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© 1995-2004 Michael Reichmann
Understanding Exposure
A Complex Subject Made Simple for Beginners
History & Moonshots
When I was in my teens (in the mid-1960s) and just starting to make my way in the world of photography
in a serious manner, brought out the . This was an SLR, very much in the modern
Pentax Spotmatic
idiom, but it contained the world's first behind the lens metering system. One had to manually close down
the aperture to take a reading, and notwithstanding the name it wasn't a spotmeter, but everyone who
bought one thought that they'd died and gone to heaven. Imagine  a camera that could read exposure
through the shooting lens, with filters, bellows extension and all. Damn, technology was advancing so
much we might even land a man on the moon one day.
Tables, Charts and Selenium
Prior to this taking accurate exposure readings was problematic. There were, of course, clip on meters,
hand-held meters, extinction meters (you're definitely over 50 if you remember these) and various
exposure charts and tables. There was even that extremely esoteric from Ansel Adams
Zone System
that everyone in the know was talking about  and not understanding.
Hand-held meters primarily had selenium cells  large honeycomb matrixes. Downsides were that they
could be dazzled by very bright light and they didn't work worth a damn at light levels much lower than a
cloudy day. cells (cadmium sulphide) were just coming out and their small size allowed them to be
CDS
placed in shutter-coupled clip-on meters and then eventually to become in-built, as with the
Pentax
.
Spotmatic
All of this with the purpose of helping photographers expose their film correctly.
Anyone getting into photography today takes having sophisticated matrix metering systems in their
camera for granted. But even these are fallible, and unless one understands what the automation is doing
getting accurately exposed photographs under difficult lighting conditions can be problematic and
frustrating. Let's start with some basics.
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Sunny 16 & Beyond
Tumbleweed Dunes, 2000
The extremes of brightness that one encounters in the natural world are not that varied. For this reason
there is the so-called Sunny 16 rule. This says that on the brightest day normally encountered the proper
exposure is roughly the reciprocal of the film speed at f/16. Thus, if you are shooting ISO 200 film then
the exposure will be 1/250 second @ f/16. This is the same whether you're in Auckland or Amsterdam,
mid-summer or mid-winter.
From the extremes of a sunny day outdoors down to typical indoor room lighting covers a range of about
10 stops. With the exception of seldom encountered situations like fireworks, cityscapes and moonlight
scenes these 10 stops encompass every lighting situation you are ever likely to encounter. Only on the
ski slopes or at the beach will you need to stop down one more stop beyond Sunny 16 because of
reflections off the snow and sand.
So, that being the case, why is exposure so difficult? Most people should have no problem in recognizing
10 different light levels, shouldn't they?
The Eye's Autoexposure
Unfortunately (or fortunately, depending on your point of view), the human eye and brain have a superb
autoexposure mechanism built in. This means that once your eyes have adjusted to the current lighting
situation, and without clues as to what is causing the light level encountered, it is almost impossible to tell
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Understanding-Exposure Page 3 of 8
how bright things are on a relative basis. As long as the light level lies somewhere within that 10 stop
range for most people it all appears the same.
This is why light meters, whether built-in or handheld, are such vital tools. But before exploring light
meters and how best to use them it's worthwhile to have in ones mind a firm idea of what "proper"
exposure settings are for the ten light levels normally encountered. This way you're not a blind slave to
the meter.
Let's assume an F stop of and a (film speed) of . Here's what these 10 light levels are and the
f/8 ISO 400
shutter speed that would be needed.
A Sunny day outdoors  1/2000 sec
A hazy bright day  1/1000 sec
A bright cloudy day without shadows  1/500 sec
An overcast day, or open shade on a sunny day  1/250 sec
A heavily overcast day  1/125 sec
Deep shade. The woods on an bright overcast day  1/60 sec
Just before a thunderstorm or late on a heavily overcast day  1/30 sec
A brightly lit store interior  1/15th sec
A well lit stage or sports arena  1/8th sec
A well lit home interior  1/4 sec
Of course you would vary the F stop and shutter speed combinations to whatever would be most
appropriate. In the case of a home interior, for example, instead of 1/4 second at f/8 you might choose
1/30 sec at f/2.8. The point is though that these 10 brightness levels represent 95% of the conditions
under which we all do our shooting.
What a Meter Does
A light meter does one thing. It tells you what the correct exposure is for 13% Gray. This is approximately
the tonality of green grass or concrete. The meters that are built into almost all cameras today are
reflective meters. This means that they are measuring the light being reflected off the subject. This is a
convenient way to determine proper exposure but there are potential problems because grass and
concrete usually aren't our main subjects.
Fortunately, most of time the mixture of objects in a scene; grass, sky, people, trees, rocks and so forth,
when averaged together usually are pretty close to an 13% gray. But, because much of the time the
things that we photograph are not so conveniently neutral in tonality manufacturers of TTL metering
systems on cameras have had to go to great lengths to design multi-zone patterns and sophisticated
software algorithms to enable their metering systems to provide pleasing and accurate exposures.
Of Black Cats and Snow Storms
The classic examples of how reflective meters can be lead astray are the examples of a black cat in a
coal mine or a white cat in a snow storm. In both cases if you were to trustingly use even the most
sophisticated multi-zone matrix metering system you would end up with very badly underexposed and
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overexposed negatives or transparencies. The reason, of course, is that the meter sees the black cat and
coal, or the white cat and snow as being 13% gray. Even the smartest computer algorithms can't (yet)
understand what the subject is, and since there is nothing else in the scene they will provide "correct" but
inappropriate exposures.
The smart photographer, knowing this, will dial in some exposure compensation. In fact, because most
amateurs use colour negative film with its wide exposure latitude even this is seldom necessary and no
one is the wiser.
But, the pro and the serious fine-art photographer typically shoots colour transparency material which has
a much more limited exposure latitude. With these, if your exposure is more than a half-stop off,
particularly in terms of overexposure, you've forfeited the shot.
Incident to The Rescue
Using the L508 as an incident meter
An meter capable of taking incident light readings, like the Sekonic L508 reviewed on these pages,
features what looks like a half of a golf-ball-sized hemisphere, usually on a swiveling support. To take an
exposure reading instead of pointing the meter at the subject, as one does with in-camera and reflected
meters, you instead place the meter in the same light as the subject.
The hemisphere, or lumisphere as some call it, is designed as a 13% gray object and thus provides a
reading equivalent to that which you would get if you took a reflected reading off a theoretically perfectly
integrated scene, or a Kodak 18% gray card. (It really should be 13%, but don't ask!?)
The beauty of the incident metering approach is that you needn't carry a large gray card around with you
on location, and you don't have to worry that your subject matter  whether because of its colour or
reflectance characteristics, will give an erroneous reading.
On The Spot
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A third approach after straight reflective and incident metering is . Strictly speaking spot
spot metering
metering is no different than other forms of reflective metering. You point the camera or handheld
spotmeter at the subject to be photographed and take a reading. But interpreting the reading and
knowing what to read are the trick. From a practical point of view I regard it as the most useful form of
light metering practice.
A spot meter reads a very small area of the scene being viewed. In the case of a meter like the Sekonic
L508 it can read a variable area as small as 1 degree. While using a standard reflective meter, a
camera's built in meter or an incident meter is straightforward and doesn't requite much thought on the
part of the photographer, using a spot meter implies having an understanding of film latitude, contrast
ranges and other aspects of exposure control.
Most frequently a spot meter is used in conjunction with the . Rather that delve too deeply
Zone System
into this sometimes arcane topic, here's an explanation of how to use a spot meter to both understand
and control exposure using the system's basic concepts. If this sparks your interest you may wish to read
the article on this site on a simplified zone system approach.
In The Zone
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The above example photograph consists of three images. On the left is a straightforward photograph
taken on a hazy bright morning. At ISO 100 my Canon D30's built in meter read exactly 1/125 second at
f/8 taking into account the entire scene.
As you can see from the histogram at the bottom right of the frame the exposure is technically perfect.
The range from shadows to highlights is smooth and the shadows are just clipped, while there are clearly
some extreme highlights at the far right.
Now look at the image of the Sekonic meter's LCD screen. I used it at the same time to take 1 degree
spot readings of various parts of the scene. It shows a digital readout of 1/125 second between f/8 and
f/11 when reading the cedar shingles on the building's roof; about a half stop off from the Canon's
integrated reading of the whole scene.
If you look at the meter's reading again you'll see a small square above the point between f/2.8 and f/4
and another one above the point between f/22 and f/32. These resulted from taking readings at the points
shown  in the sky and in the shadow area on the side of the building.
If you count the number of F stops between these two extreme readings you'll see that the range is
6
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. The square at the center just above the point between f/8 and f/11 is the result of pressing the
stops
button on the meter, integrating the two extreme readings. It matches exactly the reading taken
Average
from the cedar roof, which is close to medium gray in tone.
What does this all tell us? First, you have to understand that most colour transparency films have a
contrast range of about 5 stops. The best current digital SLRs about 6 stops and colour negative film
about 7 stops. Carefully processed B&W film can have 8 stops.
The shot that you see above, because it was taken with a moderately high-end digital SLR can just
encompass the range needed for this exposure. There is just a hint of detail in the hazy sky and some
modest detail in the deep shadows. The frame you see above has been imported in 8 bit mode and the
only adjustment made was a bit of . No or adjustments were made. If I had imported
USM Levels Curves
it as a 16 bit file and spent some time with it in I could have extracted worthwhile detail in
Photoshop
both the highlight and shadow areas.
Working with What Nature Provides
Clingmans Dome Sun. Great Smoky Mountains NP. September, 2000
Photographed with a Rollei 6008 and 300mm Schneider lens on Provia 100F.
In the studio one can adjust lighting ratios and be in control of the entire lighting strategy. But, for us
landscape and nature photographers we have to take what's served up and sometimes it's a dog's
breakfast.
For this reason most serious photographers working outdoors carry a handheld meter in addition to
whatever metering capability their camera may offer. A meter like the or new is ideal
Sekonic 508 608
because it provides two meters in one, an incident and a spot meter. (In fact they are also flash meters,
but that's outside the scope of the article).
I personally prefer to use my Sekonic in incident mode for most types of photography. When this isn't
possible I use the meter in spot mode and do what I did in the above example  take highlight, shadow
and mid-range readings and average them. Of course this presupposes that I have the luxury of taking
the time to do so.
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An Experiment
Try an experiment. Turn off the autoexposure mode of your camera and turn it to manual. If you don't
have a handheld meter use your camera's meter to take close-up readings of various parts of a scene 
the highlight areas that you want to hold detail and the shadow areas where you also want to have detail
 and then see how many stops range are encompassed. Can your film handle it? What's the average of
the two readings? How does it compare to the camera's integrated reading of the whole scene?
Now, take several exposures, at 1 stop intervals, making notes of what you're doing. Use transparency
film because you'll then be able to judge directly what the effects of varying exposure are.
Why do this? Because it will make you think about light, contrast and what film is capable of. Just
remember, don't let the highlights become overexposed. Clear film contains no information and there's
little to nothing that can be done to salvage an overexposed frame. A bit of underexposure is always to
be preferred.
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