Understanding Light and Lenses

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Explaining Light and Lenses document.

Light

Light is a form of electromagnetic radiation and is the part of the electromagnetic spectrum that the human eye is the most sensitive to. All light travels initially in spherical waves, from a single point source. The most common is the sun but the same can be said for any light source.

The first part of measuring light is the wavelength. When light waves move away from a source, its electric field when plotted against a distance or time will have measurable peaks and troughs. The distance between each peak or trough is the wavelength.

Amplitude is the half of the full wave height, or simply the distance from the resting position of the electromagnetic field to the top of a peak. Frequency is then the total number of waves per second.

Finally, velocity of light is defined as the speed of which light moves in a medium. This is a key constant and light within a vacuum is, “exactly 299,792,458 metres per second, or about 186,282 miles per second.” (Light, 2017)

(tjconnects, 2016)

Example Values for Colours of Light

Colour Wavelength Frequency
Red 620 – 750 nm 400 – 484 THz
Orange 590 – 620 nm 484 – 508 THz
Yellow 570 – 590 nm 508 – 526 THz
Green 495 – 570 nm 526 – 606 THz
Blue 450 – 495 nm 606 – 668 THz
Violet 380 – 450 nm 668 – 789 THz

Bruno, T. Svoronos, P. (2005)

Understanding that different colours within the visible colours of Light is hugely useful in video production as it allows for the use of filters as well as manipulation of light in different ways, using the theory explained below.

Another key aspect of light is how it propagates through different materials. This is governed by reflection and refraction of light.

(Pedrotti, 1993)

Reflection is when light meets a surface, along with the light being transmitted as a refracted ray, you can get the light reflected off the surface. The angle of reflected rays will be the same as the angle of incident rays. Refraction is where the wave bends when it enters a medium where its speed will be different. The amount that this occurs is calculated as the mediums’ index of refraction. This index is calculated by dividing the velocity of light in a vacuum by the velocity of light in the medium.

This interaction with light is useful when looking at the critical angle of refraction. This is where, rather than a percentage of the light being reflected away from the surface or travelling through the surface, the light is instead directed along the plane of the surface itself. This can be useful in several ways. Firstly, once you know how light is going to interact with a surface, you can manipulate the light itself. When an incident ray impacts upon a surface internally in a medium, this can result in what is called total internal reflection. This is where rather than the light passing through a medium, it is instead reflected within it.

C:\Users\chris\AppData\Local\Microsoft\Windows\INetCache\Content.Word\TIR_in_PMMA.JPG

This effect gives rise to uses like optical cabling where light can be used to transmit data. However, this technique also has uses when it comes to lens design and use.

Lenses

Using the basic understandings about light above is useful when understanding lenses. The main idea of a camera lens is to focus the light onto a film plane or digital sensor, which results in the image being generated. This is done both with refraction of the various lenses to angle the light and often also includes reflection after the lens to allow photographers to see what is being shot.

Because the angles of internal reflection can be carefully created by design of the lens itself, you will often find multiple types of lenses being used to channel the light in this way. At the most basic level, you have a lens which is a flat surface, as well as concave and convex lenses. A convex lens is one with an outwards curved surface and as such is thicker in the middle. When light passes through this, the light rays are converged to a single point (called the focal point of the lens) and the distance the light travels from the middle of the lens to this point is called the focal length. A concave lens is essentially the opposite of this where the outer surface curves inwards, resulting in the lens being smaller in the middle. When the light rays pass through this type of lens they diverge. For this lens the focal point is where the light appears to be coming from if you observe the diverging rays and the focal length is again the distance from this point to the middle of the lens.

lens elements diagram

(Lens Elements, n.d.)

Although the main aim of the lens is to accurately represent the image that is being shot, depending on the amount of refraction taking place can result in errors once the light has been redirected onto the film or sensor. These errors include image blurring, reduced contrast, misalignment of colours as well as uneven, radially decreasing image brightness or distortion.

These issues occur when points in the image do not translate back onto single points after passing through a complex lens element. When a lens is referred to as having lower optical quality than another lens, this is when they are more prone to some combination of the above artefacts. Some of these lens artefacts may not be as objectionable as others, depending on the subject. As this is determined by the quality of the lens production, obtaining a lens that does not have these artefacts because of a poorer quality manufacture tend to cost significantly more to purchase.

For example, this is seen in the cost of purchasing larger diameter lenses. Along with some of the details below, simply having a larger diameter lens can be useful in allowing more light to travel to the sensor and prevent a decrease of light around the edges of the image (vignetting mentioned above) which can also help with the overall sharpness of the image as it is transposed more accurately. However, for images this is not the whole story as you do have to consider either the film or the sensor the light is being directed down onto. The smaller that surface, the greater chance of detail being lost or the image suffering from one of the other above errors.

Aperture

The Aperture is used to change the amount of light entering a camera. It is described with the term “f/stops” which have an f number. Small apertures (for example f/22) only let through a small amount of light, whereas large apertures (for example f/4) let through a lot of light. It is using f/stops that a photographer can control the amount of light moving through the lens to the sensor or film. The f/stop is calculated by dividing the focal length of the lens by the diameter of the aperture. F-stops increase and decrease (inverse-) geometrically in powers of the square root of two because when the aperture diameter increases by the square root of two, the size of the area of the aperture is doubled. This allows for an easy to remember scale of each change of f/stop either halving or doubling the amount of light entering the camera. Changing the aperture of a camera also changes the amount of the image that is in focus – this amount is called the depth of field. Large apertures give shallow depths of field, whereas small apertures give deep depths of field (Ray, S.(n.d.)).

Focal Length

Another key aspect of lens design is focal length. Wide angle lenses have a short focal length to allow for a wide angle of view. Telephoto lenses have longer focal lengths with a narrow angle of view. Outside of this, you can have lenses which are classed as prime lenses. Unlike zoom lenses which have a range of focal lengths to allow the photographer to adjust the focal length, a prime lens has a fixed focal length only (Ray, S.(n.d.)) .

Exposure Value

Exposure value (EV) is a number that represents a combination of a camera’s shutter speed and f-number, such that all combinations that yield the same exposure have the same EV. Its intent was to simplify choosing among equivalent camera exposure settings by replacing combinations of shutter speed and f-number with a single number. Although all camera settings with the same EV nominally give the same exposure, they do not necessarily give the same picture. The f-number determines the depth of field, and the shutter speed determines the amount of motion blur.

Essentially, a user estimates the general brightness of a scene, for example a light level of 12 is a daylight scene under heavy clouds with no shadows. This LV value then gives you a range of available F-numbers and shutter speeds that will give you a shot with the same overall exposure, although as noted above might produce a different effect on the shot.

http://easy-exposure.com/wp-content/uploads/2012/06/light-metering-033.jpg

Using the chart for reference, photographers can create a variety of shots. For example, with a light value of 9 (light just before sunrise), you could have a 1/500s shutter with f/1 aperture or a 1 second shutter with a f/22 aperture. Both of these would result in the same exposure value, however, the 1 second and f/22 aperture would result in a shot with significant amounts of motion blur and a deep depth of field, and then 1/500s with f/1 aperture would result in a shot with little to no motion blur at all (outside of very fast-moving objects) and a very shallow depth of field.

Filters

There are several common filters which can be used to change the light that impacts onto the sensor. Some cameras (DSLRs) can have these filters be a screw-on which attaches to the front of the lens, whereas video cameras can often hold these filters within the lens system until they are needed.

Lens Filter Photography Type Purpose
UV/Clear/Haze Filter Any Protects the front element of a lens from dust, dirt, moisture and potential scratches. High quality UV filters can be permanently mounted on lenses with a minimum impact on image quality.
Polarizing Filter Any Filters out polarized light, dramatically reducing reflections, enhancing colours and increasing contrast. Can be used for any type of photography. Polarizing filters are typically circular, allowing for easy control of the effect of polarization.
Neutral Density (ND) Filter Landscape, Flash Photography Reduces the amount of light entering the lens, thus decreasing camera shutter speed. Useful for situations where motion blur needs to be created (rivers, waterfalls, moving people) or large apertures must be used with flash to avoid overexposure.

Mansurov (2017)

Like how you can obtain many different types of lenses, there is a huge range of available filters outside of the more common examples given above, as well as various strength grades of a single type to allow for a significant amount of customisation for the user.

References

Light. (2017). Britannica Online Academic Edition, Encyclopædia Britannica, Inc.

tjconnects (2016) The Wavelength, Frequency, and Amplitude of Light Waves [online]

Available at: http://www.tjconnects.com/the-wavelength-frequency-and-amplitude-of-light-waves/

[Accessed: 17th October 2017]

Pedrotti, F. (1993). Introduction to optics (2nd ed.). Prentice Hall.

Ray, S. (n.d.). Camera lenses-Chapter 10. In The Manual of Photography (pp. 175-197).

Bruno, T. Svoronos, P. (2005) CRC Handbook of Fundamental Spectroscopic Correlation Charts. CRC Press.

Sai2020 (2008). Total internal reflection in a bar of PMMA [online image]

Available at: https://commons.wikimedia.org/wiki/File:TIR_in_PMMA.jpg

[Accessed: 1st November 2017]

[Lens Elements] [image online] (n.d).

Available at: http://www.cambridgeincolour.com/tutorials/camera-lenses.htm

[Accessed: 1st November 2017]

Mansurov, N (2017) Lens Filters Explained [online blog]

Available at: https://photographylife.com/lens-filters-explained

[Accessed: 6st December 2017]

Chris

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