Some Basic Elements of
by Chuck Doswell
Latest update: 20 May 2002: fixed some errors in the discussion
about f-stops ... thanks to Quinn Cheung for pointing
Notice: This material
is copyrighted by Chuck Doswell. Any commercial use of it
in any medium without his expressed written consent is a
violation of Federal Copyright Law. Please contact him for
permission to use it. All photographs are copyrighted, as
well, so any use of these images, commercial or not, without
permission will be liable to prosecution under Federal copyright
law. Save all of us the hassle of litigation and get my
I have enjoyed
photography for a number of years. Being a meteorologist,
I have put a special emphasis on outdoor photography, notably
clouds and storms ... but it also means that I have a technical
bent. I have enjoyed the technical aspects of photography
and believe it might be of some value for me to develop
some materials meant to explain the process. Hence, I have
developed these notes as a way of describing and explaining
a number of these technical points. I hope you find them
useful and interesting.
have been developed here for typical, simple 35 mm cameras.
The details will vary from one manufacturer to another.
camera is a means of exposing film to light in measured
quantities. How does photography work? The key to photography
is the film; photographic film is basically a coating (known
as an emulsion ) on a transparent film backing. The emulsion
contains a number of very tiny grains of light-sensitive
chemicals suspended in an inert medium. These very tiny
grains of chemicals respond to the light falling on them.
When the film is exposed to the light, these chemical grains
change in a more or less permanent way. Before the exposed
film is developed, we say the image is latent in the altered
chemical grains. Development alters the grains in such a
way that it creates a real image (either a negative or a
positive image, depending on the type of emulsion) on the
Emulsions and film speed
Not all emulsions behave the same way to a given amount
of light. Some respond to light more readily than others,
and so are called "fast" films. That is, it takes
less light to produce a properly exposed film than a "slow"
film. Film speeds are expressed in ISO numbers . ISO-50
film is slower than ISO-100, and ISO-100 film is slower
than ISO-400. The rating of a film in terms of ISO numbers
increases with the speed of the film. We will be talking
about f-stops shortly, but there is a relationship
between f-stops and ISO numbers ... a change of ISO
by a factor of 2 corresponds to a change in the f-stop
of one full stop. That is, if we double the film speed (in
ISO units), we can decrease the aperture size by one f-stop
and get an equivalent exposure.
Sounds like you
always would want to buy the fastest film available, right?
Well, unfortunately, it's not that simple. The faster the
film, generally speaking, the larger the grain size. Large
grains can give an image a "grainy" look ... "fast"
film often shows the grainy character of the emulsion that
can detract from image clarity. Some typical ISO film speeds
are shown in Table 1.
1. A selection of typically-available film speeds, rated
in ISO numbers. Note that not all film speeds are at one
full f-stop intervals.
The notion of an emulsion's latitude is an important one.
The human eye is a remarkable instrument for seeing ...
one of its characteristics is a large dynamic range . That
is, when we look at a scene, some parts of it are dark and
some parts of it are light. What we mean by dynamic range
is the range of light and dark between seeing something
as totally black and seeing something as totally white.
If we can see structure and detail in shadow areas as well
as in brightly lit areas ... at the same time ... then we
have a large dynamic range. Of course, if it gets too bright
or too dark, our eyes can adjust to the amount of light,
but that's another issue. Emulsions vary as to their dynamic
range, but virtually all films have a much narrower dynamic
range than what our eyes can see. If the contrast between
light and shadow in the scene is large, then the film needs
to span a large dynamic range, or latitude, if all parts
of the recorded image are to retain detail in both shadow
and brightly lit areas at the same time.
Slide films (positive
images) typically have a latitude that is less than typical
print films (negative images). Some slide films have only
about a 2 f-stop latitude. Some print films have
perhaps 6 f-stops of latitude. Our eyes can see with
a range of much greater than this, but I don't know the
actual value in f-stops.
When an emulsion
has relatively little latitude, then scenes with a lot of
contrast between light and dark pose a problem: if the shadow
areas are properly exposed, the highlights "wash out"
(lose detail) and just look white; if the highlights are
properly exposed, the shadow areas "block up"
(another form of losing detail) and just look black. There
are ways to compensate for this at times, but if you are
using film to record what you see, remember to account for
this whenever possible.
When using print
film, remember that even if it has more latitude, the final
image has to account for the latitude of the emulsion being
used to make the final print (usually on paper). Print emulsions
typically have less latitude than the negative emulsions
used to record the image!
|3. The Camera as a light measuring
is the primary light measuring device, but it also acts
to focus the light onto the film. I am not going to discuss
focusing very much ... details of focusing depend on the
lens elements (usually glass of one sort or another). The
glass in a typical lens involves several elements (2 or
more), in an attempt to account for various kinds of distortion
(or aberration ) that a single element produces. The complex
structure of a good quality lens is the result of trying
to design a lens to minimize simultaneously several different
kinds of abberation in an attempt to get the maximum clarity
(focus) possible and still be affordable.
However, there is another critical element of the camera
that is usually contained within the body of the camera:
the shutter . Basically, the shutter is a mechanism that
stays closed most of time. It is only open for a precisely
measured amount of time, usually measured in fractions of
a second, called the shutter speed (or exposure time). Most
cameras have a range of shutter speeds more or less as shown
in Table 2.
Table 2. Typical lens shutter speeds.
Some cameras have much faster and/or much slower set shutter
Observe that, roughly
speaking, the pre-set shutter speeds change by a factor
of 2 (more or less), so that slowing the shutter speed down
to the next value provides twice the exposure. In real cameras,
the shutter often is of the "focal plane" type
and not part of the lens ... it is a slit that travels across
the image at a fixed speed. The effective shutter speed
is determined by the size of the traveling slit ... if it
is wide, the film is exposed for a longer time than if it
Apertures and f-stops
Not only is the time of the shutter's opening is controlled,
however. Also, the size of the lens's opening is variable;
the size of the opening is called the lens's aperture .
Basically, the aperture is controlled by a sort of diaphragm
with a circular hole that increases or decreases in size
according to how the lens is set. A simplified version of
the lens/aperture is shown in Fig. 1.
Fig. 1. The lens/aperture
combination, showing how the aperture determines how the
light is restricted by the aperture. For convenience, the
focal length and focal plane are illustrated on this schematic.
In (a) the aperture is large, in (b) the aperture is small.
In the following,
I have included some equations. If you are capable of understanding
the equations, so much the better, but it is not absolutely
necessary to do so in order to read the conclusions ...
go ahead and skip down to the paragraph beginning "The
largest possible aperture ..."
discussion of the f-number
The aperture is not described
in terms of its actual size, however. Rather, the
aperture is described in terms of its f-number (or
f-stop), which accounts for the focal length
of the lens ... the f-number (f ) is the ratio
of the focal length (f) of the lens to the diameter of the aperture (D
, where the aperture is assumed to be circular).
Thus, if a lens has a focal length of 50 mm and an
aperture with a diameter of 25 mm, then its f-number
is 2.0, usually written as "f /2" so that
The area of the aperture A
that determines the amount of light let through the
shutter is given by
Suppose the area of the aperture
at one setting of the lens's aperture happens to be
two times the area at another setting; that is A2
= 2A1 . Hopefully, it is easy
to see from the preceding formula that in this case
the diameter must have increased by a factor of = 1.41421....,
such that D2 = D1. But that means
That is, every time we double
the area ("open up" the aperture), the f-number changes
by a factor of ( ). Recall that for a 50 mm lens with a 25 mm aperture, the f-stop
f /2. With twice the area (i.e., opening up by
one full stop), the f-stop changes from 2.0 to
2 /=1.41421... . This would be denoted as f
What if we go the other way,
and cut the area in half ("stop down" the aperture)?
It should be clear by now that each time we cut the
area in half (i.e., stop down by one full stop), the
f-number changes by a factor of = 1.41421... , so for half
the area of the aperture initially set at f
/2, the new f-stop would be 2 *= 2.8285, written
as f /2.8.
The largest possible aperture is for the diameter is when
D = f, or f /1.0.
We say that each factor of 2 in aperture size is considered
a full stop . That is, when the aperture area changes
by a factor of 2, that corresponds to one full f-stop. It
is also common terminology to refer to the speed of
a lens in terms of its maximum aperture. Thus, a 50
mm lens with a maximum aperture of f /1.4 would be
a full stop faster than a 50 mm lens with a maximum aperture
of f /2. That lens would be described as a 50 mm f
/1.4 lens. A lens two full stops faster than an
f /2 lens would be rated at f /1, the fastest
possible lens speed for that focal length. Many lenses have
aperture rings that have click stops at 1/2 stop intervals,
such that it takes two clicks to equal a full stop.
It is important to note that as the f-numbers increase,
the size of the aperture decreases. When we say we are "stopping
down the lens" that means we are increasing the f-number
or decreasing the aperture area. If we are "opening up"
the lens, this corresponds to decreasing the f-numbers or
increasing the aperture area. [Note: if this seems silly
to you, just remember that I am only reporting on what terms
mean ... I didn't develop this terminology!] The following
table shows the f-numbers associated with full stop differences.
Table 3 shows f-stops as they are typically marked on lenses.
3. f-numbers at full stop intervals, beginning
I will leave it
as an exercise for the interested readers to figure out
what the f-numbers would be at 1/2-stop intervals;
also, to calculate (for example) how much faster an f/3.5
lens is than an f/4.0 lens, as measured in f-stops.
I have been talking about focal length, f; what does focal
length mean? If you can imagine a lens focused on an object
at an infinite distance, so that light rays from the object
are coming in as straight parallel lines, the focal length
is simply the distance from the center of the lens to the
point where those rays converge after passing through the
lens (see Fig. 2)
Since items must
always be focused on the plane of the film (which, therefore,
is known as the focal plane ) Longer focal length lenses
typically are longer in length. The longer focal length
means that a narrower region of the subject is projected
onto the film. This means that longer focal lengths result
in an increase in the "magnification" of the resulting
image ... see Figure 2. Therefore, "telephoto"
lenses have long focal lengths and "wide angle"
lenses have short focal lengths. A "normal" lens
for a 35 mm camera is about 50 mm; such a lens is neither
a telephoto nor a wide-angle. Its images correspond roughly
to the perspective we would see with our eyes.
2. A schematic illustration showing how focal length corresponds
to magnification. Some exaggeration has been used to make
We are now ready to explain exposure reciprocity . The total
amount of light that the camera allows to reach the film
is determined by the combination of the shutter speed and
the aperture. Let's suppose that some combination of exposure
time and f-stop produces an ideal exposure ... for
example 1/125th of a second at f/8.0 ... but wait
... if we doubled the aperture size (which would let in
twice as much light ... a full stop, corresponding to f/5.6)
and cut the exposure time in half (to 1/250th of a second),
that would correspond to exactly the same total exposure!
This is the reciprocity rule ... if you double the aperture
size and cut the exposure time by half, you get the same
exposure. Or, you could stop down by a full stop (cut the
aperture size by half, in our example to f/11) and
double the exposure time (in our example, to 1/60th of a
second) ... bingo! ... the same total amount of light, once
O.K. ... that's
all very interesting. Why would we want to do such a thing?
To see that, we have to consider two other topics: depth
of field and the motion of the subject.
If all subjects were absolutely still all the time, then
having the capability to vary the shutter speed would not
be very important. However, most real subjects move, to
a greater or lesser extent. If we want the capacity to "freeze"
that motion, we need a fast shutter speed. For example,
suppose you wanted to take a close-up shot of some wildflowers
that were waving in a slight breeze. If your shutter speed
is fast enough, say 1/250th of a second, the image will
appear to have "frozen" the wildflower's motions
and it will be sharp and clear.
On the other hand,
you might want to photograph a moving stream and have its
motion turn the water into a sort of blur. This is a very
common thing to want to do ... in order to do this, you
would want a slow shutter speed, say 1/2 a second.
ability to vary the shutter speed is an important factor
under your creative control.
Depth of field
If you have your lens focused on some subject, it turns
out that there is a range of distances, both larger than
and smaller than the distance to the subject, where other
objects in view are also shown clearly. They are very nearly
in focus, as well. This is called the depth of field, and
it turns out to depend mostly on the focal length of the
lens and on the aperture setting. As illustrated schematically
in Fig. 3, if you increase the aperture size, you reduce
the depth of field, and vice-versa. If you want objects
nearby and far away to be in focus at the same time, this
is going to require you to stop down the aperture (increase
the f-number). There also are times when you want
the depth of field to be small; for example, if objects
in the background of a photograph's subject would be distracting,
it is common to have them so badly out of focus that they
simply become a blur, allowing the image to emphasize the
subject rather than the distracting background. In such
a case you would want to open up the aperture (decrease
3. Schematic showing the effect of changing the aperture
on depth of field, where the horizontal lines indicate the
range within which objects will be in focus for each aperture
setting. The X- marks indicate the hyperfocal distances
(only schematically!). At f/2, only the near
foreground objects would be clear, while the midground and
background would be blurry. At f/5.6, at this particular
hyperfocal distance, only the midground and background would
be in focus, while foreground objects would be blurry. At
f/16, objects from the foreground to the background
would all be in focus. Shifting the hyperfocal distance
alters which objects are in focus, and can change the range
in which objects are in focus. See Fig. 4 caption below.
Note that since
there is a range associated with the depth of field at a
particular f-stop, if you want the foreground and
background to be in focus simultaneously, you would want
to focus the lens at some distance in between the nearest
and farthest object. This distance, which optimizes the
focus to achieve the desired result is referred to as the
hyperfocal distance. Many lenses have depth of field marks
on the focusing ring, showing the depth of field range for
several different apertures. Decide the distances to the
nearest and farthest subjects you want to be in focus and
the hyperfocal distance will be shown clearly on the lens.
Simply focus at that distance and you're in. A schematic
example is shown in Fig. 4.
4. Schematic example of the depth of field marks on the
focusing mechanism of a lens, as seen looking down on the
lens as it is mounted on a 35 mm camera. In this example,
the lens's pre-set aperture is f/16; the depth of
field marks indicate that at f/16, everything from
infinity (€) to about 2.5 meters (m) is in focus, with
a hyperfocal distance of 5 m already set on the focusing
ring. Observe that the distance scale is not linear ...
that is, the depth of field range (as measured in feet [ft]
or meters [m] changes as the focus of the lens changes ...
to the right of the hyperfocal distance (5 m), the focal
range is from 5 m to infinity, whereas to the left of the
hyperfocal distance, it's from 2.5 m to 5 m (only 2.5 m).
If the focus were to shift from 5 m to, say, 10 m, the range
to the right would be 10 m to infinity, but the range to
the left would be from about 4 m to 10 m (about 6 m).
is to use your camera's depth of field preview feature,
if it has one. Most modern cameras use the maximum aperture
for focusing and through-the-lens (TLL) metering, and the
lens is stopped down to the pre-set aperture only when the
shutter is pressed. Otherwise, what you see in the (TTL)
viewfinder would, in general, be darker than what you see
at a wide-open aperture ... this facilitates focusing. However,
it can prevent you from seeing what the aperture's depth
of field does to the final image. Some cameras permit you
to stop down the lens to "preview" what the depth
of field at the pre-set aperture looks like. If something
looks fuzzy when you preview the depth of field, it will
be out of focus in the image. Thus, you can experiment with
different apertures and see for yourself what the shot will
look like ... and you can experiment with trying to find
the hyperfocal distance this way, as well.
The drawback to
using the depth of field preview feature is that the image
can be too dark in the viewfinder when the lens is stopped
down using the depth of field preivew. The darkness of the
view through the viewfinder can make it difficult to see
what ther resulting image might look like. I have used the
depth of field marks on the lenses quite successfully to
find the hyperfocal distance ... but of course, this depends
on how accurately the lens manufacturer was able to determine
the depth of field marks on the lens!
By now, we have enough information to explain the term "reciprocity
failure." The reciprocity rule says we can trade shutter
speed for aperture, and vice-versa and still obtain a properly-exposed
image. But it turns out that there are limits to this. One
limit is the lens and camera combination.
A lens has an aperture
range between its largest and smallest aperture. For any
given lens, then, there are limits to how much depth of
field control you have. If you must stop down or open up
beyond the lens's capability to achieve the desired depth
of field, then it simply cannot be done with that lens.
The same can be
said of shutter speeds, especially as you go toward high
shutter speeds. There is a fastest shutter speed pre-set
into your camera's shutter and you simply cannot use a faster
speed. What about at the slow end of the range? Well, there
is some slowest shutter speed that is pre-set into the shutter's
mechanism, of course; say 1 or two seconds of exposure time.
Can you take a picture with a slower shutter speed (i.e.,
a longer exposure)? If your camera has a "B" (for
"bulb") setting on its exposure speed dial, then
you can. On the B setting, once the shutter release is pressed
and held down, the shutter opens and stays open. When you
let go of the shutter release, the shutter closes. Therefore,
you can extend the exposure times virtually without any
limit (other than a practical one ... you cannot take an
infinitely long exposure, after all!).
In some low light
situation, then, in order to achieve a large depth of field,
you would have to stop down the lens. But by the reciprocity
rule, this would necessitate a very long exposure time,
perhaps several seconds. In principle, this is no problem
... use the B setting and you're in business. But wait!
It turns out to
be a property of film emulsions that the reciprocity rule
only works for a limited range of exposure times . The limits
include both a high-speed and a low-speed limit. Beyond
that range, the emulsion's capacity to get a proper exposure
using the reciprocity rule breaks down. Such situations
are called reciprocity failure . In practical terms, this
is most commonly encountered at the low-speed end. That
is, most emulsions permit the use of the reciprocity rule
for virtually all permissible fast shutter speeds, say out
to 1/1000th of a second. But the reciprocity rule no longer
applies beyond some relatively long exposure time, say for
exposure times exceeding 1 or 2 seconds. In such cases,
you may have to provide an even longer exposure time than
required by the reciprocity law to obtain a proper exposure.
With color films, reciprocity failure is seen as a color
shift that usually can be corrected simply by compensating
with a longer exposure. Of course, the color shift may not
be something that ruins the image ... this is a matter of
Note that this
is property of the film , not of the camera and lens. Most
film manufacturers provide information about the range where
the reciprocity rule holds ... often called the coupling
Now that you've
completed this presentation, you may want to move on to
another that discusses some more advanced
skills you will need as a photographer.
Corrections? Useful information? Send me an e-mail!
Some Basic Elements of
Some More Advanced Photgraphic