EXPOSURE TUTORIAL PDF
The first (and still in many ways the most important) thing I learned when I bought my first camera was the concept of “Exposure”.
We all want to take photographs that are pleasing to our eyes, and show as much tonal range (individual shades of brightness between the extremes of white and black) as possible to retain detail, but with extremes that “just” touch the white and black ends of the tonal range. This gives us an image where the tones match what our own eyes tend to “see”… Whites that look white, shadows that are suitably dark, midtones that look natural etc….
For the image that your camera records to be “correctly exposed”…the light sensitive sensor (or film if you still shoot film) needs to be exposed to the correct amount light for the correct amount of time for the speed that the sensor reacts to light.
These things can all be adjusted by you, and any increase in one of these requires a decrease in one or both of the others for the exposure to remain balanced and unchanged…
for example: if you double the time that the sensor is exposed to light, you either need to halve the amount of light coming in, or make the sensor react half as fast to the light to end up with the same exposure (or to a lesser extent you could reduce both)
If one or more of these are too high, and reciprocal changes are not made in the others then your image will be OVER EXPOSED… Too much light data recorded at the sensor…
These images will be very bright, and shadows will look washed out. Blacks will end up grey, and light areas will retain little or no detail…..
If one or more of these are too low, and reciprocal changes are not made in the others then your image will be UNDER EXPOSED… Not enough light data recorded at the sensor…
These images will be darker than the scene you have photographed. Shadow areas will be too dark and will lose detail and may even be blocks of solid black. Whites will end up grey….
Now remember I said that you can adjust these things!
Lets go back and look at what I said here…
“For the image that your camera records to be “correctly exposed”…the light sensitive sensor (or film if you still shoot film) needs to be exposed to the correct amount light for the correct amount of time for the speed that the sensor reacts to light.”
WELL...
Correct amount of light = APERTURE
Correct amount of time = SHUTTER SPEED
Speed that the sensor reacts to light = ISO (“Film” speed…)
Next I’ll talk briefly about each setting, and also how else it might affect your image….
APERTURE
The APERTURE is just that…it’s a hole.
It is in fact the hole in the front of the camera that the light from your lens passes through into the camera body and on to the sensor. An adjustable set of blades (sometimes in the lens) arranged in a ring allows you to govern the size of the aperture and therefore control the amount of light that can enter the camera!
Now… Just to make it complex for people learning, they decided to label the aperture sizes as fractions (eg. f8, f11, f16…..). So as the hole gets smaller (less light) the f number gets BIGGER.
So remember …
Bigger numbers = less light
Smaller numbers = more light
It stands to reason that for a given exposure…if you change the aperture for a BIGGER number (smaller hole = less light) you need to get that light back by opening the shutter for longer and/or making the sensor react quicker to the light. If you change the aperture for a SMALLER number (bigger hole = more light) you need to compensate by opening the shutter for shorter amount of time and/or making the sensor react more slowly to the light.
What else does it do?
A side effect of APERTURE is Depth of field (DOF). This basically is an indication of how much of your image will be in focus. When you focus on a spot in any scene, the distance from you to that spot is the focal distance.
High fnumbers tend to produce a large DOF (where large distances in front and behind the focal distance remain sharp and in focus). This is useful for landscapes etc…
Low fnumbers produce a lower DOF where objects further away and in front of the focal distance will blur off or “bokeh” and focussing all the attention on the subject in focus. This can be nice for portraits etc…
SHUTTER SPEED
The SHUTTER SPEED is the amount of time that the shutter opens and allows light to enter the camera.
This is an easier concept to grasp as it is expressed in seconds (a NORMAL measuring tool!!!!!!! YIPPEEE!) although often this will also be so fast as to be expressed as a fraction of a second (eg. 1/4s would be one quarter of a second, 1/60s would equate to one sixtieth of a second…)
Obviously the longer the shutter remains open, more light will enter the camera and hit the sensor. So as the shutter speed increases (smaller time) less light can enter so the other values would need to be increased to maintain the same exposure. If the shutter speed is decreased (longer time), then the other values would need to be decreased to compensate.
What else does it do?
Shutter speed is what gives an image it’s sense of movement, action …or calmness.
Fast shutter speeds freeze time still, and stop motion in it’s tracks. This helps capture a sharp image, and enables shots to be taken by hand rather than on a tripod. Slower shutter speeds allow some movement within an image and create the illusion of motion. Get it right and you have a powerful image, BUT over cook things and your subject will suffer from motion blur and you wont have a sharp image.
Very long exposures, allow faint objects to register on the sensor, (eg. night / low light photography) or completely obliterate motion from water. (Ripples and waves disappear, leaving a flat calm effect)
ISO (Film speed)
The ISO represents how quickly the sensor (or film) reacts to the light entering the camera.
IF you shoot film (and lots of people me included still do shoot film! I still wholeheartedly recommend it too!) this is pretty much pre-determined by the speed of the film that you have loaded, but if you shoot digitally then you can change this setting on a shot to shot basis.
As this goes up, then less light is required to create an image on the sensor so the other values can be reduced. As it goes down, then the opposite is true.
I don’t often change this from the lowest possible setting unless I specifically need a shot within a certain shutter speed and aperture setting and I can’t get it. This happens to me in low light when I need a sharp shot without flash… (eg. bride and groom in a dimly lit church)
What else does it do?
As you increase the ISO the image is affected increasingly by grain (film) or digital noise. This basically means the image becomes grainier. Although often regarded as a bad thing, in some cases it can produce very nice effects. Some images just suit grain and it’s all about personal choice.
STOPS?????
It’s all very well saying “adjust this, reduce that, increase that…” but by how much?
Some bright spark developed the idea of “STOPS” as a uniform measure of exposure value. This allows us to accurately adjust these settings to compensate for each other.
To put it simply to go up “ONE STOP” means to DOUBLE the exposure. To go down “ONE STOP” means to halve it. To go up “TWO STOPS is to increase it 4 times (Double it, then double the result!) etc… To go down the reciprocal conversions apply…
STOPS AND....
SHUTTER SPEED…
Its measured in seconds so if the time doubles then you have gone up a stop, if it halves then you’ve gone down a stop.
eg. Separations of one stop from less light to more light would be..
1/500s 1/250s 1/125s 1/60s 1/30s 1/15s 1/8s 1/4s 1/2s 1s
APERTURE…
Separations of one stop from less light to more light are for example…
f22 f16 f11 f8 f5.6 f4 f2.8
ISO
As ISO doubles, then you go up one stop, as it halves you go down a stop.
eg. Separations of one stop from less light to more light would be..
50 100 200 400 800 1600 3200
PART of a stop? Most cameras will allow you to vary exposure by half a stop! So you may see values in between the ones I’ve shown here! Don’t worry the rules are the same…..
I Hope this is useful, and any comments are welcome! Remember, the "rules" are there to be broken sometimes too!
