Here is an example that shows how a camera maps its
tonal scale, given in half f-stops around 18% gray, to its histogram.
In addition, the grayscale version of the image below is shown with
selective tones mapped to that cameras 1/2 f-stop step-scale.
In the center of the
graphic below is a 13 bar step chart that was created by exposing an 18% gray card
in 1/2 f-stop intervals. This step chart represents the camera's
response to exposure variation over -3 to +3 f-stops around the central
value, i.e. correct exposure of the 18% tone. These 13 tones are, in
turn, mapped to their associated histogram slots on the standard 0-255 scale
(see blue arrows). The 13 exposures were not taken in an experimentally
perfect way, and at least two of the data points appear to be a bit off the
mark. Nevertheless, the data still shows the non-linear relationship
between exposure and histogram position. As set, this camera produces
a file with noticeable highlight separation, with no apparent roll-off as
pure white is approached. As a result, overexposure latitude is
limited. On the dark end of the spectrum, there is noticeable
compression of the tones and apparently considerable room for exposure
error. This underexposure latitude is, no doubt, tempered by
increasing image noise in the darkest tones.
The tones of the grayscale version of the sample image are shown mapped
to the camera's step scale and the histogram for that sample image is shown
at the top of the graphic. This sample image spans most of the usable
tonal range of the camera and fits neatly into the 6 + f-stop dynamic range.
We have tones from textured black (-3 f-stops) to a pure white specular
highlight on the earring (see orange arrows). All other tones fall
pleasantly in between.
Exposing for RAW and JPEG Capture
How you set your exposure should depend on how you are capturing.
In general, you want to expose for jpegs such that the rendered file closely
matches the scene, just as film shooters do when using transparency (slide,
reversal) film. The goal with jpeg capture is to produce the finished product
in camera. When capturing in raw, most experts suggest using the
greatest possible exposure, such that the brightest printable tones are
moved as close as possible to the right side of the histogram. Specular highlights, such as those from jewelry, may or may not fall within
the printable range. By doing this, you retain the greatest amount of
tonal information in your raw file. Photoshop and digital-capture
expert Bruce Fraser has written an excellent white paper explaining the
underlying reason for this approach. Click the highlighted link to the
right to access it:
Bruce's article at Adobe.com.
Dynamic Range and Exposure
Dynamic range is a measure of the difference between the faintest
luminance and the brightest luminance recordable by a medium. In
digital cameras, this would range from the faintest tones recordable above
the inherent electronic signal noise, to the point where the pixels reach
saturation and blooming. For film, it would be the minimum and maximum
useable transmission densities, and for printing paper, the minimum and
maximum reflectance The number of the f-stops of light that can be
packed between these extremes is the dynamic range.
Dynamic range must be assessed at several levels, from the luminance
range of the original scene, to the maximum recordable range of capture, to
the maximum displayable range of the output medium. Usually, it is the
output stage that limits the total dynamic range that can be utilized.
When the luminance range of the scene matches the dynamic range of capture,
and that in turn matches the dynamic range of the output medium, life is
good. To get there, is not so simple.
Scene Luminance Range
If you are working in a studio, you can usually adjust your lighting to
keep the total dynamic range of the scene to within the dynamic range of
both the capture and output media. This is usually accomplished
through an adjustment of the fill lighting. If you are photographing
under available light, you may not be so lucky. Outdoor scenes can
easily exceed the luminance recording range of most capture methods and
photographers must decide which parts of the scene must fall outside of the
recordable zone, i.e. which areas will be relegated to pure white and black.
And, even if you can capture those tones, you may not be able to print all
of them. Scene luminance can range from a few f-stops to more
Capture Dynamic Range
The luminance range you'll be able to capture with your camera will
depend on the specific medium that you've chosen. In general, black
and white negative film is the king of dynamic range. With appropriate
development and exposure, up 14 f-stops can be recorded. This is
followed by color negative film, which can record approximately 10 f-stops,
though one can expect some color distortions and tonal compression at
the extremes. Digital capture comes next, with premium SLR cameras
capable of 6-7 f-stops in jpeg mode, and 8 or so f-stops in raw.
Fuji's S3 and S5 cameras can capture about another 1.5 f-stops in raw, and
medium format digital backs somewhat more. At the bottom of the
capture heap is color reversal film. You can expect up to 6 f-stops of
range with color reversal film. Even if a medium is capable of
capturing a very wide range of tones, there may not be an output medium that
can display all of them.
Output Dynamic Range
The range of viewable tones or output dynamic range is also strongly
dependent on the chosen medium. Newspaper images have the lowest
viewable dynamic range, topping out at about 4 f-stops. Fancy art
magazines approach photographic print paper in total range, nearing 6
f-stops. Photographic print paper can reproduce, at most, between a 6
and 7 f-stop range. Computer displays and, optical or digital
projection reproduce a noticeably larger range.
Putting It All Together
The total viewable dynamic range is a function of the capture dynamic
range, the output dynamic range, and any adjustments that may be applied to
either via chemical, optical, or digital means. In general, the stage
in the the process that has the least dynamic range, dictates the total
viewable range. When adjusting lighting, this stage will dictate the
maximum scene luminance appropriate for the task. For instance, if
your image is destined for a magazine page which can only hold a dynamic
range of 5 f-stops, your lighting will have to be somewhat flat for best
results, and the total scene luminance should fall within 5 f-stops .
Of course this is a bit oversimplified, and through digital manipulation,
for instance, one can expand or contract tonal range significantly.
Certain media have a relatively fixed dynamic range and characteristic
gamma. Color negative and positive film fall into this category.
Likewise, optical printing to color paper is generally limited to a known
dynamic range and characteristic gamma. As a result, portrait
photographers who use color negative film and RA-4 c-prints work with known
quantities and can plan accordingly. They know that their printing
paper limits their total dynamic range to 1:100 or somewhat over 6 f-stops.
As all their media are adjusted to work together and deliver a rather
average total gamma, for best results, the dynamic range of the scene should
come close to that of the printing paper. If the scene dynamic range
is noticeably less, the prints will look flat. If it is greater,
prints will be snappy, but deep shadows and bright highlights may be
rendered as black and white respectively.
Other media offer the photographer significant
flexibility in the adjustment of gamma, and to a lesser extent dynamic
range. Photographers who use black and white film can make significant
adjustments via adjustments to development and exposure. Similarly,
digital photographers, especially those who capture in raw formats, can do
so through image-editing software. For these media, the link
between the scene luminance range and the dynamic range of the
range-limiting medium, is not so clear.
Underexposure is demonstrated in much the same way. The same target
was photographed, but at one f-stop less exposure than normal.
Notice how the histogram tones have been pushed down and compressed toward
the black end of the histogram. Clearly, as configured for these
captures, this camera delivers more latitude at the dark end of the
spectrum, but with considerable compression of tones.
Target Underexposed One F-stop
One lesson learned from this experiment is that the tonal steps of a
histogram are not necessarily linear with respect to exposure.
To demonstrate how overexposure is represented on a histogram, the same
target used above was re-photographed, but with the lens aperture opened
by one f-stop. Notice how black has been pushed up to dark gray and
most of the tones associated with the white portion of the target
pushed right off the end of the histogram. Tonal separation in the
lighter tones appears steep and, clearly, there is not a lot of overexposure
Target Overexposed One F-stop
Using a Camera Histogram to Establish Correct Exposure
A camera histogram is a very useful tool, especially for photographers
who shoot in RAW mode. It can be used as a rough check of exposure
accuracy and to ensure that important tones remain within the range of the
What is a Camera Histogram
The histogram is a representation of tonal frequency against tonal
position. The vertical axis of the histogram represents the number of
times a tone occurs in the image (frequency) and the horizontal axis
represents the tonal value, from pure black at the left to pure white at the
right. Histograms can generally display a composite of all color
channels or a breakout into the red, green, and blue channels. The
camera histogram is usually based on the tonal rendering for a jpeg file,
with contrast and white and black points already adjusted. As such, it
doesn't really show the entire range of capture, but rather a typical
rendition of it. If you are shooting RAW, you may have a bit more
exposure latitude than appears in your histogram.
Setting Exposure with a Histogram
Several manufacturers now make targets consisting of a white, a gray, and
a black area that can be used to set both exposure and white balance.
For the examples below, a homemade version was assembled and photographed in
soft, but unfortunately uneven light. A digital capture of the
target is shown below along with its histogram taken from Adobe Photoshop.
Notice how the white, gray and black areas can be identified as three peaks
and are indicated by the letters W, G, & B. Unfortunately, those peaks
are a bit broad and jagged due to the uneven surface and illumination.
For the tones selected for this target, a centering of the peaks within the
histogram generally results in near-perfect exposure. By the way, the
gray tone on this target is slightly lighter than an 18% gray. Some
suggest placing 18% gray as middle gray on the histogram. When
shooting in jpeg mode, I have found my best exposure occurs with 18% gray a
little to the left of center on the histogram.
Target at Normal Exposure
Normal Exposure Histogram
The difference between an exposure for a continuous source and a flash
source is shown in the graph below. The continuous source is graphed
as a red line and the strobe as black. The areas under these curves
represent the total energy of the exposure or effective exposure.
Curves that enclose equal areas provide equal effective exposure, whether
they result from a flash pulse or a continuous source whose duration is
constrained by a camera shutter. If both of these exposures were
applied to the same capture, as might happen when adding flash to an ambient
light exposure (e.g. strong fill flash), the total exposure would be the sum
of the areas or about twice the exposure (~ +1 f-stop).
Flash and Continuous Exposure
Metering Strategies for a Reflective Meter
Now that it is pretty clear that a reflected-light
meter will calculate shutter and aperture settings that will render any
metered object as an 18% percent midtone, what do you do if there aren't
any 18% midtones in the scene, or at least any you can get to with your
meter. One approach is to take a reading or several readings of an area
that contains a balanced mix of dark and light areas. Chances are the
average reflectance is not that far from 18%. Another approach is to
meter a very dark area and a very light area and split the difference.
If you have a neutral gray 18% card (Kodak, Gretag Macbeth, etc.), place
it in the scene and meter off of it when practical.
The more you work with a reflectance meter under a
variety of lighting conditions, the better will be your sense of where
the various tones fall on the exposure scale. Under normal contrast
lighting for instance, you may find that tones in your captured image
render as pure white when they are 3 f-stops brighter than something
that reflects at 18%. Similarly, tones 3 f-stops darker than 18% may
yield a deep black with only the faintest detail. Using this
knowledge, you might meter off of object that should be rendered as a
textured white and open up the lens aperture 2.5 f-stops more than
recommended by the meter. To the same end, you could meter off of a
deep, textured shadow and close down the lens aperture approximately
Flash and Continuous Light Meters
We've covered the difference between incident and
reflected meters. Meters can also be differentiated by the type of
illumination they measure. Some measure only continuous lighting,
others only flash, and quite a few measure both. The best of the
combined flash/continuous meters can discriminate between the flash and
continuous portions and accurately indicate the strength of both. The
cheaper combined meters are less reliable in this regard and are best
used for one illumination type or the other.
The circuitry needed to measure flash illumination
is significantly more complex than that used for continuous
illumination. A simple meter to measure continuous light can be cobbled
together from a selenium photovoltaic cell, a few resistors, and a meter
movement--that's all. However, flash meters must have circuitry that
both detects the sudden rise of a flash pulse and then calculates the
total energy of that pulse.
Reflective reading on center strip
Reflective reading on left strip
Most reflective meters are calibrated to return perfect exposure when the object or
scene reflects, on average, 18 percent of the light that strikes it.
Some manufacturers calibrate their meters for a slightly different
reflectance. As an example, if we have an evenly lighted scene and we
take a reflective reading of an object with an 18 percent reflectance and
set our camera accordingly, all of the objects in the scene should fall into
their proper place and good exposure should result. On the other hand,
if we point our meter at a white object in the scene and use the resulting
exposure recommendations to set our camera, we will find the result too
dark. In fact, the white object should look gray, 18 percent gray.
Similarly, if we point our meter at a black object in the scene and use that
reading, the result will be too light, and the black object will appear
gray, 18 percent gray. This is shown in the three photographs below.
A board consisting of a white, a gray, and a black strip was photographed
under flat lighting. In the first image (left), a reflective reading was
taken of the center (gray) strip and camera set accordingly. In the
second image (right), a reflective reading was taken of the white strip and the
camera set accordingly. In the third image (bottom), a reflective reading was
taken of the black strip and, again, the camera set accordingly. As
expected, things fell pretty much into place in the first image, but in the
second image, everything was greatly underexposed and the white strip is
gray. In the third image everything is greatly overexposed and the
black strip is now gray.
Reflective reading on right strip
In this section we'll tackle exposure basics and
take a look at how exposure, the camera histogram, and the dynamic
range of the scene are related.
Camera Exposure Controls
and Studio Flash
When using studio strobe, there is generally no
means of interfacing with a camera's through-the-lens exposure and flash
controls. You will have to determine and set exposure manually.
Manual Exposure Control and Measurement
Exposure Control with Continuous Lighting Sources
Whether you are shooting film or capturing digitally, the basic
principles of exposure control are the same. You have to get the right
amount of light energy to the capture medium. With continuous sources of
light (daylight, light bulbs, etc.), the photographer has basically two
means for doing this: adjusting the intensity of the exposure (lens
aperture), and adjusting the duration of the exposure (shutter speed).
With studio strobes, the duration of the exposure is set by the strobe's
pulse width, limiting adjustment to intensity only.
Adjusting Exposure Intensity via Lens Aperture
Light intensity is adjusted via the camera lens aperture. Lenses
are produced with standard aperture settings known as f-stops. Typical
whole f-stops are: f1.4, f2.0, f2.8, f4, f5.6, f8, f11, f16, f22, f32.
Each succeeding f-stop lets in half of the light of the preceding f-stop and
halves the effective exposure. For instance, the light intensity
at f4 is half that at f2.8. Of course, you could look at it the other
way round and say that the intensity at f2.8 is twice that at f4.
Applying the successive halving of intensity, you'll find the intensity at
f8 to be 1/8 that at f2.8. You may have noticed that this strange sequence
of f-numbers is actually the powers of the square root of 2, albeit rounded at
the odd powers.
Adjusting Exposure Duration via Shutter Speed
When working with continuous lighting, the time that the capture medium
is exposed to light is adjusted via the camera's shutter speed. The camera's shutter system is
calibrated for standardized durations. Typical whole shutter durations
are: 1 sec., 1/2 sec., 1/4 sec., 1/8 sec., 1/15 sec, 1/30 sec., 1/60 sec.,
1/125 sec., 1/250 sec., 1/500 sec., 1/1000 sec. From this point
forward we'll refer to the duration of the exposure as the shutter
speed. The faster the shutter speed, the shorter is the duration. Similar to the
intensity halving of f-stops, each succeeding shutter speed is 1/2 the
duration of it's predecessor, and cuts the effective exposure value in half.
How Shutter Speed and Lens Aperture Interact
The intensity and duration of the exposure act in equal measure.
Halving either, halves the total exposure. When doubling one and
halving the other, exposure remains the same. Except at the exposure
extremes, several combinations of duration and intensity can be chosen to
yield identical effective exposure. As an example, the following
combinations yield identical exposure: f5.6 & 1/125 sec., f8 & 1/60 sec.,
f2.8 & 1/500 sec., and f11 & 1/30 sec.
The ability to trade aperture for shutter speed and visa versa is great
for the savvy photographer. For instance, to minimize motion blur, a
sports photographer might choose the widest lens aperture paired with the
highest shutter speed. At the other end of the spectrum, a
landscape photographer might pair a slower shutter speed with the small aperture
necessary to keep the image
foreground and background in apparent focus.
Exposure Control with
Studio Flash Systems
Adjusting Exposure via Lens Aperture
The lens aperture is used to change the intensity of illumination for
strobe sources just as it is for continuous light. See the explanation above
for continuous sources.
Shutter Speed and Strobe Illumination
For strobe-only illumination, the exposure duration is almost always set
by the strobe's pulse width. Strobe pulse durations are typically less
than 1/500 sec. and most shutters are either unable to synchronize with
flash above this speed or, in the case of leaf shutters, are unable to
exceed this speed. As a result, the strobe pulse sets the duration by
There are two types of shutters found in most high-quality cameras today:
leaf shutters, and focal-plane shutters.
Leaf shutters, which are found primarily in compact,
medium-format, and large-format cameras, synchronize with studio strobe
systems at any shutter speed. Leaf shutters operate in the same manner
at all speed settings, triggering the flash just as all the blades of the
shutter swing fully open. With an exception or two, leaf shutters attain a
maximum shutter speed of 1/500 sec. As leaf shutters cannot open and
close faster than the duration of a flash pulse, they cannot reduce the
duration of illumination.
Focal-plane shutters present another issue; most can be set to durations
shorter than strobe pulses, but not while synchronizing properly with the
flash. Most SLR digital and 35 mm film cameras
are equipped with focal-plane shutters. Unlike leaf shutters which
operate in the same manner at all speeds, focal-plane shutters operate in
one manner up to and including the synchronization speed, and somewhat
differently for higher speeds. Focal plane shutters consist of two
curtains, so called as earlier versions consisted of two cloth curtains, one
which opened to start the exposure and other which closed to complete it.
Today's shutters are more likely to be made from aluminum alloy or
carbon composite. Nevertheless, they operate in much the same way.
For speeds through the synchronization speed, the front curtain opens fully
to expose the film or sensor. If the camera is operating in
front-curtain synchronization, the flash is triggered just as the front
curtain completely clears the frame. The rear curtain then closes at
the end of the exposure duration. For rear-curtain synchronization,
the front curtain opens and the flash triggering is delayed until just
before the close of the rear curtain. In either case, the image frame
is fully cleared when the flash is triggered. Above the
synchronization speed, there is no time during which the whole image frame
is clear of the shutter curtains. The front and rear shutter curtains
move together for most of the exposure, traveling as a slit that traverses
the image frame. If a shutter speed above the maximum synchronization
speed is used, only a fraction of the full frame will be exposed to the
Most photographers who use a variety of strobe lighting in their work
rely on a handheld flash meter. For film users, a good flash meter is
nearly a necessity. Digital users can get by using the histogram of
their camera, but a meter is far more convenient.
Exposure meters can be classified by how they measure light (incident,
reflective) and what type of light they measure (continuous, flash).
The simplest and least expensive meters measure continuous reflected light.
The most expensive are capable of measuring it all, and then some.
Most meters can display exposure in terms of shutter speed and aperture,
holding either the aperture or shutter speed constant (priority metering)
while varying the other parameter.
The illustration to the right shows a reflective meter reading being taken.
Reflective meters measure light reflecting back from an area. As with
camera lens, reflective meters have an angle of view, often referred to as
the angle of acceptance. Some reflective meters gather light over a
broad field and others, such as spot meters, from solid angles down to 1
degree. If you take reflective readings of various objects in an
evenly lighted scene, the reflective meter will return a variety of
different results depending on the reflectance of the individual objects.
An incident meter moved about the scene would return the same exposure
result for all positions. This begs the question; if different objects
give different reflective readings, which reading do I choose to get a good
Incident Light Measurement
The illustration to the right shows an incident
meter reading being taken at the subject position. Incident meters are
usually equipped with a hemispheric diffuser that averages the incident
light over an 180 degree solid angle. Incident meters measure the light
falling on an area, not the light reflected from it. For objects that
reflect a large portion of the light incident upon them in a diffuse
manner, incident meters normally provide excellent results. Both skin
and clothing are predominantly diffuse reflectors, so incident
measurements work well for portrait photographers. Incident meters will
not give ideal exposure for objects that either transmit, absorb, or
directly reflect much of the light that hits them.
Reflected Light Measurement