What is Exposure? A Tutorial



What Is Exposure?


In photography exposure is the moment when the light strikes the film or sensor and the image is recorded. Three factors combine to determine the
correct exposure for a digital image: the amount of light in the scene that
strikes the image sensor (controlled by the lens aperture); the length of
time that the sensor is exposed to the light (controlled by the shutter speed); and the sensitivity of the sensor (determined by the ISO setting).
The importance of a good exposure cannot be overstated. If an image is overexposed, the highlights will be completely white without any tonal information


If an image is grossly underexposed, the image will be
dark and lack shadow detail


Although it is possible to improve moderate exposure mistakes using image editing software, no amount of software magic can save a picture that
is extremely overexposed or underexposed. Fortunately, camera light meters
are very sophisticated instruments and do an excellent job of determining
the settings for a proper exposure in most common photographic situations.
However, by changing any of the three main settings, you can gain control
over certain characteristics that can influence the look of the image.


Aperture refers to the opening of the iris, or diaphragm, in the lens that can
be adjusted to let more or less light hit the image sensor. One way to think
of aperture is to imagine water running through faucet. The opening of the faucet (aperture), controls the amount of water (light) flowing in a period of time. The vessel is the sensor.

2-001 Aperture.jpg

Aperture is measured in f-stops, and each full stop represents a factor of two in the amount of light admitted. Thus, “opening up” a lens from f5.6 to f4 will admit twice as much light, and “stopping down” from f11 to f16 will cut the amount of light in half.
Now “What’s in an ƒ-stop?”


The f-stop is the ratio of the focal length of the lens to the diameter of the opening in the aperture. On a 50mm lens, for example, when the aperture is opened up to a diameter of 12.5mm, this results in an f-stop of f4 (50/12.5 = 4).


Some of the f-stops used on lenses for film cameras are f1, f1.4, f2, f2.8, f4, f5.6, f8, f11, f16, and f22
Each change in f-stop either halves or doubles the amount of light that
enters the lens. In this, a larger opening and larger numbers indicate a smaller
opening. If you wanted to adjust the aperture one stop down from f4, to let in half as much light, common sense would tell you that the next stop should be f8, not f5.6. The reason for this seemingly odd number thrown in between f4 and f8 is that apertures are circular openings, and the mathematical realities of dividing the area of a circle in half sometimes produces a fractional
number instead of a whole number.
Apart from controlling how much light passes through the lens, the aperture
is also one factor that affects depth of field (the others are the focal
length of the lens, the size of the image sensor, and the distance between
objects in the scene). Depth of field is the area of the image that appears in
focus from foreground to background and is one of the main ways that you
can change the appearance of an image




Shutter speed
If we continue with the same faucet analogy that was used to explain the
aperture, the shutter is the valve that controls how long the light flows
through the lens and onto the image sensor. The smaller the faucet opening (aperture), the longer it will take a given amount of water (light) to flow and
the longer the required exposure will be.

3-001 Shutter.jpg

Shutter speeds are measured in extremely small fractions of a second, and speeds on DSLRs range from 30 seconds up to 1/8000 of a second.


The function of the shutter is similar to aperture in that each successive change in the shutter speed either halves or doubles the exposure time. Using standard shutter speeds as an example, 1⁄125 of a second is half as much exposure as 1⁄60 but twice as much as 1⁄250.
In addition to controlling how long the light is exposed to the sensor, the
shutter speed also impacts how motion is rendered in a scene. Speeds below
1/40 of a second are likely to result in motion blur when moving subjects are
photographed, and very fast shutter speeds of 1/1000 or higher can do an
excellent job of freezing even very fast movement in a scene. Just as depth
of field can be used creatively to affect the look of an image, shutter speed is
also an important creative control




Aperture and Shutter work together in Partnership

Aperture and shutter speed work together to create a proper exposure in
a given lighting situation. Because of the way they function, you could take
several shots, each with a different aperture and shutter speed, and produce
several images that all had equal exposure.

4-001 Aperture_Shutter.jpg

You could use a wider aperture
for a shorter amount of time, for instance, or a smaller aperture for a longer
amount of time to admit equivalent amounts of light. Another way to look
at this is that opening up the lens aperture by one stop is exactly the same
as decreasing the shutter speed by one setting; each doubles the amount of
light for the exposure. And increasing the shutter speed by one setting has
the same effect on exposure as stopping down a stop to a smaller aperture.


This give-and-take nature of the aperture-shutter speed relationship we call as 'Partnership', and it’s one of the most effective exposure tools available
to photographers



The benefits of 'Partnership'y come into play when your camera meter recommends a certain exposure, but you need to change either aperture or shutter speed to produce a desired creative effect. Let’s say you’re photographing a flower, and the camera’s light meter indicates that it will use a shutter speed of 1⁄2 of a second and an aperture of f11. Although this might yield a correctly exposed image, an aperture of f11 would produce too much depth of field, making the background details too distinct and distracting. If you were using a manual exposure mode, you could take advantage of 'Partnership' to quickly (well, reasonably quickly) calculate an equivalent exposure that would give you a wider aperture and throw the background out of focus.
If you decided that an aperture of f2.8 would produce the desired shallow depth of field, you’d increase the aperture by 4 stops (f8, f5.6, f4, f2.8); that would require an equivalent adjustment of the shutter speed. Opening the lens aperture to f2.8 lets in more light (16 times as much in this case); so to balance out the exposure you would need to shorten the amount of time the shutter is open by 4 stops—to 1⁄30 of a second. For the final exposure of the flower, the shutter speed is at 1⁄30 and the aperture is at f2.8. This produces exactly the same exposure (in other words, the same amount of light reaches the sensor) as the initial camera meter’s suggestion of 1⁄2 at f11, but the differences between the two images is significant



Fortunately, unless you’re operating on full manual mode or you just enjoy the
intellectual challenge, when used in Aperture or Shutter Priority mode, cameras will automatically calculate the reciprocal aperture and shutter speed values for you. This makes it easier to concentrate on the image and choose the settings that will give you the right creative look.


Full Auto Mode


Nearly all modern cameras provide a fully automatic mode that does everything
for you but compose the shot and decide when to press the shutter
button. Full Auto mode evaluates the lighting; selects the ISO, white balance, aperture, and speed settings; and even decides whether the scene needs a little extra light from the built-in flash. This is a good mode to use if you’re new to digital photography and you still don’t know much about your new camera but you want to take pictures right away—or when you need to hand the camera to someone else to take a picture of you.
Keep in mind that some camera features, such as the abilities to change the
ISO, adjust the exposure with exposure compensation, and shoot in RAW format, may not be available in Full Auto mode. To gain an extra level of control and customization while enjoying the ease of automatic operation, you may have to use another automatic mode that is commonly called Program.

Program Mode


Program mode is similar to Full Auto mode in that the camera selects the
appropriate aperture and shutter speed to deliver the correct exposure for
the scene you’re photographing. You also have the ability to modify the settings
the camera has chosen by shifting the aperture–shutter speed combination
to select a mix that better serves your creative goals ('Partnership'in action).
On DSLRs you usually make this adjustment by dialing a control
wheel until you arrive at a desired aperture or shutter speed, something
you can do without taking your eye away from the viewfinder.
Program modes also offer access to more advanced features of the cameras, such as shooting in RAW format, exposure compensation, higher ISO settings, and choosing a custom white balance. Because it offers the convenience of being fully automatic with the flexibility of changing some of the settings, you may find that Program mode works well for many situations.

Aperture Priority Mode


Aperture Priority can be thought of as a semiautomatic mode because it relies
on you to decide which aperture to choose while the camera supplies the appropriate shutter speed. Once you select a given aperture, the camera constantly adjusts the shutter speed in response to changing exposure conditions, but the aperture remains the same. This mode is an excellent choice for images where depth of field issues take precedence over shutter speed. A wider aperture
causes the background to be more out of focus, and a smaller aperture yields
a photo with more areas of the image in focus. Aperture Priority is excellent
for portraits where you want only the subject in focus (use a smaller ƒ number
for a larger aperture) and for scenic shots where you want good depth of field
throughout the scene (use a larger ƒ number for a smaller aperture).

Shutter Priority Mode


Like Aperture Priority, Shutter Priority is a semiautomatic mode. You decide
what shutter speed you want to shoot with, and the camera chooses the correct
aperture. Shutter Priority is ideal for situations where exposure time
is more important than depth of field. If you need to freeze motion, such as
with sports or birds in flight, using this mode allows you to select an appropriately
fast shutter speed. If your aim is to use motion blur creatively, such
as the classic rendition of moving water in a stream, you can also use Shutter
Priority to choose a slow shutter speed. Depending on the speed of the
object you’re trying to blur, you may need to use a tripod so that stationary
elements in the image remain sharp.

Manual Mode


With Manual mode you have to do all the work. Well, maybe not all the
work. The camera does provide a light meter to tell you if your settings will
give you a properly exposed image, but you have to turn the dials or push
the buttons and make sure that aperture and shutter speed are set correctly.
Although a Manual mode is essential for photographic control and those who want as many creative options as possible, it’s not as spontaneous as some of the other modes, and realistically you may only need to control either aperture or shutter speed to achieve the effect you want. For some situations, however, such as night photography and in the studio, having a Manual mode is critical.

Scene Modes

5-005 .jpg

Scene modes are preset configurations that are designed for you to use under
specific shooting conditions to achieve good results without having to think
about the optimal camera settings. They’re not exposure modes you would
use all the time. You’ll find these modes on many digital cameras, from compact
point-and-shoot models all the way up through entry level DSLRs. You'll not find this on a pro-DSLR.
The actual names and modes vary from camera to camera (other terms
include Best Shot and Creative Assist modes), and depending on their
features, some cameras may offer more sophisticated interpretations.

Happy shooting
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Wow! Sensational article Henryji! Thanks a lot! I loved that explanation of 'partnership' part. I had a tough time using the full manual mode. Now this makes a lot of sense. Thank you so much :) Rating this 5 stars!


Wow! Sensational article Henryji! Thanks a lot! I loved that explanation of 'partnership' part. I had a tough time using the full manual mode. Now this makes a lot of sense. Thank you so much :) Rating this 5 stars!
You are welcome, thank you.