Fundamentals of Light and Photography

May 30, 2013  •  Leave a Comment

 

Photos: Greek for light

Graphos: Greek for written

Writing with light: Also known as photography…

According to Wikipedia the definition of photography is “the art, science and practice of creating durable images by recording light or other electromagnetic radiation”. Perhaps not the most inspiring resource for a definition, but I consider it adequate, because quite simply without light there would be no photo. In addition the three words “art, science and practise” subtly suggests that there is a large chasm between good and bad photography.  Now, if you wanted a more creative description of what defines “good” photography then a professional would probably respond along the lines of “The ability to see the light for its quality, direction, how it interacts with the subject and then ultimately capturing a creative composition of the scene”.  To create beautiful photos the professional has (or should have) mastered the art and science of photography by understanding light through lots of practise. In some sense a professional needs to have the ability to visualise mentally how an image will appear in order to transcend what is been seen by the eye. Anyone who has ever taken a photo knows that what we see with our eyes and what gets reproduced by the camera is not always the same thing. When we are starting out this can be quite frustrating. However, as our understanding increases with respect to how light interacts with our camera sensor we begin to realise that this phenomenon gives us the ability to create beautiful and artistic images.

So, no matter whom you ask or how it gets described the common denominator is that photography is about light. To progress as a photographer it is important to have an understanding of light and how the settings on our camera are influencing the final image. Every setting we adjust on our camera when preparing to take a photo is an instruction to control the light from the moment it enters the lens until the last photon ends its journey on the camera sensor. The lens aperture controls the amount of light passing through the lens (as a consequence of light properties the aperture also controls depth of field). Shutter speed controls how long the shutter curtains remain open and thus how long the sensor gets exposed to the light (shutter speed will thus control motion blur). ISO setting determines how sensitive the camera sensor is to the light. The higher the ISO the more sensitive the sensor will be to light but it will also cause grain in the final image to increase. All three of these setting will influence exposure but from a creative point of view it’s useful to think of aperture as controlling depth of field, shutter speed controlling motion blur and ISO controlling light sensitivity.

A natural progression after understanding how camera settings influence the final image is to question why it happens (well, for me it was the natural progression but I could well be straying in to camera geek territory now). I wanted to know why a small aperture gives a small depth of field. Why does a small aperture not “crop” the image hitting the sensor? Why does a lens have an optimal aperture for sharpness? Why are expensive lenses expensive? Why does the drop-off in light from a source follow an inverse square law? Why do zoom lenses have large diameters?

To begin to answer these questions it is necessary to have a basic understanding of light. Although when it comes to light there is very little about it that is basic! Broadly speaking light is a form of electro-magnetic radiation consisting of photons that travel in a wave type motion. Photons have no mass, move at the speed of light (obviously) and are able to travel through a vacuum (unlike sound). Next time you look up at the stars in the night sky spare a thought for the photons that are ending their journey in your retina after a few million years of travelling through space (the energy of the photon is used to stimulate a photochemical reaction which enables vision).

Let’s begin by considering a single free-standing point representing a light source. Light will be emitted from the point source in all directions. In an attempt to get a crude understanding of this phenomenon it’s analogous to the waves generated when a pebble is thrown into a pond. The waves travel away from the source in a circular manner with an ever increasing diameter. However, for a free-standing light source this would be a 3-dimensional event. To represent this graphically the image below shows a cross-section of the point light source (red point) and the light waves travelling away from the source (think of the layers in an onion when cut through the middle).

For a given “sphere” of light travelling away from the point source the number of photons within the sphere will remain constant. However, as the light sphere travels away from the source it will increase in surface area (in a similar way the ripples in a pond increase as they travel from the source). Given that the number of photons within the sphere must remain constant the photons per unit area will decrease as the sphere’s surface area increases. The surface area of a sphere is given by the equation: Area=4.pi.r2 (r=radius of sphere). This requires that the energy intensity of the light (how bright it appears to our eyes) decreases proportionally to 1/r2 in order to ensure the conservation of energy. This phenomenon is also commonly referred to as the inverse square law of light and can be a useful property to utilise in flash photography. The graph below shows how light intensity decreases with distance from the source. 

With reference to the above graph, consider the scenario where you correctly expose for your subject who is standing 2 meters in front of a background wall. The light source (camera flash) is at 2 meters in front of your subject. So in summary the camera flash is at distance 0 in the above graph, the subject is at distance 2 and the background wall at distance 4. Then the “light intensity” on your subject will be 0.02 compared to 0.005 on the background (i.e. your background will be two stops darker). Now let’s say you move your flash a further 4 meters away from your subject (total of 6 meters) and correctly expose for your subject. What you will now notice is that even though the subject is still 2 meters in front of the wall the exposure will be approximately the same for both the subject and the background wall; 0.0025 & 0.002 light intensity respectively. This principal of light is very useful to understand when you want to either lighten or darken the background relative to a stationery subject. By just moving the flash closer or further from your subject you can maintain the same exposure on the subject but have vastly different exposure on the background.

Understanding that the number of photons within a sphere of light must remain constant will help in explaining why zoom lenses have relatively large diameters. We have all experienced a light source appearing to decrease in size as it’s moved further away from our eye position until at some distance it will disappear altogether from sight. The reason for this is that as we move further away from a light source we subtend a smaller angle of the emitted photons. I know when I first heard this explanation it did not make much sense but basically what is happening is that fewer photons are hitting the eye. The fewer photons hitting the eye from a point light source will make the object appear smaller. Using the diagram below to explain this is probably simplest.  Once again the red dot is the light source and the spheres represent the waves of emitted photons. The first sphere of light will contain a large number of photons per unit surface area due to its close proximity to the source. The purple area highlighted on this first sphere will contain, for example, 100 photons. Due to the conservation of energy the second sphere will contain fewer photons per unit area. Thus the purple area highlighted on the second sphere will have to increase in size in order to contain 100 photons. Likewise for the third sphere the purple area would need to increase in size once again. In fact the size of the patch will be increasing in size exponentially. However, our eyes are a fixed diameter and therefore as we move further away from a light source we receive less and less photons. Consequently the object appears to get smaller.

The same principal applies to zoom lenses. In order to magnify an object more photons are required to enter the lens. To achieve this magnification the lens diameter has to increase (incidentally the length of the lens also needs to increase to allow re-focussing of the widely distributed photons onto the sensor). So, for example, you can be four times further away from an object with a 200mm lens in comparison to a 50mm lens and still capture the same field of view. However, the lens opening (aperture) of the 200mm lens has to increase in order to capture the same amount of light. This is where the beauty of the f-number (also called focal ratio or f-stop) designation for aperture settings becomes so useful. Newcomers to photography are sometimes confused about the f-number designation but, once understood, it really is very simple. The f-number is the ratio of the lens focal length to the size of the aperture opening which explains why the f-number gets smaller for larger aperture diameter size. So, for example, the aperture size of a 200mm and 50mm lens at an f-number of f4 will be as follows:

Aperture at f4 for 200mm lens: 50mm

Aperture at f4 for 50mm lens: 12.5mm

The beauty is that for any lens the exposure will be the same for an equivalent f-number but the aperture size will be different. Compare an 85mm f/1.4 and 85mm f/1.8 prime lens and the size and weight is immediately apparent. The price of the 85mm f/1.4 is also about 3 times more expensive! Is this justified? Well, by doing the maths you will notice that the f/1.4 has approximately 400mm2 more area of lens glass (the f/1.8 lens has approximately 600mm2 area of glass in total). Given that the greater area of lens glass requires higher quality to ensure imperfections are avoided and the demand for such a professional lens is relatively low the price differential starts to make sense.

This brings me to the next question. Why does the field of view not get smaller as the aperture is reduced in size (otherwise known as stopping down)? To answer this it needs to be understood that the purpose of a lens is to focus light and the aperture controls the “intensity” of light to focus and not the angular field of view. As previously mentioned light is emitted in all directions from a point light source. So a lens focussed on a point light source will focus all the incoming light waves onto a point on the sensor. As the aperture is stopped down the amount of incoming light waves from the point source will decrease and this is why the exposure will change. Consider the images shown below of a single point of light emitting photons. These photons pass through the lens and aperture (the image only shows the light waves that will hit the camera lens but in reality it would be emitting in all directions). Also remember that this image is only a 2D cross-sectional representation of what in reality is a 3D phenomenon. As the aperture is stopped down it is apparent that more light waves will be “stopped” from passing through to the camera sensor. From the image it can be seen that less of the lens is actually being used to focus the light. 

 

A positive consequence of utilising "less" of the lens is that the image will become sharper as there is less possibility of a lens imperfection “disrupting” the light path. However, at some point, a small aperture will begin to soften the image as diffraction (which is a property of light) “scatters” the light. It is interesting to note that there is not much difference in image sharpness between a medium and high quality lens towards the lower aperture values. Often a lens is referred to as having a sweet spot which is simply the aperture value (typically between f/5.6 to f11) that produces the sharpest image.

On the other hand as the aperture is increased a greater area of the lens is being used and lens quality (or lack of lens imperfections) becomes critical in determining the image sharpness. You will often hear of lens aberrations limiting sharpness at maximum aperture. This is simply a fancy way of saying that a very small imperfection in the lens glass prevents the light waves (from a point light source) perfectly converging on the sensor.

Anyway, besides for aperture being a very important setting to control exposure it also influences the depth of field in a photograph. The definition of depth of field is the distance between the nearest and furthest points in a photograph that appear to your eye to be in focus. In fact a lens can only perfectly focus at one single distance at a time for a given setting. The areas immediately in front and behind this focal point will be slightly out of focus when viewed on the photograph. Instead of forming a perfect point on the sensor these out of focus points will form a small circular disk or optical spot of light due to the fact that the light rays are not perfectly converging (you can visually see this principle by focussing a beam of sunlight through a magnifying glass). This “spot” size increases for areas further away from the focal distance and that is why out of focus points in a photo look like circular disks, also known as bokeh. The “circle of confusion” refers to those out of focus spots that are too small for the eye to resolve and are perceived as points (and thus appear sharp). An example of light rays passing through a small aperture is shown below. The focus point is point B and all the other points on the object are effectively out of focus. However, because the aperture is very small in this instance only a very narrow cone of light passes through the lens from each point. Effectively, the light rays passing through the aperture in this instance are quite close to being parallel (otherwise known as collimated). Thus, although the out of focus point don’t converge to a point on the sensor the “spot” size from the narrow cone of light is very small on the sensor. As mentioned, our eyes are unable to discern this small spot and thus it effectively appears as a point and appears in focus.

As the aperture increases a greater angular degree of the light cone from each point will pass through the lens and onto the sensor. The incoming light from the out of focus points is consequently a lot less collimated. The angular degree of the cones of light from a point has increased and this in turn means that the size of the light spot on the sensor will increase in size. The size of the spot becomes visible to the human eye and appears out of focus. So in summary the more light that hits the sensor from the out of focus areas will increase the blurriness of that point due to size of the “light spot” increasing. 

It needs to be remembered that the above images are in effect trying to demonstrate what is in reality a 3D phenomenon. So if you were to view the sensor “front on” for the two different aperture sizes the light spots can be demonstrated resembled by the following diagrams.

 

I original started writing this article to help me understand some of the fundamental principles of light and photography. As I spend a large proportion of my time using a camera I wanted to really understand the physics behind it all. I have done my best to ensure the explanations are as accurate as possible – even to the extent of speaking to a Professor of Nuclear Physics at Oxford University. Initially I did not really think understanding the principles would help to improve my skills as a photographer. However, having now researched some of the topics I do find myself utilising some of the principals to my advantage. I suppose most importantly it also helps justifying why spending over £1000 on a 85mm f/1.4 prime lens is well worth the money!

 

 

 


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