Controls
for Exposure
There are Three things that control how much light enters into the camera:
ISO, Shutter Speed, and Aperture
While these three things control the quantity of light (exposure) they also have
visual differences which can be used creatively, which is why you would select
to change one instead of the other.
ISO
Changing the ISO is probably the easiest thing to change
on most cameras. The only creative decision you are making when you
change your ISO is how grainy or noisy the image will be.
Basically, to be able to take images in low light situations and still be able
to hand-hold the camera, you may need to make your sensor more sensitive to the
available light. By raising the ISO you make your sensor react to light quicker,
but the trade-off is not as good image quality.
ISO is often referred to as film speed. It is a misnomer as it is
more akin to light sensitivity. The higher the number, the more sensitive the
film or sensor becomes to the available light, meaning you can use it in lower
light situations. The trade-off is that due to increased amplification of the
light, the sensor produces poorer image quality with more noticeable noise (grain
and/or color abnormalities). ISO is an abbreviation of the International
Standards Organization which is an independent organization that produces
world-wide industrial and commercial standards.
Because your sensor's true ISO is the lowest number available, you want
to try to shoot at that ISO as much as possible. Only change the ISO when absolutely
necessary, ie. indoors with no flash photography allowed, or when flash will
change the nature of the image, or when flash isn't strong enough like inside
Notre Dame Cathedral. This is ISO 1600, which is aweful in my particular camera,
but better than nothing.
Most cameras
are set to auto by default, which means it changes whenever it
wants to, but usually it strives for the lowest ISO.
Because this setting is usually the first one anyone changes, and because you
can change it in Manual, Shutter Priority, Aperture Priority and in Program P
mode (and even automatic in some cameras) it tends to be fairly easy to access;
usually by one button labled ISO on the back of the camera, or by clicking your
function button and scrolling down to the option.
Shutter Speed
Shutter speed is a slight misnomer because the shutter has a finite mechanical
speed that it can physically move at, and it is not adjustable. What is really
being measured is the duration the shutter stays open measured in fractions of
seconds. The longer the shutter stays open, the more light that enters the camera.
Shutter speeds look something like this:
B
1 2 4 8 15 30 60 125 250 500 1000 2000 4000 8000 |
So the larger the number, the shorter the duration since it is a fraction of
a second (8000=1/8000 of a second).
Setting Shutter Speed
"To blur or not to blur?"
There are many different kinds of blur, but usually we are concerned
with motion blur (blur due to subject/object motion) and camera-shake (blur due
to photographers movement when pressing the button). Both types of blur can be
avoided by choosing the correct shutter speed for the relative type, direction
and speed of the movement causing the blur.
When you select to change your shutter speed before any other setting, you are
making the creative choice that deals with blurring. How long the shutter stays
open determines how blurry an image, or a subject in the image, will be. If the
shutter stays open for a relatively long period of time, then the subject has
time to move during the exposure, thus reflecting light back to the camera in
several different places; the resulting image will have motion
blur. If the shutter speed is relatively fast, then the subject has not had
time to move during the exposure, thus the movement is "frozen", resulting
in no motion blur.
The ability to control the amount of blur helps to alleviate the phenomenon known
as "Camera Shake". Camera shake occurs when the shutter
speed is so slow that the image is blurred not due to the subject's motion, but
because the photographer could not hold the camera still; just the act of holding
the camera or pushing the button blurs the image. Setting the camera down or
using a tripod usually alleviates the problem, but sometimes that is not possible
or desired, so to be able to "hand-hold" the shot, use this rule of
thumb:
Slowest
Hand-Hold Shutter Speed = 1/Focal Length |
For example: My 28-135mm lens is zoomed out to 100mm, then my slowest shutter
speed would be 1/125 of a second since there is no 100 on my camera.
For Lenses made for 35mm film size being used on a camera with a smaller sensor
(digital rebel), then I need to remember my focal length multiplier (canon=1.6X
and Nikon=1.5X) meaning in the above example, I need to use at least a 1/160
of a second or faster shutter speed.
Aperture
The aperture is the opening into the camera.
Think of it as a fancy word for hole. Actually it is a ratio of the size of the
opening to the distance of the focal plane, which is why we have such odd numbers.
They are also commonly called focal stops or f-stops.
|
200 ÷ 50
= 4
Focal Stop = 4
=f4 |
Apertures look like this but may be different because they are specific to the
lens:
f2.8
f4 f5.6 f8 f11 f16 f22 |
The bigger the opening, the more light that enters the camera. Most people get
confused at first because the smallest number is actually the largest opening.
To keep from getting confused while you are learning your camera, I suggest writing
down your apertures and shutter speeds and indicating which does what.
Setting the Aperture
You set the aperture by selecting the mode, either manual M or
aperture priority Av, and then using the jog dial or menu buttons
to select which aperture you want (in Av the camera will set
the corresponding shutter speed).
Depth of Field
When you select to change your aperture, you are making the creative decision
to limit the range of what is relatively in focus; this range of focus is called Depth
of Field. The larger the aperture (opening) the shorter or shallower the
range of acceptable focus. This range of focus is dependent on how close the
camera is to the subject that you are focusing on; the closer you get, the shorter
the range of focus.
Where you are focused is called the "plane of critical
focus", and is the sharpest point of the photograph, but that doesn't
mean that other things closer and farther away from the subject are not also
in focus. The range of focus depends on the lens, the sensor size, and the distance
from the camera to the subject and tends to be about 1/3 of the range in front
of the plane of critical focus, and 2/3 of the range behind.
In order to help
determine the near and far points of focus, some cameras come with a Depth
of Field Preview button that closes the aperture down to the size it
will be when you take the image, thus allowing you to preview how the depth of
field will look.
Because depth of field is determined by your distance to the subject you are
focusing on, you can emphasize your depth of field by using a zoom or telephoto
lens. The more you zoom in, the more you compact the space between foreground
and background.
The Reciprocal Nature of Photography
This is the basic relationship between shutter speeds, iso, and apertures that
allows you to get the right exposure when making a change to any of the controls.
This means that if you change one thing brighter, then you can make one of
the other things less bright and you will have the same exposure, just a different
creative look.
Every complete change in ISO, Aperture or Shutter Speed is in a factor of 2.
This means that when you make a change, you are either doubling the amount of
light or halving the amount of light reaching the sensor. Knowing this allows
you to make setting based on creative choices and compensate the other two controls.
Everything in photography is a trade-off. This means while you change one setting
to get the creative look, you will sacrifice some other setting.
For Example: Your camera's light meter says f22 @ 1/4 second is the correct amount
of light. I know that f22 = a long depth of field which isn't necessary for
my shot. I also know that I will need a tripod because the shutter speed is
soooo slow. For me to be able to "hand-hold" the shot, I need to change the
shutter speed to at least 1/60th of a second.
1/4, 1/8, 1/15, 1/30, 1/60 that's four jumps; x2x2x2x2=16 times less light to
be able to hand-hold the shot. In order to get enough light to expose correctly
I need to change either the ISO or the Aperture to allow 16x more light
into the sensor.
I would choose to change the Aperture since I really don't care about depth of
field as much as being able to hand-hold this shot. f22 is a small aperture letting
in very little light. Since I need 16x more light, I will open up the aperture.
f22, f16 ,f11 ,f8, f5.6 With the aperture opened up four jumps, I now have the
same amount of exposure, but I can hand-hold the shot.
f22 @1/4 is the same exposure as f5.6 @1/60
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