Photography is based on light, which modifies the chemistry of film (or
the electrical state of an electronic sensor) in much the same way that
light affects the cells in the retinas of our eyes. The end result
is a pattern or image that resembles what we saw.
A successful image combines several
First, the exposure must be correct.
This means that the overall picture is neither too dark nor too
light. There should be details in both the highlights and in the
shadows. Whites should be white, and blacks black.
Second, the picture must be sharp. This
means good resolution of at least the main subject. There
must also be sufficient depth-of-field, which means that important
subjects close to the camera and further away from it are all in sharp focus.
Third, the image must be aesthetically
pleasing. For scientific imagery, aesthetics must include the
ability of the image to convey the essential information to the viewer,
as well as simply being pleasing to the eye. Aesthetics involves
artistic judgment as well as pictorial accuracy; it includes
composition, cropping, balance, color, and so on.
Let's discuss these 3 elements separately,
starting with exposure
Exposure is controlled by 3 factors: the sensitivity of the film (or
digital chip), the amount of light reaching the film, and the length of
time the light is allowed to strike the film.
Film sensitivity is a function of its chemistry. As in
everything else in photography, there are tradeoffs in selecting
film. The more sensitive the film is, the more grain it
has, and grainier film has less resolution. Film is rated by the ISO
number; most consumer films range from ISO 50 to ISO 800. For more
information on film, jump to the films
The amount of light reaching the film is affected by many factors:
the source of the light the direction, things that are blocking the
light, the reflectivity of the subject, and the lens of the
camera. The photographer has limited control over many of these
factors. You can move about to take maximum advantage of natural
light (it's usually best to have the sun behind you, for instance), or
you can add light with reflectors or flash (or subtract it with
shades). You also have some control over the amount of light
going through the lens.
A lens has a maximum aperture.
The aperture is usually measured in f-stops. F-stops
range from an absolute maximum of 1 to about 64 for most lenses.
Now, this is tricky - the smaller the f-stop
the more light it lets in; the larger the f-stop
the less light it lets in. Most lenses have a diaphragm
built into them that reduces the amount of light that reaches the
film. Older cameras had "traditional" f-stops of f/2.8,
f/4, f/5.6, f/8, f/11,
f/16, f/32, and f/64. Each larger
number lets in 1/2 of the light as the previous stop, that is a lens
with its diaphragm set to f/16 lets in half as much light
as it would with the diaphragm set to f/11, and twice as
much light as it would with the diaphragm set at f/32.
Newer cameras have any number of "intermediate" f-stops
in addition to the traditional ones.
Why the different settings?
Sometimes you do want to cut the amount of light reaching the
film. In most cases, however, the best way to do this is by
reducing the time of the exposure; this leads to sharper pictures.
Using a smaller (bigger number) aperture does two other things,
however. First, it increases the clarity (resolution) of the image
slightly. The smaller diaphragm opening blocks light coming from
the subject that has been scattered, allowing only a clear image to
form. Second, the smaller diaphragm opening increases the depth
From a practical standpoint, it comes down
to this: One way you can control the amount of light reaching the
film is to adjust the aperture of your lens. Using a larger
aperture (small f-stop) allows you to take pictures in dimmer
light (or at a faster shutter speed), but costs you clarity and depth of
field. Using a smaller aperture (large f-stop) gains the
clarity and depth of field, but requires more light, or slower shutter
speeds. More expensive lenses, by the way, tend to have larger
The other factor affecting exposure that is under
your control is the shutter
speed. Think of the shutter as a door
in the camera which allows light from the lens to hit the film. It
is closed except for when you are taking a picture; at that time it
opens, usually for a fraction of a second. The amount of time it
is open is controlled by you (and/or the camera). Typical shutter
speeds are 1/15, 1/30, 1/60, 1/125, 1/250, 1/500. Like the f-stops,
moving from a slower speed (1/15 of a second) to a faster one (1/30 of a
second) reduces the amount of light. In this case, cutting the
speed in half reduces the amount of light by 1/2.
Again, from a
practical standpoint, slow shutter speeds let in more light and allow
you to use smaller apertures for grater clarity and depth of
field. Faster shutter speeds let in less light. They also
tend to freeze motion - whether it be the movement of a tree branch in
the wind, the movements of an animal, or the shake of your hand holding
the camera as a bear approaches you.
Putting exposure all together,
you have to find a compromise between shutter speed and aperture.
Ideally, you want to use the fastest shutter speed and the smallest f-stop.
This can be one by using a faster film, but then you get grain and a
reduction in image quality. Remember, if you change either shutter
speed or aperture, you have to change the other in the opposite
direction to maintain the same exposure; otherwise your image will be
too dark or too light. So, if you decrease shutter speed (letting
light in longer), you must compensate by using a smaller aperture
(letting less light in). Or, perhaps you might move to a larger
aperture - to compensate you must use a faster shutter speed. In
practice, with modern cameras you set one or the other of these and the
camera picks the other - in some cases the camera can pick both.
Still, it is very important that you understand the effects of altering
shutter speed or aperture.
is a much simpler matter than exposure, and in fact we have already
dealt with some aspects. Resolution is affected by clarity of
the atmosphere or water, quality of the lens, movement of the subject
or the camera, graininess of the film, and the f-stop. As
we have seen, smaller apertures (larger f-stop numbers) give
greater resolution, as do higher shutter speeds (which allow for less
movement to be recorded on the film).
To maximize resolution, one needs to compromise. We saw that
in exposure, f-stop and shutter speed work against each other -
sharp pictures result from high shutter speeds and small apertures,
but this is only practical at high light levels (or with fast films,
but they tend not to have good resolution).
What to do? Remember a few tips. First, there is a
point beyond which faster shutter speeds don't contribute as much to
sharpness. When you are holding the camera by hand, this is at
shutter speeds of 1/focal length of the lens. For instance, a
400 mm lens being handheld should always be used at shutter speeds of
at least 1/400 or a second. A 100 mm lens should be used at
speeds over 1/100 of a second. This sets a level you can cut the
shutter speed back to. If you need an even slower shutter speed,
you can mount the camera on a tripod or brace it against some solid
object - a tree, a fence or railing, the ground.
Consider how much depth of field you really need - perhaps you can
give some up; in some cases this helps isolate your subject from
background clutter and results in a better picture. This would then
allow you to increase the shutter speed.
Examine your film - for some subjects you may have to use a
grainier film to allow for sufficient shutter speeds or apertures.
Some subjects look better on grainy film than others do; organisms
with "fuzzy" edges like mammals and birds, for instance,
look better on grainy films than "smooth" frogs, flowers, or
Consider adding light. Perhaps you need to come back at a different
time of the day or when the weather is clearer. Maybe a
reflector will throw in the additional light, or you can move the
subject into the light. Or, you might need to use flash.
An image is useless for scientific purposes unless it conveys the
necessary information. The challenge to the scientific
imager is to produce a picture that is both accurate and pleasing to
The most important factor in obtaining an image that conveys the
information you want to get across is making sure the central subject
occupies enough of the image frame to be clearly visible. If you
want to present a picture of a bird, for instance, you want the bird
to be obvious in the image, and not just a brown blob in a tree.
On the other hand, if you want to document the forest habitat of the
bird, you need to see that wider view (although having the bird
present in the image would be a nice touch!).
In our discussions of exposure and resolution, we had to face many
tradeoffs. In terms of aesthetics, however, the news is somewhat
better in that for once the photographer is in better shape than the
mere observer. For example, if you are walking in the woods and
see a bird in the distance, you probably won't be close enough to
accurately see it. Assuming you don't have binoculars with you,
you will have to get closer to make the bird fill more of your
view. You might even have to climb the tree. On the other
hand, a well-equipped photographer will simply change lenses to change
perspective; either zooming in on the
bird for a portrait shot; or zooming out to show its habitat - all
without moving. Likewise, macro lenses and microscopes allow you
to magnify small subjects. For much more on how lenses affect
perspective, jump to the lenses page.
The photographer is not done when the image is initially taken,
however. Whether in the darkroom or the computer, the image can
be further manipulated to change the perspective, enlarge or reduce
the main subject, and cropped
(select only the portion of the image that you want). A
scientific imager is free to do all these things as long as he or she
does not distort reality (such as placing an organism in a different
habitat, altering the relative size of its features, changing its
With all this power in your hands, the remaining task is to present
the information in a pleasing way. This will be discussed in
greater detail elsewhere. Some
general rules will suffice for now:
|1. Keep the main subject out of the center of the frame.
|2. Give motile organisms the room to move into the frame, that is,
give them more room in front than behind.
3. Try to use the "rule of thirds" - place the central
point of an image (center of a flower, the eye of an animal) about
1/3 of the way from the top, bottom, left or right of the
frame. Especially appealing are the intersecting points
where a vertical third meets a horizontal third.
Frame your image with strong vertical (tree trunks) or
horizontal (horizons) elements.
5. Place horizons on the upper or lower third.
This completes the basics of imaging - move on to one of the