Pulling and Pushing: Exposure and Dynamic Range
Since my Minor Project is all about experimenting with different cameras and comparing the respective results, especially when conducting tests in low light (which can be the most challenging condition for a camera), looking at the exposure and dynamic range of the captured images will be mandatory for gauging a camera’s performance.
What is Exposure?
In film and photography – be it digital or analogue – exposure refers to the amount of light that is hitting the film or camera sensor. Since both arts are essentially painting with light, controlling and experimenting with the amount of light is the first – and I would argue most important – step in creating a piece of art such as a film or photograph.
As such, two factors are predominantly defining how much light enters the gate: shutter speed and aperture. Whilst the aperture defines how much light enters the camera in terms of quantity, shutter speed defines how much light enters the camera in terms of time. Since both factors are interdependent to one another, thus affecting the outcome, there needs to be a system with which to determine the relationship in terms of the result, which is the exposure triangle.
The Exposure Triangle
Since a triangle would not be a triangle with only two cornerstones, ISO – which affects the brightness of an image – has to be added into the mix as well. Although the ISO does not have an effect on the “luminance exposure”, it still does have an effect on the brightness of an image: Whilst in terms of film, ISO – or film speed – is defined as “sensitivity to light”, in terms of video it would be more beneficial to think about ISO as “amplification of the incoming signal”.
As digital sensors are effectively turning light into an electrical signal, thus into a current, changing the ISO on a camera is essentially telling the software to amplify the incoming signal. Whilst camera sensors for the longest time used to have only one native ISO, technology has already brought upon us sensors with dual native ISOs; allowing us to capture the best image in two different settings.
But How Is Exposure Measured?
Since relationships usually get significantly more complicated whenever a third party is involved, there need to be rules in place in order to manage the relations between all elements. As such, the relation between all three elements is governed, measured, and calculated by a system using stops. In order to describe this, I quickly need to venture into the subject of dynamic range:
What is Dynamic Range?
In order to be able to gauge and retrieve good exposure, a more detailed understanding of dynamic range as a property of any digital camera will be crucial:
In technical and mathematical terms, dynamic range usually refers to “the ratio between the largest and smallest values that a certain quantity can assume”. Slightly less technical and in terms of photography and filmmaking, dynamic range either refers to a camera’s ability to capture the range between the darkest and brightest parts of an image without loss of detail in them or to the range of luminosity in any given scene.
In order to make this phenomenon accessible (and most importantly: calculable), dynamic range is nowadays often measured in so-called stops. One stop thereby corresponds to either doubling or halving the amount of light. Thus, the bigger the range, the more values or gradations of luminance are within a certain scene or image. Once this range is exceeded, highlights will wash out to clear white and darks will turn to pixelated clusters of clear black, creating visual noise. Whilst camera manufacturers naturally seek to increase the dynamic range of their cameras, attempting to approximate the dynamic range of the human eye – which lies at about 20 stops – cameras do still have less dynamic range and the best ones are currently located at around 15 to 16 stops, although these are manufacturers’ numbers.
Lastly, next to using full stops, there is also the possibility of calculating stops based on one-half, one-third, and one-quarter, giving you a more correct reading, based on smaller increments, thus giving you more control in crafting light.
Because of the above-mentioned definition, dynamic range lends itself to be used as a relative comparison for the relation between the luminance of the real-life situation on set and the captured image as well as for the luminance between the captured images of two or more cameras. As such, the concept of dynamic range acts both as a measure for capacity as well as a measure for limitations of any camera in terms of the attainment of the correct exposure. The more dynamic range a camera has, the more freedom the filmmaker has in crafting the visuals.
So much for the LVL4 recap.
Coming back to our subject of exposure, the individual settings of the ISO, aperture, and shutter speed are now set in a relation that allows to practically controlling the exposure of any given scene. Furthermore, apart from defining the exposure and brightness of any image, all three elements also do have various side effects:
In film, the shutter speed does have an effect on motion blur. Whilst it is usually set to double the frame rate (25fps would result in an optimal shutter speed of 1/50), the effect of motion blur will change with experimentation. In addition to that, the aperture has an influence on the depth of field – the wider the aperture, the shallower the depth of field, the smaller the aperture, the larger depth of field. Lastly, the amplification of the ISO does carry importance in terms of the degree of visual noise in any given image.
Depending on the desired effect and exposure of your final image, you want to prioritise one element, which usually means dialling down the other elements in the mix.
But what is “good” exposure now?
Obtaining Good Exposure
What do you do now if you want your image to be darker – as a style, mood, or genre choice – and what do you do to still achieve what we would consider a usable image?
There is no single unified answer as to what is considered good exposure – apart from the fact that it needs to serve the image and its sujet or theme. Because of that, some images might benefit more from extreme over- or under-exposures whereas others need a more mellow contrast (dynamic range) to convey their symbolism. In order to achieve this, there are certain processes that need to be discussed:
Pushing and Pulling
In cinematography, pushing and pulling refer to the deliberate under- and/or overexposure of images in order to produce a certain effect. Pushing an image thereby means deliberately underexposing, whereas pulling an image refers to a deliberate overexposure of said image. While the concept is the same for video, it is here more often referred to as simply over- or underexposing the image.
But what do you do when you filmed with deliberately ‘wrong’ exposure. With film, if you pushed or underexposed in camera, you would usually need to overexpose (or pull) in post to counteract the effect and create an image that still encapsulated the desired effect and dynamic range. Contrary to that, if you pulled or overexposed your image in camera, you would have to underexpose (or push) in post as well.
The range of stops to which a camera sensor or film stock can be successfully manipulated without irreparably destroying the image quality, is called latitude and closely interrelated to the concept of dynamic range. However, although they are being used interchangeably, they are not the same as latitude is always smaller in range than dynamic range.
My Objective
Since pushing and pulling this is a vital process in crafting the image, and the sensor of your capturing device is the most crucial factor, I’ve decided to test my cameras in terms of their malleability in exposure and latitude tests. I figured that, especially in regard to my FMP and the genre style of Science Fiction, I would need to test and know the dynamic range as well as the latitude of the cameras at hand in order to get the best out of my project without compromising for quality.
I’m excited to find out what these limitations are!
References:
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