- Shutter Speed, Aperture and ISO Work Together to Create an Exposure :
To have a good understanding about exposure and how
shutter speed, aperture and ISO affect it, we need to understand what happens
within the camera when a picture is taken.
As you point your camera at a subject and press the
shutter button, the subject gets into your camera lens in a form of light. If
your subject is well-lit, there is plenty of light that travels into the lens,
whereas if you are taking a picture in a dim environment, there is not much
light that travels into the lens. When the light enters the lens, it passes
through various optical elements made of glass, then goes through the lens
“Aperture” (a hole inside the lens that can be changed from small to large).
Once the light goes past the lens aperture, it then hits the shutter curtain,
which is like a window that is closed at all times, but opens when needed. The
shutter then opens in a matter of milliseconds, letting the light hit the
camera sensor for a specified amount of time. This specified amount of time is
called “Shutter Speed” and it can be extremely short (up to 1/8000th of a
second) or long (up to 30 seconds). The sensor then gathers the light, and your
“ISO” brightens the image if necessary (again, making grain and image quality
problems more visible). Then the shutter closes and the light is completely
blocked from reaching the camera sensor.
To get the image properly exposed, so that it is not too
bright or too dark, Shutter Speed, Aperture and ISO need to play together. When
lots of light enters the lens (let’s say it is broad daylight with plenty of
sunlight), what happens when the lens aperture/hole is very small? Lots of
light gets blocked. This means that the camera sensor would need more time to
collect the light. What needs to happen for the sensor to collect the right
amount of light? That’s right, the shutter needs to stay open longer. So, with
a very small lens aperture, we would need more time, i.e. longer shutter speed
for the sensor to gather enough light to produce a properly exposed image.
Now what would happen if the lens aperture/hole was very
big? Obviously, a lot more light would hit the sensor, so we would need a much
shorter shutter speed for the image to get properly exposed. If the shutter
speed is too low, the sensor would get a lot more light than it needs and the
light would start “burning” or “overexposing” the image, just like magnifying
glass starts burning paper on a sunny day. The overexposed area of the image
will look very bright or pure white. In contrast, if the shutter speed is way too
high, then the sensor is not able to gather enough light and the image would
appear “underexposed” or too dark.
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| Underexposed-Normal-Overexposed |
Let’s do a real-life example. Grab your camera and set
your camera mode to “Aperture Priority“. Set your lens aperture on your camera
to the lowest possible number the lens will allow, such as f/1.4 if you have a
fast lens or f/3.5 on slower lenses. Set your ISO to 200 and make sure that
“Auto ISO” is turned off. Now point your camera at an object that is NOT a
light source (for example a picture on the wall) then half-press the shutter
button to acquire correct focus and let the camera determine the optimal
exposure settings. Do not move your camera and keep pointing at the same
subject! If you look inside the camera viewfinder now or on the back LCD, you
should see several numbers. One of the numbers will show your aperture, which
should be the same number as what you set your aperture to, then it should show
your shutter speed, which should be a number such as “125” (means 1/125th of a
second) and “200”, which is your sensor ISO.
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| NIKON D5000 View finder |
Write down these numbers on a piece of paper and then
take a picture. When the picture comes up on the rear LCD of your camera, it
should be properly exposed. It might be very blurry, but it should be properly
exposed, which means not too bright or too dark. Let’s say the settings you
wrote down are 3.5 (aperture), 125 (shutter speed) and 200 (ISO). Now change
your camera mode to “Manual Mode“. Manually set your aperture to the same
number as you wrote down, which should be the lowest number your camera lens
will allow (in our example it is 3.5). Then set your shutter speed to the
number you wrote down (in our example it is 125) and keep your ISO the same –
200. Make sure your lighting conditions in the room stay the same. Point at the
same subject and take another picture. Your results should look very similar to
the picture you took earlier, except this time, you are manually setting your
camera shutter speed, instead of letting your camera make the guess. Now, let’s
block the amount of light that is passing through the lens by increasing the
aperture and see what happens. Increase your aperture to a larger number such
as “8.0” and keep the rest of the settings the same. Point at the same subject
and take another picture. What happened? Your image is too dark or underexposed
now! Why did this happen? Because you blocked a portion of the light that hits
the sensor and did not change the shutter speed. Because of this, the camera
sensor did not have enough time to gather the light and therefore the image is
underexposed. Had you decreased the shutter speed to a smaller number, this
would not have happened. Understand the relationship?
Now change your aperture back to what it was before
(smallest number), but this time, decrease your shutter speed to a much smaller
number. In my example, I will set my shutter speed to 4 (quarter of a second)
from 125. Take another picture. Now your image should be overexposed and some
parts of the image should appear too bright. What happened this time? You let
your lens pass through all the light it can gather without blocking it, then
you let your sensor gather more light then it needs by decreasing the shutter
speed. This is a very basic explanation of how aperture and shutter speed play
together.
So, when does ISO come into play and what does it do? So
far, we kept the ISO at the same number (200) and didn’t change it. Remember,
ISO means sensor brightness. Lower numbers mean lower brightness, while higher
numbers mean higher brightness. If you were to change your ISO from 200 to 400,
you would be making the photo twiceas bright. In the above example, at aperture
of f/3.5, shutter speed of 1/125th of a second and ISO 200, if you were to
increase the ISO to 400, you would need twice less time to properly expose the
image. This means that you could set your shutter speed to 1/250th of a second
and your image would still come out properly exposed. Try it – set your
aperture to the same number you wrote down earlier, multiply your shutter speed
by two and set it to that number, then change your ISO to 400. It should look
the same as the first image you took earlier. If you were to increase the ISO
to 800, you would need to again double your last shutter speed from 1/250 to
1/500.
As you can see, increasing ISO from 200 to 800 will allow
you to shoot at higher shutter speeds and in this example increase it from
1/125th of a second to 1/500th of a second, which is plenty of speed to freeze
motion. However, increasing ISO comes at a cost – the higher the ISO, the more
noise or grain it will add to the picture.
Basically, this is how the Three Kings work together to
create an exposure. I highly recommend practicing with your camera more to see
the effects of changing aperture, shutter speed and ISO.
- Set "ISO" in Camera :
If your camera is equipped with an “Auto ISO” feature
(known as “ISO Sensitivity Auto Control” on Nikon bodies), you should enable
it, so that the camera automatically guesses what the right ISO should be in
different lighting conditions. Auto ISO is worry-free and it works great for
most lighting conditions! Set your “Minimum ISO/ISO Sensitivity” to 100 on
Canon cameras and 200 on latest Nikon cameras, then set your “Maximum
ISO/Maximum Sensitivity” to 800 or 1600 (depending on how much noise you
consider acceptable). Set the “Minimum Shutter Speed” to 1/100th of a second if
you have a short lens below 100mm and to a higher number if you have a long
lens. Basically, the camera will watch your shutter speed and if it drops below
the “Minimum Shutter Speed”, it will automatically increase the ISO to a higher
number, to try to keep the shutter speed above this setting. The general rule
is to set your shutter speed to the largest focal length of your lens. For
example, if you have a Nikon 70-300mm f/4.5-5.6 zoom lens, set your minimum
shutter speed to 1/300th of a second. Why? Because as the focal length of the
lens increases, so do the chances of having a camera shake that will render
your images blurry. But this rule doesn’t always work, because there are other
factors that all play a role in whether you will introduce camera shake or not.
Having shaky hands and improperly holding the camera might cause extra camera
shake, while having a lens with Vibration Reduction (also known as Image
Stabilization) might actually help to decrease camera shake. Either way, play
with the “Minimum Shutter Speed” option and try changing numbers and see what
works for you.
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| Auto ISO on NIKON |
If you do not have an “Auto ISO” option in your camera,
then start out with the lowest ISO and see what shutter speeds you are getting.
Keep on increasing the ISO until you get to an acceptable shutter speed.
- Exposure Compensation
Another great feature of all modern DSLRs, is the ability
to control the exposure by using the “exposure compensation” feature. Except
for manual mode, exposure compensation works great for all camera modes.
Whether you are shooting in Aperture Priority, Shutter Priority or Auto/Program
modes, dialing the exposure compensation up or down (plus to minus) will allow
you to regulate the exposure and override the camera-guessed settings. If you
find your image (or parts of your image) underexposed or overexposed, you can
use exposure compensation to adjust the exposure without manually changing the
aperture or shutter speed.
- Either Flash or Increase ISO
It really depends on what you are taking a picture of.
Sometimes it is not possible to use your built-in camera flash in a low-light
environment. For example, if your subject is standing far away, you might not
be able to reach the subject with your flash. In that case, the only solution
is to either come closer to the subject, or turn off flash completely and use a
higher ISO. Obviously, for landscape or architectural photography, you should
always turn off your flash, because it will not be able to brighten up the
entire scene. So in a low-light situation, the only two options are to either
increase the ISO so that you can shoot hand-held, or set the camera to the
lowest ISO and use a tripod.
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