Setting up and taking photos
Choose a sample
First things first- the sample you choose will dictate how complicated the whole process will be. Some objects are quite easy to take photos of, and lend themselves to this photogrammetry process quite well, others are atrociously difficult. Here are some questions to consider.
- How hard will it be to get most of the sample in focus - are the lots of hard/sharp edges or large cavities meaning several depths will be visible at once? Getting most of your surface area in focus is an important goal when taking your photos, as it improves the chances of photos aligning properly, and also improves the quality of your final model texture.
Two images of a welded plate. the second image show how it can be difficult to take photos from certain angles and relate them to other photos, as the two photos will have very few pixels in common.
- How will light reflect off your sample when you take photos- will the surface reflect light (is it very shiny?). A shiny surface will be more difficult to capture and might complicate the process. I've used a spray to reduce the glare on really shiny rocks like Obsidian, a trick that comes from the movie industry. Don't worry, you can always add the shine back as a post-processing effect on the model.
A photo of the spray I use called 'Kenair Anti-Reflect'
- How big is your sample and what kind of detail (final texture pixel size) do you need to faithfully replicate your model? A larger sample with a large scale texture will be easier to capture than a tiny sample with a lot of small detail.
Focus and overlap of photos
When taking photos, your main goal is the capture photos that show the level of detail you need in your final model. The most challenging part of this process is that you will find that once you get close to capture the detail, much of the model surface will be out of focus, making those part of the image blurry and losing the detail. This is where the art of photography comes in- you'll have to use tricks to make sure you get good quality photos that will work for sfm model making.
I can't stress enough how this is the most crucial step in all the process. The quality of your photos will dictate not only how good your model will be in the end, but also how much time it will take you to create the model. If your photos don't overlap enough with enough detail to find communal pixels between images, your photos will not align no matter what you do and you will waste time trying. It is well worth taking the time to set yourself up properly and take good images that have good overlap.
A screenshot from Metashape ilustrating two photos that aligned well. The lines illustrate the concept of tiepoints - points that are tied to each photo.
Taking the photos
There are several things you can do to achieve good photos:
- The camera sensor should be parallel to the sample surface as much as possible, this will ensure more of the sample is in focus.
- Manually control the camera focus point and use focus peaking (peak highlighting) to see what part of the photo is in focus
- Control your camera settings to enhance the depth of field- a larger f-stop number will increase the depth of field. It is recommended to use the following apertures to minimise diffraction, however every camera and lens is different so you may need to experiment with your specific setup: F9, F11, F16, F22.
- Going higher than F22 on most lenses will result in a poorer quality image due to excess light diffraction.
- Make sure you pay attention to image diffraction if using a higher f-stop!
- Try to fill the field of view with your sample. However, if you can't do so, use a bland background if the background is visible in the photos.
- If you need to flip the sample over to take photos of the underside, make sure your photos have plenty of overlap with communal faces.
- Lighting is crucial. Pay attention to shadows and dark patches.
While it may be seem daunting, it is worth it to turn your camera onto manual mode. While Auto mode is often great for a lot of things, you'll get much better results by controlling the fine details when doing photography for photogrammetry. In manual mode there are three main settings you should try become familiar with.
- F-stop: This changes the aperture of your camera lens. A larger F-stop number (i.e. F22) increases the depth of field (what is in focus in your image) but also decreases the amount of light the lens lets into the camera. Increasing the F-stop also increases the diffraction of light which can lower your image quality. This is discussed in more detail below.
- Shutter speed: Shutter speed is just how fast the shutter moves up and down. A longer shutter speed (5s) lets more light into the camera but also gives the opportunity for your image to become blurry due to camera shake. Rule of thumb is don't go longer than 1/80s hand held or 1/30s on a tripod unless you are comfortable with your ability and your gear.
- ISO: ISO is a camera setting that determines the sensitivity of a camera sensor to light. A higher ISO means a brighter image, however also means more grain and noise. Ideally you want to minimise how high you push your ISO. This is also discussed in more detail below.
By manually controlling settings is that you can ensure that your image looks as sharp as possible. You will have to play with lights and camera position, and your settings will constantly be compromises.
If you have a good quality camera, you'll be able to control aperture (F-stop), shutter speed and other settings such as ISO levels. Bear in mind that you can increase the field of depth by increasing the f-stop number, but it also increases image diffraction, meaning you loose image detail.
Two extreme close-ups of photos showing the effect of diffraction and depth of field. The photo on the left was shot with a higher f-stop, allowing a bigger depth of field. However, the detail is blurrier compared to the photo on the right. Note how the photo on the right only has a narrow band of detail, as the rest is out of focus.
A note on my preferred setup
I have a little turntable that I use, and I set up a camera on a tripod with as many bright lights as I have available all around the sample. You can try to diffuse the light, it might help on some types of samples.
I use a Nikon Z7 camera which shoots fantastic photos with a macro lens. I set the camera on manual, and set the f-stop to 22 if I need a higher field of depth, but never higher. Anything higher and diffraction makes the images too blurry. The shutter speed is determined based on theses settings. The turntable can be set to spin on a timer, so I either use a timer and time the photos, or use a remote trigger to set off the camera after I manually focus the photo. Which of those two methods I chose will depend on how much time I have and how much detail is required!