Since the advent of the early
selenium cell light meters in the 30s, light meters have come a long way. Built-in, automatic exposure control has made
exposure settings so effortless that a lot of camera users hardly give it a
thought anymore. On the other hand,
especially in professional applications, full manual control over exposure
settings is often an absolute must. A
portrait for instance might call for a white background. Obviously, a white background material, like
seamless paper, is the correct choice here.
Yet, just because we use a material which we consider white, this does
not necessarily turn into a white background in a photograph. As a matter of fact, given the right control,
a photograph taken of a subject against a white background can actually result
in a photograph which shows a very dark or even black background, just as a
black background can be rendered white in a photograph. All that is necessary is a good understanding
of exposure and a good hand-held light meter.
Most hand-held light meters are
basically designed as incident light meters, yet for full control over our
exposure settings a reflective light meter is absolutely necessary also. Let’s look at the differences: A reflective light meter, like all of the
light meters built into our cameras, reads the intensity of the light which is
reflected off the subject. An incident
light meter on the other hand will read the intensity of the light which
illuminates the subject. Let’s look at
both types and their inherent differences.
If we take a reflective light meter
and aim it at a subject of average brightness, it will give an accurate
reading. For easier understanding, let’s
aim the light meter at a square which is painted an average gray. Such a surface will reflect an average amount
of the light which illuminates it and the meter will react accordingly, giving
a certain exposure setting. Without
change in illumination, if the gray square is replaced by a pure white one,
what will happen? The white surface
obviously will reflect a lot more light which the light meter will
register. Replacing the square with a
black one, will result in just the opposite.
Without any change in relative brightness, our meter has now come up
with three distinctively different exposure settings.
Reflective light meters are all
calibrated to register (or assume) an average level of reflectance, as with the
gray square. Subsequently, the exposure
reading obtained from the gray square is the correct one. The white surface will actually fool the
meter into registering a higher light intensity, just as the black one will
result in registering a lower light intensity.
Both of those exposure settings will be wrong. The end result is actually that the meter
will render exposure settings which make all three surfaces in the photograph
grey. This is the very reason why photographs of a snow scene will usually be underexposed, while photographs at dusk or at night often come out much lighter than the actual scene.
This is where an incident light meter
has tremendous advantages. It only
registers the relative brightness of illumination. The resulting exposure settings will make an
object of average brightness average in the photograph also. Subsequently, a lighter surface will be
lighter in the photograph just as a darker one will show up darker..
So it may seem that for accurate
camera settings an incident light meter is the way to go. In most situations this actually is indeed
the case. But what about the white
background? The exposure setting we
choose has to be such that not only the subject is exposed properly, but also
the background, which in our example is to be white.
A three stop overexposure will
generally result in white, just as a four stop underexposure will result in
black. This is easily verified by taking
an evenly illuminated white back ground and taking a reflective reading. We then open the lens by three stops and take
an exposure. Then subsequent exposures
are taken in 1/3 stop increments until a four stop underexposure has been
reached. The results will then show the
various shades of gray (white and black are also considered shades of gray
here) which can be achieved with various degrees of over and under exposure. Please note that these are average settings. Digital sensors vary in their response to over and underexposure. However, a simple test like the one above will easily show the amount of overexposure and underexposure necessary to render white or black without detail.
For the example , if for a white
background it is necessary to see if it is three stops
overexposed, we must do the following:
We determine the correct exposure for
the subject with the light meter in the incident mode. This will be very accurate and therefore
establish the actual camera settings.
These settings also need to overexpose the background by three stops in
order to render it white. To check this,
all we need to do is switch the meter to the reflective mode. Now the meter will register the amount of
light which is actually reflected by the background. If this indeed shows an overexposure of three
stops, the background will be white, as would a four or 5 stop overexposure. But that is not advisable at all since
otherwise the background will reflect such a strong amount of light that it
very likely will result in flare or loss of detail at the edges of the subject. If the
reflective reading of the background registers less than a three stop
overexposure, it is ultimately a simple interpretation of what the background
will look like. A two stop overexposure
for instance would render a very light gray background, a one stop overexposure
would still render a noticeably darker grey. The same exposure would render an average or
neutral grey, while underexposure would render progressively darker shades of
grey until the background is rendered black.
For example: The basic exposure for the subject is 1/60
sec @ f/5.6. For the background to be
three stops over, the meter would have to read 1/60 @ f/16. If we were to expose at f/16, the background
would be medium gray, but since the actual camera setting for the subject calls
for f/5.6, that is indeed a three stop overexposure compared to f/16. Does it matter if the background material is
actually white? No it does not! Regardless of color or brightens, if the
background is illuminated such that it registers a three stop overexposure, it
will be white. On the other hand, if the
overexposure is less than three stops, the background will show various degrees
of light grey, red, green or whatever other color the background may be, but
with an underexposure of four stops, all will be black.
The above photograph is a good
example of predicting the outcome of the photograph with the help of a good,
hand-held light meter. The bright light,
outlining the profile of the face and the hands was purposely chosen at +2 1/2
(2 1/2 stops over). This was determined
with an incident light reading of just the main light source and then opening
the lens by 2 1/2 stops. The rest of the
face needed to be quite dark, but also
needed to show detail. For that an
underexposure of 2 1/2 stops was chosen.
This again was done with an incident reading by adjusting the intensity
of the light source until it registered –2 1/2.
This photograph was actually shot in a studio with all white walls and
no black background material was used.
The subject was approximately 30 feet from the background. With that distance, so little light reached
the background that the reflective meter did register an underexposure of 3 ½
stops. In the original enlargement one
can actually see a slight separation between the dark part of the hands and the
background which is ever so slightly lighter.
This kind of control would not be possible without a good, handheld
light meter offering both incident and reflective reading capability, and
certainly no automatic metering system, however sophisticated, would allow this
to be done.
A similar technique was applied for
the above color photograph. Via incident
reading the correct exposure for the subject and subsequent camera settings
were determined. A spot with blue gel
was used on the background. For the
relatively intense blue an overexposure of one stop was chosen. With a reflective reading the background
light was adjusted until this exposure was reached.
Could the same background exposure be
achieved with an incident reading? Not
at all. While an incident reading can
determine the relative brightness of the light source, it cannot give any
information about how much of that light is actually reflected by the
background material. A reflective meter
on the other hand will read exactly that.
Therefore, to have the total control described above, both incident and
reflective meters are absolutely essential.
It must also be mentioned here that a
lot of light meters require the removal of the incident sphere and replacement
with a reflective reading attachment. In
situations as described above, those meters have proven to be very
cumbersome. Light meters which allow the
incident sphere to slide aside and thus changing to a reflective reading have
proven to be much more practical.
Could the subject exposure be
determined with a reflective meter? Yes,
but it is much more difficult.
Especially with portraits, we have to deal with a large variety of skin
tones. Most Caucasian skin tones are too
light and would lead to a slight underexposure with a reflective reading, just as very
dark skin tones would lead to overexposure.
An incident meter on the other hand will automatically render exposure
settings which will render the skin tones accurately, regardless of how bright
or dark they might be. An incident meter
will render true tonality in both color and black and white.
A good example is the photograph
below. A simple, diffused light source
(umbrella) was used as the only form of illumination. This, of course, rendered a mostly black
photograph. The incident meter, aimed at
the light source easily gave the correct camera setting. Most reflective light meters would have
failed with the possible exception of a spot meter. Any automatic exposure control would have
been fooled by the mostly black scene, including any of the computer controlled
matrix metering systems, regardless of manufacturer.
The question is often asked about
where to aim the incident meter when multiple light sources are used. That ultimately depends on the approach to
lighting. I know that a lot of
photographers set up lighting, especially for portraits, by establishing
certain lighting ratios, which then are set up with the help of a light
meter. I prefer to exercise full visual
control. It is my philosophy that no
meter “knows” what lighting is right for an individual. Further, I strongly feel that the lighting
should be set to accent the individual.
Preconceived lighting ratios can’t always do that. Therefore, I set up lighting to best accent
the task at hand. Once I decide on a
certain lighting, as in the above examples, I start with the main light and
then use fill as necessary. With other
words, I adjust the lights until they look good. Then, for the reading, I aim the incident
meter toward the main light source, may it be a soft box, umbrella, scrim,
window or any other. That will give the
correct camera settings, and everything else will literally fall into place.
A word of caution about measuring
range of the light meter. Using large
format cameras often necessitates very small apertures like f/32 or f/45. To apply the same control as described above
to get a true white background would subsequently require a light meter capable
of reading apertures of f/90 or f/128.
While some light meters are capable of doing this for continuous light,
few are able to do the same for electronic flash. The same also applies to maximum exposure
time. While very long exposure times
usually are not necessary, it is however, a good idea to have ample reserve in
this respect in order to be able to handle all photographic situations. For my own use the longest exposure time has
been one hour.
The above photograph is not computer
manipulated. It is a double exposure of
the microscope and a carefully positioned rear projection screen with the image
of the earth projected onto it.
The exposure for the microscope, once
lighting was set up, was determined with an incident reading and the first
exposure was done accordingly with just a black background. After carefully positioning the rear
projection screen, a reflective reading of the projected image was taken. An incident reading will not work in this
situation since there is no illuminating light source. Instead an actual reading of the true
brightness of the projected image had to be taken, which can only be done with
a reflective meter. The meter was placed
flat against the rear projection screen over the area showing the brown area
visible in the center. No adjustments to
the reading were necessary because this color of the earth is of neutral
density. This reading was then used to
expose the background.
A common practice among professional
photographers is to use Polaroid film or to use the viewing screen of digital cameras to verify exposure. While there is nothing inherently wrong with
that, it is still beneficial to be able to take an accurate and reliable
exposure reading since some photographic situations simply don’t allow for
enough time to do so. The
photograph below is such an example.
The front elevation of this building
is facing west. Therefore late afternoon
/ early evening setting sun lighting was chosen. I also decided to use tungsten film to
accentuate the blue sky (a tungsten setting for the white light adjustment on a digital camera would do the same). As it became
darker, spot readings of the entry way and main stair case of the building were
taken. The interior was very much
neutral in density; subsequently the reflective spot readings established the
actual exposure settings. These were
checked periodically to make sure that no changes took place. For the intensely blue sky exposures of
neutral to +1 are necessary. At that
time of day lighting changes very fast.
To get the correct exposure, spot readings were taken of the sky right
above the building. As soon as the sky
density had reached the +1 level, exposures were taken in short intervals until
the sky darkened to neutral. These
changes take place so quickly that no Polaroid test exposure would have
rendered usable exposure settings.
None of the above examples were
exposed with the help of Polaroid film or the almost instant feedback of the
viewing screen on a digital camera. But
they do show how a good hand held light meter can be an invaluable tool for
total exposure control. Yet there is
more.
Many photographers use seamless paper
to achieve different colored backgrounds, and at times this is indeed a good
choice. However, colored gels can in a
lot of cases do very much the same, again with absolute control. In general, a white background material will
usually lead to relatively light, pastel colors since it does reflect a large
amount of the ambient light in addition to the background lighting. Gray surfaces are helpful in this
respect. If, however, really intense, pure
colors are needed, this is best done with a flat black background material like
black seamless. The same test, as
described above for backgrounds from white to black, can be done here. Regardless of the color of the gel, the three
stop over exposure will render white as a four stop underexposure will render
black. Exposure changes in 1/3 stop
increments will reveal the various color densities which can be achieved. This incidentally is repeatable with
virtually absolute accuracy if the same gels and background materials are used.
Photography inevitably requires
exposure settings. The more control we
are able to exercise over these exposure settings, the better the outcome of
our photographs. Besides the camera, the
most important tool here is the photographer, his or her knowledge about
exposure control and the help of a good reflective/incident light meter.
