|
Understanding ISO,
Aperture, and Shutter speed
By Kevin Hoffman
Here we will try to explain the relationship
between ISO speed, Aperture setting, and Shutter speed. These three
things combined have an enormous effect on the quality of an image. And
while most cameras offer an “Auto” mode that may make these things seem
irrelevant to you, knowing how to properly combine and use these items
can often make the difference between a good photo and a great one. One
note we should probably make here is that not all cameras will allow you
to manually adjust some or all of these items. So you may want to check
your cameras manual to find out. Even if your camera does not allow you
to manually adjust these settings, this information may still give you a
better understanding of why your photos turn out the way they do.
Before we jump in and find out how these settings
interact with each other, and your finished photos, let’s take a moment
to give a brief description of what these settings are.
ISO
ISO stands for International Organization for
Standards, the group that helped establish how the film-numbering scheme
works. In a traditional camera, film is differentiated principally by
it’s speed, or ISO number. The ISO number refers to how sensitive a film
is to light. The lower the number, the less sensitive it is. Let’s
assume that a typical ISO number for film used in ordinary daylight
photography is ISO 100. Increasing the ISO to 200 doubles the films
sensitivity to light. Meaning it would take the film half as long to
capture the same picture. Whereas decreasing the ISO to 50 would double
the amount of time it takes to capture the same picture, as compared to
an ISO of 100.
Aperture
The lens of a camera is equipped with a diaphragm
called an Aperture. This Aperture is basically the size of the lens
opening and determines the amount of light that will be let in. the size
of a cameras Aperture at any given moment is referred to as the f/stop.
F/stops are represented by numbers that start with f, such as f/2,
f/5.6, f11. The larger the number the smaller the opening, so an f/22 is
very, very small, and not much light gets through to the film, while an
f/1.2 is a huge opening and will flood the film with light.
Shutter Speed
A traditional film camera has what is known as a
shutter. Think of it as a door behind the lens of your camera that opens
and closes to allow light to enter the camera and activate the film. How
fast the door opens and closes correlates to the amount of light that
will reach the film and is referred to as the shutter speed.
A note on digital cameras
Although digital cameras do not have actual film,
they do have a computer chip that acts as a sensor. This sensor makes a
picture by noting the variation in light rays that pass through the
cameras lens. It then passes this information on to the microprocessor
witch in turn stores the information on a memory card. Acting in much
the same fashion as a roll of film. However, unlike the film in a
traditional camera, the sensor in a digital camera is not easily
replaced. Because of this, manufacturers have equipped many of their
cameras with an ISO setting that will allow you to increase or decrease
the sensors sensitivity to light, essentially mimicking different grades
of film. Also, most digital cameras do not have actual shutters like
their 35mm counterparts, instead their sensors are turned on and off
similar to the action performed by a shutter.
Ok, so now that you have a rough idea of what these
things are, let’s see if we can get a basic understanding of how they
interact with each other. In order to do that we need a base scenario to
start with, so let’s assume for a moment that it is midday and we are
outdoors in our back yard, it’s a nice sunny day, we’ve brought our
camera (good photographers always have their camera with them right?),
and we see the one flower that we’ve actually gotten to grow in our back
yard (reminding us why we are photographers and not gardeners). So we
decide to take a picture. Now let’s assume for the sake of argument that
under these conditions our camera’s film speed needs to be ISO 100, our
Aperture setting is f/8, and our shutter speed set at 1/250 (one 250th
of a second), to achieve a proper exposure on our photo. Again this is
hypothetical so don’t go telling everyone that we said these are the
settings to use for every photo on a sunny day. Every situation is
different, and only you and your camera know what settings work best for
you. But for our base scenario we will assume that these settings are
correct.
Now let’s assume that it is later that afternoon
and we decide that we want to take another picture of our lonely little
flower. Unfortunately for us, it is now late afternoon and some clouds
have decided to move in and cover the sun. Because of this we will have
to do something to compensate for the decrease in light, or our picture
will turn out underexposed (Way too dark). We could try changing our
shutter speed from 1/250 to something like 1/30 (one 30th of
a second). This would give us the proper exposure, but that shutter
speed is a bit on the slow side and we would have to risk moving the
camera, or the wind blowing our flower around while we took the shot.
Both of which would result in a blurry picture. A better alternative
would be to change our ISO setting (or film as the case may be). As you
may recall, we said that increasing the ISO setting would increase our
sensitivity to light. Thus by stepping up to ISO 200 we can have a
shudder speed of 1/60 or with ISO 400 we can maintain a shutter speed of
1/125, a much more manageable shutter speed. All of witch would give us
the same exposure on our photograph.
At this point you may be wondering why not just use
the highest ISO setting all the time and save from having to
continuously change settings? Well unfortunately even in photography all
good things have a price. And when it comes to ISO settings, the higher
the setting, the greater the susceptibility our image is to noise
(Little specs that appear in an image similar to static or snow on a
TV). Because of this we recommend you always use the lowest ISO settings
you can get away with.
Back to our base scenario, midday, outside, camera
settings of; ISO 100, Aperture f/8, and Shutter speed 1/250. Only this
time instead of photographing our little flower (because we are a little
disgusted it’s the only one we can get to grow.), We decide we are going
to take photos of the cars traveling up and down the street. So what do
we do? We see a car coming and we grab our trusty camera (Again, a good
photographer always have their camera with them right?), we focus in on
the car and we quickly take the shot. Well, since we are using a digital
camera, lets take a look in our viewfinder and see how we did. Hmmm,
that’s odd, unless we have some experience using our digital camera, we
probably have a good shot of the house across the street, but the car we
were taking a picture of isn’t in the photo. That’s because we didn’t
compensate for what is commonly referred to as Shutter Lag. Shutter Lag
is a common trait of most digital cameras, and although we are not going
to go into in depth here, essentially, when you take a photo with a
digital camera there are a lot of little things that must take place
between the time you press the shutter button, and the time the camera
actually takes the picture. This brief delay is referred to as shutter
lag. And it has lost many photographers some very good shots. The amount
of shutter lag is dependant upon your particular camera, and is
something that you will need to get a feel for as you experiment with
it.
Ok, so let’s say we took the shot again, and we
compensated for our shutter lag. Let’s look in our viewfinder and see
what we’ve got. More than likely, we’ve ended up with another good
picture of the house across the street, only this time there is a blur
across it where the car should have been. That’s because we have just
captured motion in our photograph. That blur that we see across the
picture is essentially the motion of the car as it passes in front of
the lens. And while motion can sometimes add great strength to an image,
it is outside the realm of what we are trying to cover here.
All right, back to what we are trying to cover, to
capture a picture of our moving car, there are a number of things that
we can do, but for right now, we are trying to stay within the realm of
ISO, Aperture, and Shutter speed. So with this in mind, we will increase
our shutter speed to say 1/1000. Increasing our shutter speed allows us
to capture a faster moment in time, essentially stopping the action as
it is commonly referred. Of course by increasing our shutter speed we
are decreasing the amount of light entering our camera and we will once
again have to compensate for that. We could try to compensate for it by
changing the f/stop (Aperture setting), allowing more light back into
the camera, but doing so may cause some unwanted effects in our photo
that we will get into later. So what should we do? That’s right, let’s
increase to ISO 400, Leave our Aperture at f/8, and set our shutter
speed to 1/1000. Now if we take the picture again, we can see our car
essentially stopped (Hopefully) in front of the house across the street.
So, we’ve seen how ISO settings and Shutter speed interact with each
other under different circumstances. Now let’s find out what happens
when we throw Aperture settings into the mix.
Changing the Aperture setting on your camera can
have many remarkable effects. And there will be many different
circumstances under witch you will want to adjust this setting. But for
our brief overview we are going to focus on its ability to control Depth
of Field. Depth of Field refers to the area of clear focus in a
photograph. When you focus your camera on a subject, the subject is not
the only clear image in the photo. There will be some areas in front of
and behind the main subject that will also be in clear focus. This area
is referred to as Depth of Field. Increasing or decreasing the Depth of
Field in a photograph can have a very dramatic effect on the final
image. For instance, let’s go back to our base scenario, you remember,
midday, sunny, our camera set to; ISO100, f/8, 1/250 and our lonely
little flower (Can you believe it’s still doing well and we haven’t even
watered it). When we took our first photo of our flower, we probably
ended up with a crisp clear image of our flower as well as much of the
yard around it. Unfortunately, the image we wanted was of the flower not
of the whole yard. So let’s take the picture again, only this time we
want to emphasize just the flower. We can do this by changing or Depth
of Field so that only the flower and a small area around it is in focus.
There are three main factors involved in Depth of Field, Aperture, Focal
Length, and Subject Distance. Any one or all three of these have an
outstanding effect on the Depth of Field, but for our purposes here, we
will focus on Aperture settings.
We can change our Depth of Field to focus on just
our flower by increasing the Aperture setting on our camera. Remember we
said that when it comes to Aperture, the smaller the number the large
the Aperture opening. So if we change or Aperture setting from f/8 to
something more like f/4 we have greatly increased the size of the
Aperture, which in turn greatly decreases our Depth of field. Of course
by increasing our Aperture size we have increased the amount of light
that will enter our camera, so we will have to adjust for that. In this
scenario we are going to do this by increasing our shutter speed to
something like 1/1000. Now, with all other things being equal, if we
take the photo again, our flower should be the primary focus in the
picture with everything in front of and behind it fading into a soft
blur. Please note that Aperture adjustments are very subjective and are
greatly influenced by many conditions such as, distance from the
subject, Focal point of the lens, and the Depth of Field desired. There
is no right or wrong when it comes to Depth of Field, it is purely a
choice of the photographer. For the beginner, it may be easier to
remember that when it comes to Aperture, the smaller the number, the
smaller the depth of field.
We hope that through all of this you have come to
understand that there is a very distinct relationship between ISO,
Aperture, and Shutter speed. Changing any one of these settings without
changing at least one of the others will greatly affect the exposure of
your image. Also there are a number of different ways that these
settings can be combined and still maintain the same exposure. For
instance;
ISO100, Aperture f/4, and Shutter speed 1/1000
ISO100, Aperture f/5.6, and Shutter speed 1/500
ISO100, Aperture f/8, and Shutter speed 1/250
ISO100, Aperture f/11, and Shutter speed 1/125
ISO100, Aperture f/16, and Shutter speed 1/60
ISO100, Aperture f/22, and Shutter speed 1/30
Will all give you approximately the same exposure
only with dramatically different effects in Stop Action and Depth of
Field.
As you can see ISO settings, Aperture settings, and
Shutter speed play an important role in every picture that you take. The
ability to use these settings at the proper time and in the proper
fashion is one of the first marks of a good photographer. We strongly
recommend experimenting with these settings in different situations
until you become familiar with your camera and the effects the settings
have on it. |
