Depth of Field (DOF for short), is usually associated with the aperture which is
often used interchangeably with the word f-stop. The Aperture controls the
amount of light that passes through the lens and onto the film; or in the
digital world, onto the sensor inside the camera. This amount of light is
determined by the size of the lens opening (the aperture) inside the lens.
On the older traditional 35mm cameras, the settings on the outside of lens that
controlled this function were called f-stops. That's why even though technically
they are two different things; many writers use the words aperture and f-stop as
if they were one and the same. It's like calling the door knob, a door. You do
use one thing to affect the other, but they are NOT one and the same.
The greater amount of light the aperture allows in, the narrower the depth of
field is, and visa versa; the less amount of light the aperture allows inside,
the wider the depth of field is. Many beginning students get confused by this.
Since the f-stop is what controls this light, the easiest way to remember it is:
- Small number = small f-stop (f 1.8) = small amount of picture in focus.
- Large number = large f-stop (f 32) = large amount of picture in focus.
The Shutter also affects this same light; where as the aperture controls "how
much", the shutter controls "how long". If the shot is exposed for too long a
time the photograph will appear washed out. If the shot is exposed for too short
a time the photograph will appear too dark. This is commonly referred to as:
over exposure and under exposure. The Shutter Speed (how long things are
exposed); can be used to freeze things in midair (with a fast speed), or it can
be used to intentionally blur something like water (with a slow speed).
As a general rule if you want to freeze something you want a shutter speed of
1/500th of a second or faster. As a general rule if you want to blur something
(on purpose), you would use a slow shutter speed like 1/30th of a second or
longer. Just remember, that generally you only want part of the picture as a
blur so when you do use slower speeds, also use a tripod.
Wherever you focus your lens within a given image, there will be an area that is
in focus and other areas that are out of focus. The area that is "in focus" is
referred to as the "focal plane". The important thing to remember is that 1/3rd
of this focal plane is in front of whatever you focused on, and 2/3rd's of the
focal plane is behind whatever you focused on. By deliberately focusing 1/3rd of
the way into your landscape shot and using a high number f-stop (like f-16 or
f-22) you capture the greatest amount of the photograph in sharp focus.
Some people have the mistaken idea that if you just set your focus to infinity
that it would do the same thing. The infinity setting that looks like an 8
turned on it's side, will give you an indication that says anything from 10 feet
back will be in focus. So what happens if you want to include something that is
less than 10 feet away? That's where the hyperfocal distance comes into play.
When you use the hyperfocal distance you focus on something that is half of what
the front infinity setting indicates, in this case that would be five feet.
Since we know that 1/3rd of the area in front of your picture will be in focus,
it gives the illusion that every part of your picture is in equal focus. Whether
you use the 1/3rd in rule or set the hyperfocal distance, they both give you a
greater sense of a large depth of field which is good for both landscapes and
scenic shots.
If you always trust your camera meter and use what it tells you, you are using
what we refer to as a standard exposure value. Exposure Values are those numbers
that refer to the combinations of f-stops and shutter speeds that can give you
an accurate exposure under a given light situation. Keep in mind, meters do not
see in color, in order to make up for this it will average your scene, and give
you a reading for 18% grey. All meters do this it's not their fault, that's how
they were designed. But what happens if you actually want a white wedding dress
to look white? Or what happens if you want that great black stallion to actually
look black?
Just as many pictures look better after you take them into Photoshop and
increase or decrease the brightness or contrast settings; many images actually
look much better if they are exposed slightly more or less than what the meter
suggests. This process of shooting above, directly at, and below what the meter
indicates is referred to as bracketing. If you are in an especially difficult
lighting situation or a location that is in and of itself very hard to get to .
. . use bracketing. If your camera batteries are getting low, use bracketing. In
short, if you have any doubts in your mind at all, use bracketing.
Most photo books strongly cover the subjects of f-stops and shutter speed. These
are important, but there are other important subjects that many of them just skim over.
Understanding the Focal Plane, Hyperfocal Distance, Exposure Values, and
Bracketing will all greatly increase the tools you can use to make a better
image.
About the author: Tedric Garrison has been involved with photography for over
30 years. In college, he was an Art Major, and firmly believes that "Creativity
can be taught." Today, as a writer and photographer, he shares his wealth of
knowledge about photography at
www.betterphototips.com.
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