one third stop, and one around 0.7 is two third stops. The lens can only be set to third stops, so just pick the nearest third stop:0, 1/3, 2/3, or 1 stop.

There would seem no point of 1/10 stop meter readings for daylight, since we can only set the camera to the nearest third stop. However there are two good reasons to use tenth stops for 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 is when pondering fill level for that lighting ratio - how much is one and a third stop less than f/10? It is about f/6.3, but who knows that? 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 know 1.3 stops difference, in our heads, immediately (in use, that is really big).

F/stop Table, in Tenth-Stops


Notes: (√2 is 1.414).   f/stop = 1.414 (stop number + fraction)

e.g., two tenth stops past f/11 (stop number 7) is 1.414 7.2 = f/12.126
Or 1/3 stop past f/11 is 1.414 7.333 = f/12.698
Stop number -1 is f/0.7, and stop number -2 is f/0.5.

If interested, here is a one page printable PDF file of this tenths chart.

How the Stops are Numbered (Origin of these numbers)


Two purposes of f/stop nomenclature:
f/stop = focal length / aperture diameter

The focal length factor is about the magnification of the field of view.
A short lens (wide angle) gathers a lot of light from a wide view, and concentrates that light onto the camera sensor area.
A long lens (telephoto) gathers less light from a smaller view, onto the same sensor area.
But fstop = focal length / aperture diameter equalizes these, a larger aperture in a longer lens, giving constant exposure at equal f/stop numbers. Exposure is about Illumination per unit of scene area, which stays the same. That's why we bother with f/stop numbers, the benefit is great. f/8 is always f/8, on any lens. Our light meter works the same for any lens.

Aperture is circular, and the area of a circle is defined as Pi r². Double area is twice the light, or one stop.
For double area: 2 Pi r² = Pi (1.414 x r)² , so 1.414x radius gives one stop. √2 is 1.414).
Since f/stop = focal length / aperture diameter, then f/stop numbers increase in 1.414x steps (or 1/1.414 is 0.707x decreasing steps).
Inversely, when the diameter and area are made larger, the f/number from the ratio f/d becomes a smaller number.

See the note under the tenth stop table above for a formula computing f/stop for partial stops.

Full f/stop numbers advance in 1.414x numeric multiples. From any f/stop number, in all cases, double or half of that number is two stops (for example, f/10.2 is two stops above f/5.1). Every second stop is the doubled f/number. Or one stop is x1.414 (or /1.414 which is x0.707).

Third f/stop numbers advance in multiples of the cube root of √2, or 1.12246x the previous (speaking of f/stops).
Every three third-stop steps (from any point) is exactly 1.0 stop and a 2.0x change of the light.

Half f/stop numbers advance in multiples of the square root of √2, or 1.1892x the previous (speaking of f/stops).
Every three third-stop steps (from any point) is exactly 1.0 stop and a 2.0x change of the light.

Less (f/stop number) is More (light).

Lens manufacturers seem to truncate instead of round off. For example, f/5.6 is actually 5.66, and f/3.5 is 3.56. But we see the same f/1.2 marking for the half stop (f/1.189) and third stop (f/1.260). Point is, the markings are just nominal numbers to show us humans. The lens and camera know to try to do it right.

Shutter speed and ISO

The values of shutter speed and ISO are linear scales, meaning that 2x the number is a 2x difference, and 2x is one stop. The very important thing to the definition of our exposure system is that any span of three third stop steps (or any two half stop steps) must come out exactly 1.0 stop of 2.0x exposure difference. To force this, cube root (and square root) steps are the proper values to create and number step intervals.

The next third-stop shutter step is cube root of 2 (1.26992) greater than the previous value (but for f/stop, see above)
Every three third-stop steps (from any point) is exactly 1.0 stop and a 2.0x change of the light.

The next half-stop shutter step is square root of 2 (1.41421) greater than the previous value.
Every two half-stop steps (from any point) is exactly 1.0 stop and a 2.0x change of the light.

The next full-stop value is 2x greater than the previous value. Doubling any numeric value is one stop (speaking of shutter speed or ISO, but 2x number is two stops for f/stops, see above.)

The nominally marked numbers may not be precise, but the camera knows exactly what to do. For example, set ISO 250 or ISO 2000 in the Nikon DSLR camera. Then near the top of the Exif data will show the ISO 250 or 2000 values, but further down in the manufacturers data, it shows the precise values used, ISO 252 or ISO 2016. (The ISO base appears to be 100 instead of 1... 100, 200, 400, 800 instead of 1, 2, 4, 8. This makes third stops of 252 and 2016 instead of full stops 256 and 2048 - which we call 250 and 2000.) Auto ISO is probably using 1/6 stops, but which will still be the precise sixth root of 2 steps. The numbers we see are just convenient nominal numbers, which the number really does not much matter to us humans. We just want one stop to always be a 2x light value. The point here is that the camera typically uses numbers a little different than the numbers we see. The only time that actually matters is if we try calculating ourself, using the nominal numbers instead of the actual precise numbers.

Shutter Speed Marked As:
30 seconds30"
4 seconds4"
1/4 second4
1/30 second30
1/4000 second4000

Shutter speed is of course the time duration when the shutter is open, exposing the sensor or film to the light from the aperture. On many cameras, numerical values for shutter speed are marked on the camera using two methods with different meanings - for example, marked as either 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.

A flash, especially a speedlight flash, is typically a much shorter duration than the shutter. The flash simply must occur while the shutter is open (sync), but the faster flash exposure is not affected by the slower shutter speed. Keeping the shutter open longer does increase the continuous ambient light seen, but shutter speed does not change what the fast flash does.

Continued, Nominal and Precise Camera Settings, and charts.

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