There are many different ways to arrive at an ’ideal’ exposure, none of which are right or wrong, but each introduces their own qualities and limitations.
Getting an ideal exposure is achieved when elements of the frame are not ‘overexposed’, creating ugly highlights, but there is still enough light to show the features of your subject, making sure it is not 'underexposed'.
Features that can be used to control your exposure include;
ISO (Digital or film)
Shutter speed controls the amount of time (usually measured in fractions of a second) that your shutter is open, allowing light to fall onto your cameras sensor/film.
The shorter the amount of time, the less light is allowed onto the sensor/film. The longer it is open, the more light is let in.
The shutter speed also affects the way the camera records motion. If you imagine that a shutter speed of 100 is open for only 1/100 of a second, then the sensor will record any motion that occurs in that time. When it is open for longer periods, for example 1/25 of a second, more motion is recorded, which will often result in motion blur.
The faster your shutter speed, the less motion blur in your shots.
Motion Blur can be introduced by either the movement of your subjects, or from the movement of the camera.
Motion blur is not inherently bad, it is more of a stylistic choice, but it is important to know how to manipulate your images so that you can control motion blur. The higher your shutter speed, the less motion blur in your shots.
For video, motion blur also takes on some other qualities. Some amount of motion blur seems ‘natural’ to the eye, and helps 'blend' one frame into the next. This can be manipulated to different effects.
Generally speaking, select a shutter speed of no less than double your frame rate (so a shot intended to be played back at 25fps should be taken at a shutter speed of 1/50). This will give a ‘natural’ look to the blur in your shots. Natural in this case has to do with how the human eye perceives motion, but also the standards applied in film making for generations. Dipping below the 180-degree shutter rule (a fancy way of describing the aforementioned rule of double your frame rate, harking back to the days of rotary shutter mechanisms in film cameras) is generally a no-no, and should only be done when you are specifically looking for a jarring or creative effect.
In the golden days before electronic shutters cameras were blessed with the oft hand-cranked rotary shutters. Rotary shutters were a spinning mechanism that covered the film/sensor for a portion of their rotation, and left it exposed for the other portion. During the time it was shielded from light, the film would also track to the next frame, exposing each frame of the film. The 'angle' of the shutter determined the amount of time the frame was exposed, and the amount it was shielded. To change shutter speeds in this glorious time, one would have to physically take out the shutter, and replace it with a different shutter angle. A 180 degree shutter means that whatever speed the camera is cranked at (framerate) half the time it will be exposed, and half the time it will be hidden. It is commonly dissuaded to dip below a 180 degree shutter.
If you dip below this rule of thumb, you will find excessive motion blur is introduced, which sometimes give the impression the subject is drunk or drugged (when used as a P.O.V.) and can create light trails.
As you shorten the amount of time the shutter is open, you will get less motion blur. This can give a more ‘jerky’ and unnatural look. Some cinematographers love the look this introduces to action scenes, or chase scenes because it is unsettling. 1/100 to 1/120 introduces a spidery, mechanical look to action, but going further past this point usually has a nauseating effect, and should only be done when playing with high frame rates (slow motion).
Aperture is another way you can affect the exposure. Aperture is measured in f/stops, and is a system of measuring how wide the iris in the lens is. The wider the iris (lower the f/stop), the more light pours in through the lens.
The iris is made up of blades that expand to let in lots of light, or close up to a pinhole, significantly cutting the amount of light hitting the film/sensor. The amount of blades in the iris can also affect the shape of out of focus light points, or bokeh, but more on that later.
Each full f/stop lets in half or double the amount of light as the one before or after it. You will often hear people saying they are 'stopping up', or 'stopping down'. This refers to them letting in more or less light using aperture, shutter speed, or ISO. here is a list of full stops starting from wide open, to fully closed. You may have wider apertures, or less apertures available depending on the quality of your lens.
1.4 2.0 2.8 4 5.6 8 11 16 22
Aside from its control over exposure, the aperture, or f/stop, controls depth of field. This can be confusing at first, but once it makes sense, can be great way to be creative with your imagery.
Depth of field is the amount of your image that is in focus, a large depth of field means that a large area can be in focus, while a small depth of field isolates a small area. It’s good to think of your DoF as a continuum, with things further away from, and closer than your focus point getting more and more out of focus. Things that appear in focus to the human eye, but are in fact not at the exact point of focus, are said to sit in the circles of confusion. A lens can only hold focus at one point, and any point before or after that plane gradually defocuses. However, when something sits in the circles of confusion, it is so minimally defocussed that it is either imperceptible to the human eye, or acceptably in focus.
To control the DoF, you have to keep a few things in mind.
First of all, your f/stop. The wider open your aperture, the lower your f/stop, the less depth of field you will have, meaning only a small area around your subject will be in focus.
As you close up your f/stop more of your image will come into relative focus.
Secondly lens length. A wide lens, for example a 20mm, has more DoF, a normal lens, for example a 50mm, has a normal amount of DoF, and a telephoto lens, for example 85mm, will have less DoF. Basically, as the lens length gets longer, the perceived depth of field becomes thinner, creating blurred and softened backgrounds.
Lastly, how close you are to your subject will affect how much DoF you can achieve. If your subject is far away, it is easier to get a large DoF, but as they get closer, you can start to achieve a more limited DoF. Focus wheels are measured from their minimum focal distance, the closest you can have an object to the camera in focus, until their furthest, and then infinity. If anything is past your maximum focal range, it enters infinity, at which point, it and everything behind it will be in relative focus. In contrast, if you are focused at the minimum focal distance, then it is highly likely that everything else will be out of focus.
So why wouldn’t you want everything in focus?
A thin DoF is a good way to draw focus to a specific subject in your frame.
Out of focus areas can also be very pleasing to the eye, specifically out of focus light points, or bokeh. Bokeh are out of focus light points, like lanterns, torches, candles or light bulbs. Bokeh is generally very appealing to the eye, and looks very ‘cinematic’. The shapes of these light points are affected by their placement within the frame, (known as the cat’s eye effect) and the amount of blades in your lens. As they get closer to the edges bokeh will thin and become more elliptical, the more blades you have in your lens, the more circular and smooth the bokeh will appear. The word itself comes from Japanese, and means literally blur, or haze.
Conversely, 'Deep focus' is a style that many cinematographers love, and was very in vogue during the time of Orson Welles. Deep focus is the practice of using a very wide depth of field, but making use of a sense of depth by staging action in foreground, midground and background. Deep focus is often used to add a depth to storytelling by giving more definition to the mise-en-scène, and other parts of the world we may miss if they were reduced to painterly blurs.
ISO is another way of to adjust the exposure of a shot. ISO, or ASA, in film used to refer to the 'speed' or sensitivity of the film. The higher the ISO, the more sensitive to light, but also the more ‘grain’, or ‘noise’ is introduced into the shot.
The principles are the same for digital photography. As you increase the ISO, you increase the noise, however digital noise is considered by many to be more unsightly than film grain.
Noise and Grain are not always bad things, as sometimes filmmakers will use grain to introduce a ‘gritty’ feeling to their footage, and sometimes the trade-off (more noise for more light) is enough to justify bumping up the ISO.
When examining your shot for grain, remember that it will often look fine in your camera viewfinder, but when you view the footage on a larger monitor like your computer screen or television, the grain becomes much more visible.
A ‘normal’ ISO is around 100, and my 5DmkIII can be pushed to around 1250-1600 before noise starts to become an issue. However, every camera performs differently in lowlight conditions depending on its sensor.
If you find you want to cut more light out, for instance in the bright sun, or you want to achieve longer exposure times/wider apertures in bright environments, you can use filters.
Neutral Density Filters have no effect on the colors of your image, but cut varying amounts of light depending on their width. They are very useful when shooting outdoors. Neutral Density filters provide more room to play with your settings in the bright sun, allowing you to have your aperture wide open, or to shoot at long exposures without overexposing your shot.
You can also buy Graduated Neutral Density Filters that only cut light in certain areas of the frame, so you can get an ideal exposure for two differently lit subjects in the same location. (Often used to balance out the bright sky in landscape photography.)
Polarizing filters also cut light, but their main purpose is to remove the cast of reflective light, and so increase the saturation of colours. They are particularly effective at bringing out the colours of the sky.
There are many ways to get an ideal correct exposure;, as you let in more light with your shutter speed, you may have to cut more with your f/stop. The important thing is to understand what each individual setting does to your image, and what you want to prioritise in each circumstance.
For example you may have a group shot where you need a lot of people in focus, but at ISO 100 you can’t get enough light in to close up your aperture and still have a good exposure. So you can push the ISO up a couple of stops, giving you the room to close your aperture from f/2.8 to f/5.6, increasing the depth of field to keep everyone in focus..
You may be shooting a very fast moving subject, which would just be a blur if you had your shutterspeedshutter speed at 1/50. So you open up your aperture further to let more light in to then stop down your shutter speed to 1/100. This lessens the motion blur, allowing you to catch a split second frozen in time.
You may just want a very thin depth of field to accent your subject from the background, but you’re in a bright environment. So you push your shutter from 1/50 to 1/120 so you can have your aperture wide open. This sacrifices your natural motion (shutter speed), for your thin depth of field (aperture).
None of these approaches are right or wrong, they are just a means to get the image you want.
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