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 page.

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 of field.

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 maximum apertures.

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.

Resolution

Resolution 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 beetles.

Aesthetics

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 eye.

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 color, etc.)

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:

This completes the basics of imaging - move on to one of the advanced topics: