Both focus and exposure are wonderful 'tools' used and manipulated by professional photographers. Focus and exposure are mechanical in application, because they are mechanical camera functions. But, they may also be used to create individual expression. Everyone's perception of 'ideal' focus and exposure will vary, and here I will only introduce the basics. Any judgments between 'better' or 'bad' photos in this section are a personal opinion only.
Focus is simply adjusting the lens and camera so that the light from your center of interest is positioned correctly and sharply on the film or photocells. Exposure uses these same adjustments to insure the correct amount of light reaches the film or photocells.
A camera lens gathers and projects light. In doing so, it determines image size, adjusts the amount of light, and influences which part of the image is in focus. On a film camera with removable lenses, part of the internal workings of a lens may be see: the aperture, or opening that regulates the amount of light that reaches the film. These adjustments are measured in f-stops, such as f22 or f3.5. For example, a black puppy sitting on a brown sofa by a small window on a cloudy day would reflect such a very small amount of light, that the camera's aperture would need to be very large, in order to gather enough light to form an image. At the other extreme, a sunny day on the snow-covered ski slopes contains so much reflected light, that the correct aperture would be a very small one. The trick to remember here is, for some crazy reason, the smaller opening is the larger number, and the larger openings are the smaller f-stops. In the above examples, the indoor pet photo would use an f-stop of 2.8 or even 'smaller'. The snow photo would use an f18 or 22.
BUT--more than likely the snow scene would still have too much light for a proper image. Now the camera's shutter speed comes in to help. The shutter is the device in front of the film that opens and closes when we press the shutter to allow the light to create an image upon the film. This shutter speed is written as the length of time the shutter remains open, allowing light to hit the film., such as 30 seconds, 1/125 th, or 1/1000 th of a second. Of course, the shorter length of time the shutter is open, the less light it allows past! So in our example, the snow scene would have a very small lens opening, aperture or f-stop, as well as a very short, or fast, shutter speed.
In the same manner, the low-light scene of the puppy would need both a very large lens opening, or aperture, and a very long exposure time, or shutter speed. In this case, there may not be enough available light naturally reflected to make a good photo using a shutter speed of less than a second. In this case, there will be no way we could hold the camera still enough without the use of a support of some sort. This may be a tripod or make-shift support made of books, etc. A second option would be to use electronic flash to light the scene and insure ample light. You will notice a flash setting on the camera's shutter speed dial... this is the speed that synchronizes with the speed of the burst of light sent out by the flash unit.
If neither of the above options are used, the final image will actually be composed of more than one impression of light. As the shutter is opened, the light continues to reflect onto the film or photocells for a long period of time. As our hands move, no matter how small a movement, slightly different images are focused upon the film or photocell.
Moving objects, such as a race car or basketball player, may project very different images during the second or two that the shutter is open. The resulting photo will show several different images of the object. Still objects, such as our African violets that are rooted in a pot, will not project obviously different images, and those photos will not be obviously confused. The most noticeable sign of camera movement (as a result of a too slow shutter speed) will be fuzzy details and unclear edges between colors and the light and dark areas of the image. See photo to left. This was taken as the plant sat directly under the plant light, but the automatic setting on the camera still could not find much light and slowed up the shutter speed so that my body movement is evident. (Info on how to change the shutter speed will be explained in a following lesson.)
Focus is easiest to control by using our TTL (through the lens ) viewfinders, as what the camera sees is what you see. So pressing the shutter down halfway will set the automatic focus and exposure.
It is also worth noting here that most cameras are 'center-weighted', both in focus and exposure. In other words, it will focus on the object in the center of the viewfinder, as well as determine exposure for that object. Many digital cameras allow you to center your subject, press the shutter halfway until it focuses on that object, then moving to compose your shot, and pressing the shutter completely. Many cameras also have an 'automatic exposure lock' that works in the same way; allowing you to center on your subject, let the camera determine the proper exposure, and then moving away from the object to compose your photo.
Understanding a bit how the camera focus works will give you control over the final photo. Just a brief overview will be given here. I have uploaded several pages from texts that illustrate 'field of focus'. Right click on the links below; then select 'Open in a new window...' and continue reading these lessons while the page loads.
You can take better photos by merely being aware of the existence of a 'field of focus'. This is the area of your scene that will be sharply focused upon the film or photocells. This area is expressed as distance from the camera, (such as 10-15 ft) and is best expressed by example.
The field of focus is determined by the lens opening, or f-stop. A larger aperture (remember this is a smaller number such as f3.5) will give you a 'short' field of focus, while a smaller lens opening, such as f22, will have a 'longer 'field of focus'.
I will explain this using our standard 'auto' camera modes, but remember: control of the field of focus is the greatest skill a good photographer can achieve. You may control this by adjusting the camera settings, or by manipulating the light available on your object if your camera does not allow much adjustment.
Portrait mode is designed for situations with a centered and in-focus object against a blurry background. This will set your camera's f-stop to a larger opening and shorten the time the shutter is open. The field of focus here could be describes as 'short'. An example would be: 8-10' feet from the camera. This means the field of focus is only two feet. Only objects within that two feet will be in focus upon the film or photocells.
Sports mode sets a very short shutter speed in order to capture only one image of a moving object. The lens opening would be need to be large to gather sufficient light in that short time of exposure. Again the field of focus will be 'short'... possibly less than 2 feet.
Scenery mode is for those situations where you want near and far objects to both be in-focus and the 'field of focus' must be 'long', often 15-35' or 24'-infinity. The lens opening must be small, such as f18. Remember this will allow less light to enter the camera, so the length of time the shutter is open will have to be longer. In bright outdoor shots, this is no problem. In situations with less available light, such as indoors, the very long shutter speed will allow the objects to move before the shutter is closed. And it is nearly impossible for a person to remain perfectly still for any of these longer exposure times.
Macro mode will vary greatly from camera to camera, but most often results in a 'short' field of focus, often of just a few inches or fractions of an inch.
of insuring proper focus, as a method of using your camera.)
A very effective
exercise to try: (Best to try this in a medium to low light
situation and to place the mugs in such a way that there will be at least
four feet between the first and last. If you do not have a TTL viewfinder,
you may try this by actually taking a photo each time, and then viewing
the series of photos on your computer.)
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