The f/stop (lens aperture) numbering system: A look at how the f/stops are numbered and what the numbers mean.
A Lens Aperture Primer, Part II
Regardless of the brand or style of lens or camera you use, the f/stop system is extremely similar in all lenses.
If you were to take your lens apart (don't) this is what you'd find inside: an adjustable aperture ring. This one comes from a Contax lens.
A Really Easy Lesson in Aperture Terminology
A number of
years ago I brought a friend with me to the camera shop one Saturday
afternoon and, as usual, I got into a discussion about lenses and
f-stops and the glory of "fast glass" (I'll explain that in a minute)
with the clutch of photographers that were hanging around in the shop.
(Yes, this really is the stuff that photographers talk about.) The
conversation went on for an hour or so and when my friend and I left
she turned to me and said, "I didn't know you spoke another language."
To her, the names and numbers and descriptions of lenses that the
photographers were tossing around might as well have been spoken in a
completely unfamiliar language.
It was then that I realized
that the terminology that had become such a part of my consciousness
was, well, Greek to most non-photographers. It was a good lesson to
learn because as I began writing and teaching more I was always careful
to explain all the terms I was using so that no one was left needing a
translator. On the next few pages I'll define some basic terms and
unravel the last tidbits of math that you'll need to know to better
understand your camera's aperture system. Just think, after you've read
this chapter you'll be able to eavesdrop on the local photo bums and
understand every word.
The F-Stop Numbering System
All
lenses, regardless of brand or the focal length of the lens or the type
of camera that they are used on, use the exact same f-stop numbering
system. Some lenses may have more f-stop choices than others, but the
sequencing of the whole numbers that they share in common is exactly
the same. In the pre-digital days these numbers were always inscribed
on the lens barrel on what was known as the "aperture ring" and in
order to set a particular aperture, you simply twisted that ring until
the aperture you wanted was adjacent to a mark on the lens. Many DSLR
lenses still have an aperture ring but today, of course, apertures are
typically set electronically.
Each number in the traditional
f-stop sequence represents a whole stop difference in light gathering
from the adjacent number. If you move to a larger f-stop (a smaller
number) you double the light entering the lens; if you move to a
smaller f-stop (a larger number) you halve the light. The standard
f-stop numbering pattern looks like this: f/1.8, f/2.8, f/4, f/5.6,
f/8, f/ll, f/16, f22, f/32. There are some lenses that have larger
maximum apertures (such as f/1.4) or smaller apertures (such as f/45)
but they still represent full-aperture stops. The important thing to
remember is that among the apertures they do share, the sequencing is
always exactly the same.
One difference that you'll encounter
between older lens aperture rings and f-stops set electronically on
digital cameras is that, in addition to the whole number stops, you can
also set precise intermediate f-stops, typically in one-third stop
increments. On an older lens, for example, while you won't see any
intermediary stops between f/4 and f/5.6 (a whole stop), on some lenses
you would see tiny two tiny dots indicating third-stop increments. With
digital cameras, however, there are almost always incremental f-stop
choices between the whole stop apertures. On my Nikon DSLR, for
example, between f/4 and f/5.6 (one whole stop) there are two
incremental stops: f/4.5 and f/5 and these represent one-third stop
increments. Look in your manual and you'll find the exact sequence that
your camera uses.
Do you really need the minimal exposure shifts
offered third-stop increments? Actually, sometimes an extra third (or a
third less) of a stop is exactly what you need. As you gain experience
at working with different subjects (especially difficult or contrasty
subjects) you will find that the ability to "tweak" an exposure by a
third of a stop (or, more frequently, two-thirds of a stop) provides a
very important bit of flexibility, or what I call the "nudge" factor.
I'll often bracket an exposure in 1/3 stop increments, for example,
just to be sure I have the exposure I want Where the Numbers Come From
Until
you understand its meaning, the sequence of aperture numbers might look
somewhat random or even arbitrary but, of course, there is always the
persistent hand of science lurking just under the surface and the
numbers actually do mean something. Why should you care? Simple: once
you understand the basic principle of what the f-numbers mean it
becomes much easier to visualize the size (and comparative
relationship) of different f-stops.
The math is elegant and
simple: the f-numbers represent the ratio of the physical aperture
diameter to the focal length of the lens. In other words, if you're
using a 100mm lens and you're using an aperture of f/4, then the
physical size of the aperture is 25mm or one fourth of 100. If you
moved to a larger aperture (again, a smaller f-number) such as f/2, the
size of the lens aperture would be 50mm (because 50mm is one-half of
100mm).
You can always figure the diameter of the lens opening
by simply dividing the f-stop number into the focal length of the lens.
If you're using a 400mm lens set at f/8, for example, the diameter of
that f-stop is 50mm. If you switch to f/2, the diameter is 200mm. Which
is larger? Which will let in more light?
Will you ever need to
know the physical diameter of the aperture? Never. It will never come
up. But again, knowing how to figure it out will always help you know
which f-stops are bigger and which are smaller--and that is the only
reason to do the math.
It might help you visualize this if you
imagine flipping the lens opening sideways: If you were using a
setting of f/2 it would fit exactly twice from the center of the front
lens element to the sensor plane of the camera. By the same token, an
aperture of f/16 would fit 16 times from the center of the front
element to the sensor plane. It's easy now to visualize that an
f-number of f/16 represents a much smaller aperture than f/2--after
all, you can fit 16 f/16 openings from the front of the lens to the
sensor, but you can only fit two of the f/2 openings.
If you're
a math person the beauty of this logic is the kind of thing that makes
your morning coffee taste better. If you're not a math person, it's the
kind of thing that makes honey taste bitter and makes you a little bit
suspicious of people who say they do understand it. Not to worry
though, if understanding the relationship between f-numbers and lens
focal length leaves you cold, then ignore it. At no point when you're
out photographing a deer in a meadow at dawn will anyone (including the
deer) ask you to explain the size of various apertures.