Chapter 1: Taking the photos
So taking the photos is a very important task when doing Structure from Motion. It basically creates the data that the algorithms need to compose the point cloud. Naturally, we need to make sure, that the photos we take are of the best quality. Prior to taking the photos, we need to make sure, that our object is placed correctly and lit well.
Setup the object
Setting up the object is already half of the work. You have to consider location, lighting and accessories. For location, you have to find a place that is well lit. The best situation would be outside on a sunny day, but in the shadow. Of course, sometimes this is not possible, so we have to work with artificial lighting. The same principles apply however, the object needs to be lit very well evenly from all sides. This also means from below. In rooms, where the light comes from the top, the lower parts of the object normally aren't lit very well. Lastly, you have to take care of a colour checker and a measurement. A colour checker is important in order to colour correct the images taken. This way you will get a more realistic texture as a result. The measurement is important to be able to scale the final model to the correct dimensions. As you surely remember, the models created by the SfM algorithms are usually not scaled. We have to correct that.
Setup the camera
The camera setup is a difficult thing to do. If you are using a good DSLR, which offers the best results, finding the best options is something that takes time. If you are a pro, you probably know what you are doing, but if not, you need to do some experimentation. The settings you can play around with are basically three: 1. The aperture, 2. The ISO and 3. The Shutter Speed. Setting your camera to A or Av enables you (normally) to play around with the aperture (F-numbers) and the ISO values. To understand what these mean, we need to do some explanation.
The Aperture
This value basically determines how much your lens will open up when taking a photo to let light in. Typical numbers are between F2 - F16, while the higher the number, the smaller the lens opening is, meaning the lesser light comes into your camera. The effect of this is a mixture of depth of field (the background gets blurry) and diffraction (overall sharpness). If you photograph with low f-numbers, you will get a blurry background, but the sharpness of the foreground is better. If you photograph with higher F-numbers, the background will be less blurry, but the overall sharpness does not focus on your object. You want to find a value that completely renders your object sharp (everything right up to the edges).
The ISO
This value basically determines the quality of your photos. The lesser the value (like ISO 100) the better the quality. But for this to work, you will need to have good lighting. If the lighting is bad, a higher ISO value can help, sacrificing the quality of the image. The higher the ISO value is, the more "grainy" your images become.
The Shutter Speed
In Aperture Mode (Av or A on your camera) you can not control this value, as it is determined by the other values. Basically, you want to keep that value as small as possible. It tells you, how long the lens will open to take the photo. A very small number (like 1/200) is good, because the camera takes the image very quickly and therefore becomes sharper. If the value is high (like 1/2 or more) the lens takes a long time to take the photo. In that time, while holding the camera, you will make some micro movements. These movements translate into blurry images.
Examples
So, to understand better what I mean, take a look at the following list. Here, I experimented with the values a bit. You can see, that the lower the F-number is, the lower the shutter speed goes. But you can also see, the lower the F-number is, the more blurry the image gets. Even to a point, where part of the object is not sharp anymore (around the edges).
| Image | F-number | ISO value | Shutter Speed |
|---|---|---|---|
 |
F8 | ISO 800 | 1/8 |
 |
F5 | ISO 800 | 1/20 |
 |
F2,5 | ISO 800 | 1/80 |
 |
F8 | ISO 200 | 1/2 |
 |
F5 | ISO 200 | 1/5 |
 |
F2,5 | ISO 200 | 1/25 |
Take a closer look at the photos and pay attention to the outer rim of the object. The best setting here is the first one, as the object is sharp enough even around the edges and the shutter speed of 1/8 is do-able, although not perfect. The lighting conditions here are not very good, but we have to work with that.
Take the photos
When everything is set up, just start taking the photos. A good procedure is to start at one point and go around the object in circles. First, try to choose a very comfortable circle, where you simply stand in front of the object and go in 20Β° steps around the object. Each circle should be roughly 18 photos, so take small steps. The second circle can be a bit closer and from above, so you have a steeper angle. Again, try to take at least 18 photos for this circle. The last circle should be from below, so maybe you have to knee down or even crawl (see photo above!). It doesn't matter, a good archaeologist needs to take good photos!
Make sure, that you do not play around with the settings as soon as you have started taking the photos. All the images of one set, should be made with the same set of settings. Also, remember, that you do not move the object or the table the object is standing on. Also don't move anything in the background like a chair or water bottle. The only thing moving should be you with the camera. Everything else needs to stay very still.