Guide to Photography and Photogrammetry

Written as a guide for archaeology students

Contents

Go straight to:

  1. Introduction
  2. Photography Basics
  3. Balancing ISO, Shutter Speed and Aperture
  4. Using a Tripod
  5. Lighting
  6. Lens Choice and Perspective
  7. Post-Processing Basics
  8. Summary of Photography Tips
  9. Photogrammetry
  10. Further Information

Introduction

This guide gives an overview of basic photography for technical use, such as record keeping in archaeology. Topics include the basic functions of cameras, and how to capture good quality images through the use of lighting, perspective, composition and post-processing. There is also a section of photogrammetry, a increasingly common method of producing highly-detailed 3D models from 2D images. Most of the general tips in this guide are relevant to producing photogrammetry models, but the dedicated section provides more specific information.

Many of these tips will be useful to photographers regardless of what equipment they have available, but the guide is focused on using professional DSLR or mirrorless cameras. The quality of images captured with a professional-style camera has the potential to significantly surpass that of images taken with a compact or phone camera, but only if the user understands how to utilise the additional controls that such cameras offer.

Why Use a Professional-Style Camera?

DSLR cameras and high-end mirrorless cameras differ from other camera types by their features that allow the user to manually control how the image is captured. Such cameras are designed with the expectation that the user will adjust these controls before the majority of images are taken. Although most cameras do come with an ‘automatic’ mode, where the camera’s computer makes an approximation of the best settings to use, having a basic understanding of how to manually control a small number of the camera’s settings can help the user to capture significantly better images. This is particularly important when taking images for record keeping: an image that is sharp, in focus, and which properly reflects human perspective and scale, can be relied upon to convey the scene accurately.

Photography Basics

Any professional or high-end consumer camera will share the same three basic manual controls most important to any photograph: ISO, shutter speed, and aperture.

ISO 

The sensitivity of the camera’s sensor when the image is taken. 

A low sensitivity (usually an ISO setting of 100, the camera’s base sensitivity) ensures that the image will be captured with greatest possible quality with that camera, all else being equal. A higher sensitivity will result in an image with more noise, giving the appearance of a grainy image lacking in detail. ISO values up to around 800 will usually produce an image with small amounts of noise which are difficult to perceive at first glance; ISO values above this amount will typically cause a noticeable degradation in the image’s quality.

Shutter Speed

The amount of time, in seconds, for which the camera shutter exposes the camera’s sensor to light

When photographing a fast-moving subject, it may be necessary to have a very fast shutter speed to capture the scene without any blurring. Additionally, when light levels are low within the scene, the camera needs to gather more light over a longer duration to capture a properly exposed image. Shutter speeds range from very slow (longer than 30s), to very fast (shorter than 1/8000s).

Aperture 

The size of the opening within the lens through which light passes before hitting the camera’s sensor when an image is taken, which directly controls the intensity of light entering the camera. 

The aperture value, also known as f-stop is always stated as a ratio to the focal length of the lens: an aperture of f/2 means that the aperture opening is 1/2 the focal length (‘f’ always replaces the numerator in the aperture value ratio). Low aperture values therefore let more light into the camera; high aperture values let less light into the camera. Professional lenses typically have maximum aperture sizes ranging from f/1.4 to f/2.8, whereas most consumer lenses have a maximum aperture of around f/4. Most lenses have a minimum aperture size of around f/22 (for technical reasons, using apertures smaller than this can significantly degrade image quality).

Changing the aperture value has a significant effect on an image’s properties: it defines how much of the image is in-focus. This explains the reason for wanting control over the aperture size, rather than just letting the maximum amount of light hit the sensor (if all else is equal, this would almost always be desirable as it would produce either a sharper image by allowing a faster shutter speed, or produce less image noise by allowing a lower ISO setting, or any combination of these two benefits). Higher aperture values (a smaller physical aperture) are used when a greater range of the scene is required to be in-focus. Smaller aperture values (a larger physical aperture) are used when sharpness is not required throughout the scene, resulting in a blurred background & foreground around the point of focus, which is sometimes desirable for artistic reasons. Depth-of-field is the term used to describe the range in-front and behind the image’s point of focus within which the image appears sharp.

Balancing ISO, Shutter Speed and Aperture

The most important thing to understand in photography is how the three main control elements – ISO, shutter speed, and aperture – interact with each other, and how they must be balanced to obtain a high quality or suitable image. For archaeology, the aim is usually to get realistic, in-focus images with plenty of detail throughout the scene. The most important setting of the above to have complete control over is aperture: for the reasons mentioned above, choosing the correct value for this setting will allow you to capture a high level of sharpness and detail throughout the image.

The major drawback of using a smaller aperture to ensure greater sharpness is that less light will be hitting the camera’s sensor. In order to achieve a good exposure it is necessary to compensate by either increasing the camera’s sensitivity to light through the ISO setting, or by using a slower shutter speed to allow more light to enter the camera over a longer duration. Both of these options have drawbacks: increasing ISO will introduce more noise into the image, and using a slower shutter speed will increase the chance of blurring due to camera movements while the shutter is open. 

However, both of these drawbacks can be overcome by stabilising the camera, which means a much longer shutter-speed can be used (as there is now no chance of camera-shake), and, in turn, the ISO can be set to its lowest available setting, resulting in an image with very low noise. Many cameras and lenses have in-built stabilisation which helps to overcome these problems, but the best way to achieve good results is by using a tripod, which the next section deals with in detail.

Using a Tripod for Optimal Image Quality

When taking images of static subjects and when a fast shutter speed is not required, one way around the problem of having to compensate for the lower amount of light from using a higher aperture value is to use a tripod. By using a tripod to which the camera is fixed, we are free to use any length of shutter speed as there is no risk of blurring from movements that occur when the camera is held while the shutter is open. As a result, a low ISO setting can be used to ensure optimal image quality.

All DSLR and high-end cameras will have a shooting mode called Aperture Priority, often abbreviated ‘A’ or ‘Av’. This mode allows the user to control the aperture value, whilst automatically adjusting the shutter speed required to achieve a good exposure. This mode is recommended when using a tripod, as a longer shutter speed length is unlikely to negatively affect the image.

In order to maximise the benefits of using a tripod, it is important to avoid introducing camera movements that may occur when the shutter release button is physically pressed. Using a remote-controlled release or cable release ensures that the camera is not disturbed when the image is taken. Alternatively, a timer can be set within any camera to delay the image capture until any internal camera  movements have subsided.

Note: When using a tripod, it is important to disable any in-built stabilisation within the camera or its lens. The stabilisation mechanism is usually built to expect movement and may therefore negatively affect image sharpness when the camera is stable.

Other Benefits of Using a Tripod

Using a tripod has other benefits, too. Because an image typically takes longer to set up using a tripod, it encourages proper framing of the subject, more accurate focusing, and making sure that the image is level, which is particularly important to accurately record the scene (most cameras come with a built-in digital spirit level; alternatively, spirit levels that attach to the camera’s flash mount are easily available).

When a Tripod is not Available

When a tripod is unavailable or impractical to use, there are other ways to ensure that images are captured with sufficient sharpness and low noise.

The camera’s ISO setting can be adjusted to ensure that the shutter is not open for long enough for camera-movements to appear on the image. Raising the ISO has the effect of allowing a shorter shutter-speed whilst allowing enough light to enter the lens to get a good quality image. However, as discussed in the section on ISO above, this will have the effect of introducing more noise into the image; on most cameras, this noise will begin to appear most aggressively after an ISO of around 800.

Other tricks to ensure sharp images when a tripod is not available or impractical to use include using your other hand to stabilise lens, taking the photo while crouching, leaning your body or the camera itself against something, and using any automatic stabilisation within the camera or lens itself.

Lighting

Understanding how lighting affects an image is one of the most important aspects of photography. Any object or scene can be dramatically changed by how light sources vary in intensity, direction, and colour. For outdoor photography, the light source is the sun, which can introduce many difficult problems for the photographer in capturing the scene accurately. 

Problems with Direct Sunlight

Direct sunlight usually has the effect of highlighting edges and generally giving greater contrast, which often results in a more attractive scene at first glance. The problem is that, when we look at a scene naturally – not through a photograph – our brains are doing a significant amount of adjustment that the camera cannot do: shadows, for example, are made much brighter;  a camera sensor does not do this, so shadows often appear much darker in an image than they do when we are physically looking at a scene. This greater contrast between light and dark areas can result in an image that lacks detail and is very unrepresentative of the real scene, even when adjusted using software.

Photography and Photogrammetry Guide - Dappled Light - Jordan Mansfield
Dappled light makes it difficult to accurately interpret section textures and colours

This problem is perhaps best illustrated by the characteristic dappled light that is created by direct sunlight hitting a tree or other foliage. The resulting shadows can severely affect the accuracy of the surface, and make it very difficult to interpret texture and edges. Our brains are very good at compensating for such situations when physically there, but cannot do this when looking at an image on a screen. The image on the right demonstrates this effect.

Direct light is not always negative: when an object or surface’s texture is important to capture, the sun – when at a suitable angle – can help to highlight small details that may be difficult to otherwise see. However, this should usually be used with caution for the reasons above: any shadows created can look very different to how we would see the scene in reality.

Using Diffuse Light

The solution to the problem of direct sunlight is to ensure the light is more even – or diffuse –  throughout the scene. Cloudy weather is excellent for this purpose, and depending on where you are this may be the situation most of the time. Clouds have the effect of diffusing the sun’s direct light and essentially making the light source much bigger, so that it hits the scene from multiple angles rather than just one, which ensures there are fewer areas in shadow.

It may not always be possible to wait for cloud cover, so you can also remove shadows by, for example, putting up sheeting or getting people to stand in the sun’s path – unfortunately this method will be very difficult for most of the middle of the day when the sun is directly overhead.

Photography and Photogrammetry Guide - Shadows and Diffuse Light 1 - Jordan Mansfield
Direct light casting direct shadows
Photography and Photogrammetry Guide - Shadows and Diffuse Light 2 - Jordan Mansfield
Diffuse lighting, giving an evenly-lit, high quality image

The two images above show the same area with direct light and diffuse light. The left image (direct light) perhaps looks more striking as in image, but the details in the pit are incredibly difficult to interpret because of the shadows and high contrast areas. The right-hand image provides an accurate view of the pit, where the colours and layers of sediment are easy to see, which is much more useful for providing a record of the site.

The section on photogrammetry later in this guide will explain the even greater importance of using diffuse light when producing 3D models.

Lens Choice and Perspective

Another area of photography that is useful to be aware of is how the lens that you are using affects the properties of images. Generally, lenses fall into one of three categories: wide-angle, standard, and telephoto. Standard focal lengths (typically around 35-55mm on a DSLR-style camera) are considered to roughly correspond to how we see in reality, thus creating normal-looking images. Lenses with focal lengths higher than this, known as telephoto, create a zoomed-in looking image, which, again, look quite normal to us, as in reality the brain effectively zooms in on objects or portions of scenes all the time, filtering out the surrounding detail even though it is still within the field of view.

As a photographer, the most important element of perspective to be aware of is when using wide-angle lenses. Because these lenses can fit more into the scene that we would naturally see*, they can distort the subject drastically. Although this is often used for artistic effect, it can easily give a false representation of a scene, which can be a problem when accuracy is important. Often, however, it is very difficult to use a longer focal length – particularly in archaeology where photos typically must be taken in a trench or other confined space – so the best approach is to minimise the distortion as much as possible. 

Perspective distortion using a wide-angle lens

The same scene with the camera tilted parallel to the subject

The two images above illustrate how the effects of using a wide-angle lens can be improved. Because of the shape of camera sensors, the distance between the top and bottom of the first image is significantly over-emphasised when the camera is tilted to anything other than parallel to the subject. The second image shows how levelling the camera to be perpendicular to the ground fixes this problem and minimises distortion.

Post-Processing Basics

With images taken correctly following similar steps to the above, very little image correction will need to be done by software before the photographs can be used in documents, and many elements of an image will be outside the control of any correction software by this stage. However, there are a few useful corrections that can be applied to any image that should enhance its appearance and more accurately reflect the original scene. 

The tips below do not refer to any particular image-editing software. The controls discussed can usually be found in any software that manages images, including document editing programmes such as Microsoft Word.

Exposure Correction

One of the most common reasons for poor images within documents is photography that appears either too dark (under-exposed) or too bright (over-exposed). When an image suffers from incorrect exposure, it will be difficult to perceive detail and the photograph will appear uninteresting. By adjusting the image’s exposure (also often called ‘brightness’), a more natural looking image that better represents how we would see the real scene can be achieved. Due to the way our eyes are constantly adjusting their sensitivity to light, we expect a certain average level of light within any scene; when an image departs from this standard by appearing too dark or too bright, it usually appears unrealistic.

The images below illustrate an underexposed image, a correctly exposed image, and an overexposed image. The middle image is most accurate to how we would view the original scene.

Photography and Photogrammetry Guide - Exposure 2 - Jordan Mansfield
Under-exposed
Photography and Photogrammetry Guide - Exposure 1 - Jordan Mansfield
Correctly-exposed
Photography and Photogrammetry Guide - Exposure 3 - Jordan Mansfield
Over-exposed

Level Correction

A common problem with photographs is when the image has been taken with the camera sloping to one side. It is best to avoid this when taking the photograph — many modern cameras have built-in level indicators — but the problem can also be easily corrected with software. The horizon, if present in the image, is often the easiest guide to level correction, but any other object that is reliably parallel or perpendicular to the horizon can also be used. 

Often no such object will be within the image frame and it will therefore be difficult to tell if the scene is represented accurately, so it is always best to make sure the camera is as level as possible when taking the photograph. 

Saturation Correction

Sometimes images may appear to lack colour when viewed on a computer screen. The saturation control can be used to make colours appear more vibrant if this is the case. However, whilst this is a useful control, it is typically the most destructive to an image’s overall accuracy when compared to the real scene. It is important not to add too much saturation as colours quickly become unrealistic to the human eye. 

The images below illustrate a correctly-saturated image that reflects how we see the object in real life, and an oversaturated image with colours that appear unrealistically vibrant.

Photography and Photogrammetry Guide - Saturation 3 - Jordan Mansfield
Under-saturated
Photography and Photogrammetry Guide - Saturation 1 - Jordan Mansfield
Realistically-saturated
Photography and Photogrammetry Guide - Saturation 2 - Jordan Mansfield
Over-saturated

Cropping

Finally, images can also be improved by cropping out any unnecessary areas. When you are the photographer, it can be very obvious to you what the image’s main subject is, but the purpose of the image will be much less obvious to someone else if other objects and distractions are present. Additionally, humans find images with an obvious subject much more pleasing in general, as the mind does not have to work hard to interpret the scene. One advantage of modern photography is that images typically have a very high resolution, and can be cropped significantly whilst retaining a sufficient amount of detail. 

Basic Field Photography – Summary of Tips

To summarise the previous sections, here are some important points to keep in mind when taking photographs and preparing them for use:

  • Use a professional-style camera to have complete control over your image, and familiarise yourself with the basic controls — ISO, Aperture Value and Shutter Speed.
  • Use a tripod for optimal image quality. As you are no longer limited by setting a fast enough shutter speed to capture a sharp image, you are free to let the camera set this variable to achieve a good exposure, whilst also using the camera’s lowest ISO setting for optimal image quality. 
  • To maximise the effectiveness of a tripod, use a cable or remote release to eliminate any camera movements when pressing the shutter button. The camera’s timer function can also be used for this purpose.
  • Combine tripod use with Aperture Priority mode: an aperture of around f/11 – f/16 will ensure a large depth-of-field for good sharpness throughout the scene.
  • Use the camera’s built-in or external spirit level to correct the image before capture. This will ensure that the scene is most accurate to reality.
  • Capture images when ambient light is diffuse. Harsh shadows can make it difficult to see detail and uneven or dappled lighting can give a false impression of the subject.
  • Use exposure correction to adjust images that appear either too dark or light to achieve a more natural-looking photograph; use level correction when the scene’s horizon is not straight; cautiously use a small amount of saturation correction when an image’s colours appear too dull; and use cropping to remove unnecessary portions of the image. 

Photogrammetry

Photogrammetry is an increasingly useful way of recording archaeology, producing incredibly accurate and detailed 3D models of anything from large excavation sites to small artefacts. This section gives a brief overview of how photogrammetry works, explains why the technology is so valuable for both record-keeping and analysis, and provides some basic tips for its use.

Key Points

  • Photogrammetry is the process of constructing a 3D model of any object or scene from photographic images
  • It involves taking a series of images of the subject from multiple angles
  • These images are then analysed by software: by interpreting the same scene from different angles and positions, the software can relate common features, then estimate how the scene appears in 3D
  • The resulting model can be incredibly accurate, allowing reliable measurements and analysis to be made within the model
  • By recording a small number of GPS markers, the model can be easily integrated into other geographic software such as ArcGIS.

How Photogrammetry Works

The two images below show the same scene from different angles. By identifying how certain features – such as the dark, well defined paleosol layer – differ between the two images, the photogrammetry software can interpret the scene as a 3D object. 

Photography and Photogrammetry Guide - Photogrammetry Images 1 - Jordan Mansfield
Original image of a scene
Photography and Photogrammetry Guide - Photogrammetry Images 2 - Jordan Mansfield
The same scene after a position change

For example, as a camera revolves around a scene, the foreground will change more than the background; the software can estimate the relative distance between the foreground and background by analysing their relationship as the camera angle changes.

Increasing the number of image angles improves the accuracy of the 3D model by providing more and more visual information to the software’s process. The render below shows Area III during the 2017 palaeolithic excavations at Barnham, Suffolk, constructed from around 1000 individual images. Due to the large numbers of images taken and their high resolution, the models can be used to view the site at any magnification or angle, providing much greater detail and flexibility than capturing individual section images, as is traditionally done at excavations. As the model is mapped with GPS coordinates, accurate distance and volume measurements can also be made.

Photography and Photogrammetry Guide - Photogrammetry - Jordan Mansfield
A finished 3D photogrammetry render from the British Museum palaeolithic dig at Barnham, Suffolk

Doing Photogrammetry – Basics Tips

If you need to capture images for photogrammetry, a few basic tips can significantly improve the accuracy and overall quality of the resulting 3D model:

  • Diffuse light: it is best to capture any scene when light is flat to accurately capture surface colour and texture. Shadows created by strong sunlight – particularly when they are moving – can confuse the software, so it is better to wait for overcast conditions, or, if possible, doing the photogrammetry indoors or under cover. 
  • The photography should be completed as quickly as possible: light can change in colour throughout the day, and if there are shadows within the scene, their position will change as the sun moves.
  • Similarly, moving objects of any type should be removed from the scene, as they will confuse the software while it attempts to model the static features. For the same reasons, any artificial lighting should be static throughout the capture process and flash should be avoided.
  • High quality images are vital to let the software properly interpret the scene. Using a tripod to get stable images with a wide depth of field (the area of the image that is sharp and in-focus), will ensure that the software is given an accurate picture of the scene to work from.

Further Information and Questions

If you have any questions about photography, photogrammetry, or would like more information about any of the topics that have been covered in this guide, feel free to send me an email.