The one item I would add to Bob's excellent precis below is this: if
you are at all into math, then it is useful to know more about the
aperture "f stops" and the bizarre numbering sequence used. "f", or,
more properly italicized "f/" represents a ratio of the effective
size of the aperture and the focal length of the lens. It is a
continuous value, but by convention lenses are marked with "stops" at
1.4, 2.0. 2.8. 4.0. 5.6. 8, 11, 16... because these values represent
a successive halving of the area (not the diameter) of the aperture.
So, as you go from 4.0 to 5.6 you have cut in half the amount of
light falling on the sensor. [See "F-Number" in Wikipedia.] You can
compensate by doubling the sensitivity of the sensor (e.g., changing
from ISO 200 to ISO 400) or by doubling the length of time the
aperture is open (e.g., by going from 1/125 sec to 1/60 sec). Either
approach can cause some problems; sensors have an optimal design ISO
value (ISO 200 is common for most DSLR's), and increasing their
sensitivity to 400 or 800 or higher makes the electronics "work
harder" (technical term) and tends to deteriorate the quality of the
image. Decreasing the shutter speed also causes problems; faster
speeds will "freeze" movement in the scene (or compensate for
movement of your camera during exposure), images shot at slow shutter
speeds tend to be blurred unless the camera is well anchored onto a
good tripod. And by the way, any lens will provide different quality
results at different f/ stops; lenses tend to be at their best when
the chosen f/ stop is 2-3 stops closed from maximum value (e.g., f/8
for a lens with max aperture of f/4).
So, you have this mix of parameters you can play with - ISO, shutter
speed, and f/ stop - with many different combinations of the three
achieving the proper EV for the scene. You just need to learn which
tradeoffs you feel comfortable with. If you want to make sure your
lens is at a particular aperture, there is a setting which allows
Aperture Priority (Av in Pentax-speak). If you want to make sure the
shutter is at a particular speed, use Exposure Priority (Tv in Pentax-
speak). And there are other variation, all designed to enable you to
have more control over the parameter(s) you care more about while
letting the camera come up with a decent set of tradeoffs in
achieving the proper EV for the scene.
By the way, a classic way of resolving some of the dilemmas
encountered in trying to find a usable compromise of the three
parameters is to add more light to the scene. You can do that by
coming back when the sun is brighter, you can use a reflector to
reflect some of the available light onto your subject, or you can add
light via a on-camera or external flash system. Which gives you a
fourth parameter to play with, i.e., the output of the flash.
Enjoy!
stan
On Dec 31, 2008, at 3:35 PM, Bob W wrote:
[...]
What I need, is to find a web site / document / book that can
get me off the ground,
with explaining what ISO/shutter/aperture/exposure/etc.. are,
how how they relate to each other, and affect each other, and
the picture
[...]
Most beginners' photography books should teach you that in a few
pages, with
some shooting exercises. You also need to learn about exposure
metering, in
particular the difference between incident and reflected light
readings, and
their pros and cons. Michael Freeman has been writing such books
for many,
many years and they seem to work well - try something like his The
Complete
Guide to Digital Photography.
Here's a quick overview.
Exposure determines how much light hits the sensor. The ISO number
is a
measure of how sensitive to light the sensor is. The higher the
number, the
more sensitive it is. The more sensitive it is, the less light you
need to
record the correct exposure.
The ISO numbers usually double - each doubling represents twice the
sensitivity, and a corresponding halving of the amount of light
needed to
hit the sensor. In bright sunlight you typically use a lower
sensitivity
than you would in deep shade or at night - it gives you more
options for
setting your aperture and shutter speed.
The amount of light that hits the sensor is determined jointly by
the size
of the hole which lets light into the camera, and how long you keep
the hole
open.
The size of the hole is set using the aperture and measured in f
stops, such
as 1.4, 2, 2.8, 4, 5.6, 8, 11, 16 etc. The smaller the number, the
larger
the hole and the more light you let into the camera. Each number in
the
series above represents a halving of the area of the hole. So f/1.4
is twice
the area of f/2, which is twice the area of f/2.8 etc. (More
accurately, the
aperture is the hole; the thing that determines the size of the
hole is the
diaphragm.)
How long you keep the hole open is set by the shutter speed, which is
normally measured in fractions of a second, but can also be much
longer than
a second. Usually the speeds run from about 1/30th, 1/60th, ...,
1/4000th of
a second. Once again, each step represents a halving of the amount
of time
the hole is open.
The amount of light in a scene is measured with a light meter.
Meters built
into the camera measure the amount of light reflected from the
scene, and
make an assumption that the scene reflects 18% of the light falling
on it.
This assumption provides a reference point, but can fool the meter
under
certain conditions.
The other main type of meter is an incident meter. These are usually
hand-held separately from the camera, and measure the amount of
light that's
falling on the subject. Both types of meter give you the same type of
reading. For the meter to work properly you have to set it to the
same ISO
number as your sensor.
The meter reading gives you an exposure value, or EV. You can set
various
combinations of f-stop and shutter speed on your camera to let in the
correct amount of light to match the EV. For example, 1/1000 sec at
f/8 lets
in the same amount of light as 1/250 at f/16. If you change one
without
changing the other you are likely to under- or over-expose. Under-
exposure
means you haven't let in enough light, and the picture looks too
dark or
muddy; over-exposure means you've let in too much and the picture
looks too
bright and washed out.
A good deal of the creative control in photography lies in how you
combine
the aperture and shutter speed, and this is where you need to start
experimenting to understand the theory.
Varying the shutter speed makes moving subjects more or less
blurred in the
picture.
Varying the aperture determines how much is in focus in front of
and behind
the object you've focused on. So a larger aperture (smaller number)
means
less is in focus and you can get a nicely blurred background; a
smaller
aperture (larger number) means more is in focus, and you tend to get a
sharper picture.
Here endeth the first lesson. It's now playtime.
Bob
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