CSP Review - Lecture notes 1-2 PDF

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Summary

This review includes the first two lectures of the module and discusses themes like 'exposure' and 'depth of field' in detail....

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Crime Scene Photography Review

What is photography?  The core idea of photography is the process of light forming an image by means of a lens.

 The type of camera we will be focusing on is a DSLR camera.

 DSLR means “Digital Single Lens Reflex”:

o

Digital means that the camera operates with a fixed, digital sensor;

o

Single lens means the camera uses the same lens for framing, focusing and taking the photograph;

o

Reflex means a system where a mirror splits or directs the incoming light towards the optical viewfinder.

 In simple words, a DSLR is a digital camera with removable lens that uses a mirror mechanism to either reflect light from a camera lens to an optical viewfinder (observing through the lens) or let light fully pass onto the image sensor by moving the mirror out of the way (picture is taken).

 What does the DSLR cameras consist of? This type of camera

has eight elements:

o

Lens

The lens is one of the most vital parts of a camera – as soon as the light enters through the lens, the photo process begins.

o

Reflex Mirror

This mirror has the function of directing the light that comes through the lens to the pentaprism and from the pentaprism to the eyepiece in order for us to be able to see the image.

o

Shutter

The shutter is the device that allows light to pass for a determined period, exposing a photosensitive digital sensor to light in order to capture a permanent image of a scene.

o

Image Sensor

The image sensor of a camera has the function of converting the optical image (what we see) to an electronic signal, which is then sent to our memory card. There are two main types of image sensors that are used in digital cameras: CMOS and CCD. Focusing on the CCD image sensor – this is a charged-coupled device that contains an array of coupled capacitors.

o

Focusing Screen

The focusing screen is a flat translucent material that allows the user of a camera to preview the framed image in a viewfinder.

o

Pentaprism

After the reflex mirror directs the light upwards, the light hits the pentaprism, which in turn, directs the light into the optical viewfinder.

o

Eyepiece/Viewfinder

 How does the DSLR camera work? In a DSLR camera, the

photographer gets a more accurate view of the scene they’re shooting. What they see in the viewfinder is exactly what the lens sees. This is possible due to the mirror mechanism that happens inside of the body of the camera.

 Everything begins when the light from the scene you are attempting

to capture passes through the lens into a reflex mirror. This reflex mirror sits at a 45-degree angle inside the camera chamber, which then forwards the light vertically to an optical element called a pentaprism. The pentaprism then converts the vertical light to

horizontal by redirecting the light through two separate mirrors, right into the viewfinder.

 This is the mechanism that allows you to see the view of the scene

you want to capture through your eyepiece. So, what happens when you decide to take the photo? When you take a picture, the reflex mirror swings upwards, blocking the vertical pathway – in other words, it doesn’t allow the light to go in the direction of the pentaprism. Therefore, the light maintains its horizontal direction, which aims towards the image sensor.

 Then, as the light maintains its horizontal direction, the shutter

device opens up for as long as needed for the image sensor to record the image. After that, the shutter closes and the reflex mirror drops back to the 45-degree angle to continue redirecting the light into the viewfinder.

 The main reason why we cannot see the image when the camera

captures it relies on the fact that the reflex mirror blocks the light from going into the pentaprism. Therefore, the eyepiece is completely dark.

Visible Spectrum

 As we can probably assume now, photography is all about light. Without light, it is impossible to capture something. Because of that, it is important to have a basic notion about the visible spectrum.



The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye – this portion varies between the 400-700 nanometres. This automatically means that below 400nn, we have the Ultraviolet and above 700nn we have Infrared

What is the core of photography?

“What makes photography a strange invention is that its primary raw materials are light and time.”

 As mentioned before, photography has everything to do with light and time – in other words, the way a photo is developed depends on how much light we allow to enter the camera and how long we allow the light to enter the camera.

 The way these two elements are balanced will define the quality of our photos.

Exposure

 Whenever we take a photo, the goal is always to create the perfectly exposed photograph. What does this mean? It means we don’t want the photo to be too dark nor too bright, we want it to be just right. On the other hand, it is not always possible to get a perfectly exposed picture because we may have very bright areas or very dark areas. However, if the subject we are focusing on has a good exposure where we can see its details then that is best overall exposure.

 The purpose of exposure is to achieve a balance between brightness and darkness. How do we achieve this balance?

 Exposure can be affected by three main things:

o Aperture Aperture is the device that controls the amount of light that the lens allows to enter the camera. Similar to our iris.

o Shutter speed The shutter speed determines for how long the light will enter the camera and get to the image sensor.

o ISO (International Standard Organization) ISO is the measure for the sensitivity of the sensor image to the light.

1) Aperture

 The camera lens, as we’ve mentioned before, transmits the light

reflected from the subject and forms an image on the CCD sensor. The lens have an adjustable opening that can change its size electronically – this opening is defined by a set of fine metal blades that overlap each other like petals of a flower. The set of fine metal blades form what we call the aperture flower.

 The aperture flower work like the iris of a human eye - as they

adjust their diameter, it allows more or less light through the lens. These adjustments are called f-stops or f-numbers.

 The f-numbers control the brightness/intensity of the light passing

through the lens and they’re expressed by numbers (f-2.8, f-3.5, f-4, f-5.6, f-8, f-11, f-16 and f-22). These numbers are calculated by dividing the focal length of the lens by the diameter of the opening.

 On a digital camera, between f11 and f16, there’s also f13 and f14 –

these f-numbers are considered third stops, which means that they only increase or decrease a third of a stock of light.

 Note: to go from f-11 to f-16, we should click on the shutter button 3

times.

 What happens when you change the f-numbers? The main rule is

the smaller the number, the larger the opening – therefore, more light is captured by the lens - and the bigger the number, the

smaller the opening – which means, less light is captured by the lens.

 What happens when we change the f-stops by one stop? It depends.

If we go down on the f-number by one stop, the exposure reduces by half – this means, the amount of light that gets captured is cut down in half. On the other hand, if we go up on the f-number by one stop, the exposure doubles – this means, the amount of light that gets captured is doubled.

 Therefore, the higher numbers are small apertures that reduce the

intensity of the light passing through the lens and the lower numbers indicate a larger aperture that allows more light through the lens.

Question: If the correct exposure as indicated by the camera meter is 1/125th second at f‐8, what will be the effect on the exposure if the aperture is changed to f‐11? From f-8 to f-11, the size of the opening gets smaller. This means that less amount of light is going through the lens. With less light, the picture is likely to become underexposed. – confirm.

2) Shutter Speed  The shutter device sits in front of the image sensor and its composed by a series of thin metal blades that open vertically across the focal plane – there’s two parts with one set of blades opening to reveal the sensor and another set of blades to close the shutter and terminate the exposure.  The shutter only opens when the shutter release button is pressed, otherwise it remains closed.

 What we set in the camera is the shutter speed – i.e. the time that the blades take to open and close. This, evidentially, will affect the amount of light that gets captured by the sensor. So, the longer the shutter is open, the higher the volume of light that will strike the camera sensor. Consequently, if the shutter opens and closes very fast, little light will make it to the camera sensor – which will produce a darker picture.

 How are shutter speeds displayed/measured? Shutter speeds, in camera display, are shown as whole numbers: 1, 2, 4, 8, 30, 60, 125, 250, 500 or 1000. However, the number represents a fraction of a second. For example, a shutter speed 60 is actually 1/60th of a second – this means that the shutter could open and close 60 times in 1 second.

 The ideal shutter speeds are 1/30 to 1/60. It is advised to stick to one shutter speed when holding the camera but whenever we use a tripod, any shutter speed can be used.

 If a picture is too dark, it means that the shutter speed is closing too fast – i.e. the shutter speed is set in a high number, therefore it is not allowing light to go through the lens for long enough. On the other hand, if the picture is too bright, the shutter is closing too slowly – i.e there’s too much light going through the lens.

 As the shutter speed is increased by one stop, the amount of light entering the camera is either halved or doubled. From 1/60 to 1/30, the amount of light is doubled; From 1/60 to 1/125, the amount of light is halved.

 How does the shutter speed control motion? Since the shutter speed relies on time, it can also affect the blurriness of our picture. Whenever the shutter speed is set to small numbers, the picture will have motion blur – if we are focusing on a moving target specifically. However, if the shutter speed is set to a higher number, then the picture captures our moving target frozen.

 How does the shutter speed effect exposure? If we take into consideration the natural light of our environment, we can assume that whenever it is bright outside, the shutter speed should be faster (higher numbers) and whenever it is dark outside, the shutter speed should be slower (lower number).

 Remember that if you need more light in your image, one option will be to slow down your shutter speed by selecting a lower number. Conversely, if you wish to reduce the light, select a faster shutter speed.

3) ISO

 ISO stands for International Standards Organization. This setting controls the sensitivity of the image sensor to light – i.e. we can adjust the image sensor that sits in the back of the camera. Per norm, it should be set between 100-400 ISO for average everyday use. Standard setting in classes is 200.  The lower the number of the ISO setting, the less sensitive the camera sensor will be to light. So an ISO 100 will result in an image less bright than ISO 400.  The trick is that lower ISO numbers determine the quality of the image: THE LOWER THE NUMBER, THE BETTER THE QUALITY.

 What are the impacts of the ISO on the quality of the image? The lower the ISO, the better the quality of the picture (but the darker it becomes as well). The ISO also helps with visible noise on the image. The higher the ISO, the higher the noise of a picture and therefore, the lower the sharpness and quality of the picture there will be. High ISO settings may be chosen when shooting in low light conditions. What affect will this have on the image quality? Exposure triangule

Lecture 2 - Depth of Field

 When taking photographs for evidential purposes, it is important to produce photographs that not only are well exposed but also are in sharp focus to show as much detail as possible. An image that is not sharp or is blurred does not reflect well on the ability and

professionalism if the photographer, nor does it look good in court where clear, well-presented pictures are expected.  To ensure that the image is in sharp focus the CSI must have a basic understanding of depth of field (DOF) and the factors that control DOF.  As we mentioned previously, the light reflecting from the subject passes through the lens and forms an image on the sensor. Most DSLR cameras have lenses that can be focussed manually or automatically.  However, in our course, we will learn how to work with a camera in manual mode. The reasoning behind this is that when we have everything on manual, we have control of everything! This doesn’t mean that we won’t get a good picture with the automatic mode but it is worth to bear in mind that whenever we take a picture in auto mode, the camera isn’t aware of what it is capturing – the only thing that it is assessing is the lightning of the environment whereas with manual mode, we are in control of everything.  So, what is depth of field? DOF relates to everything excluding the point of focus that appears acceptably sharp within the image. It is defined as the distance between the nearest and the furthest points of sharp focus. Just like our eyes, a camera can only focus on one point. It is only possible to have one point of focus at any time. Everything else around that POF can either be blurred or acceptably sharp – and that’s when the depth of field comes in.

 Why is DOF important? DOF can play a large factor in how your image will be render and can change the meaning and intention of the image. You can choose to isolate a subject from its background by having a narrow DOF or alternatively can make sure that everything from the foreground to the background will be in focus – or at least, acceptably sharp.

 As we mentioned, as we look through the lenses, only one part of the image will be physically sharp because everything around it will be out of focus. How do we change that?

o Per norm, you don’t see the depth of field changing in the screen or in the viewfinder of the camera. You can only see it once you transfer the images to a big screen. However, some

cameras have the capability to show the depth of field through the “depth of field preview”. What the DOF preview does is that it opens the aperture as wide as it can, and it will show you exactly what you will see in the final picture.  There’re two types of DOF – as mentioned before – deep depth of field and shallow depth of field: o Shallow DOF happens when there’s a short range of the image in focus. The point of focus is usually focused and sharp (well defined) but then, everything around it is out of focus. Photography wise, a shallow depth of field isn’t something we should run away from but from a forensic photography perspective, it is not ideal to have anything blurred in a picture that will further down the road be selected as an exhibit for court. Therefore, we should always aim for what we call a deep depth of field when photographing a crime scene o Deep DOF happens when the focal point is very sharp and detailed and the rest that surrounds it is acceptably sharp but never as sharp as our point of focus.

o What are the factors that affect DOF? There are three factors that affect the DOF: Aperture, Camera and Focal Length.

1) Aperture  Aperture is one of the main considerations to have in photography, depending on what we are looking for. This setting not only affects the exposure of a picture, but it also affects the DOF.  Aperture is an adjustable hole that sets electronically and determines the amount of light that is able to enter the lenses. With this feature, we are able to make sure a picture is well exposed – specially the area of most interest to the photographer.

 Exposure triangle: While observing the exposure triangle, we have three considerations. First one is the ISO – which 90% of the time will stay at 200; Next one is aperture – we select an aperture for whatever depth of field we want to work with; Then, everything else will come down to shutter speed.

 How does the aperture affect the DOF? As we know, the smaller the f number, the larger the hole. When it comes to DOF, we have two scales: Shallow and Deep – and this can be defined according to the aperture we choose.

o If we want only the point of focus to be sharp, we will use a really small f-number; but if we want the POF and parts of the image to be acceptably sharp as I can, we can select a bigger f-number. In summary, when we use big f-numbers, we get a really deep depth of field and everything will be focused and sharp. However, when changing the aperture, the point of focus will remain sharp, but the depth of field will continue to change. o Note: F-11 or above for a really deep depth of field o As we’ve mentioned before, when we hand-hold a camera, we should set our shutter speed over 1/30th or 1/60th of a second. Whenever we start picking a really deep depth of field, consequently selecting a big aperture, that’s going to imply that the camera needs more time for the light to get into the lens and hit the sensor – thus, the shutter speed needs to be smaller/slower. o Note: this is one of the reasons why we use manual settings – because with manual settings we have control over everything. The problem with auto mode is that the camera will not adjust the depth of field properly because the camera will only pick up on the environment and will set everything according to that and NOT according to the object you want to capture. o Note: Acceptably sharp – you cannot get more than one area really sharp, just good enough to see the details sightly.  Rule of thirds: When considering DOF, the rule of thirds should be used – this rule has to do with where you’re going to focus your picture. According to this, your point of focus should be situated into a third of the way into your image. It can either be a third from top to bottom or left to right.  For example: We are at an outdoor scene, we have a body on the street, further down the murder weapon and then, a pool of blood. If I am interested in capturing these three elements in one shot, I will have to use a really deep depth of field. Once we do that, we will see that a third before the POF and 2/3 of the POF will be acceptably sharp.

 This rule is very useful if we want all the image – front to back – to be acceptably sharp.  What’s the effect of DOF on close up photography? The point of focus will remain constant but the DOF is changed even with close up photography. o Camera-subject distance: The position of the camera to the subject will have a massive impact overall on the DOF. It is important to consider where we are in relation to the subject that you want to capture. If an individual is really close to the subject, it will have little control over the DOF. The closer I am to the subject, the less control we have over the DOF whereas if we are...