It’s hard to get everything in focus for your high-speed shot, so it’s important to have good depth of field.
In the image on the left, we are trying to capture an ant and a bee interacting. They are both constantly moving, making it difficult to get both of them in focus at the same time.
Your application may require getting multiple moving mechanical parts in focus in one shot, or perhaps you are trying to capture a fast moving object for which the distance is not easily controlled.
The challenge is the same, so how do we get objects in focus when they are not located in a shallow plane? This is one of the most common questions when using a high speed video camera. The answer is simple: we need a good enough depth of field.
In a previous blog post, “Imaging Objects Behind a Screen With a High Speed Camera”, we looked at limiting our depth of field to include an object of interest, while minimizing focus on the screen that was blocking its view.
Our current goal is the opposite: how do we enhance our depth of field to include objects not equal in distance to the camera? And how does the high speed camera’s pixel size play into this?
As discussed in the previous blog, depth of field is a function of f-stop and focal length.
Below are three images taken from high-speed sequences. All of them have these parameters in common:
- Aperture setting: f:/2
- Distance from subject: 48 inches
- Shutter duration: 100 microseconds (to minimize motion blur)
- Resolution: 640 x 512
- Frame rate: 1600 fps
They also had two parameters not in common:
- Pixel size: 20, 10, and 5 microns, respectively
- Focal length: 100, 50, and 25mm, respectively in order to preserve the same field of view.
Let’s look at the results a little more closely.
The image above was taken with the 25mm lens and the 5-micron pixel high speed camera settings.
Everything is in reasonably good focus, although we know that there is about a 4” difference in distance between the fan and the wheel.
Using the depth of field (DOF) calculator, which is part of the Fastec Imaging Calculator App, we confirm that we get close to a 4” depth of field for a camera with a 5-micron pixel at 48” from an object, taken with a 25mm lens.
The image above is now taken using a 50mm lens and the 10-micron pixel setting. We are able to get the fan and the tissue box in pretty good focus, but the wheel is soft. We can see from the calculator that our depth of field has dropped to a little less than 2 inches (1.797) with this setup.
The final image above was taken with a 100mm lens and a 20-micron pixel. Only the face of the fan is in good focus. From the calculator, we see that our depth of field has diminished down to less than an inch (0.86).
What if we need to accommodate this last configuration? If we go to the calculator again we can see that if we change the f-stop to f:/8, the DOF increases to 3.443, which is very close to the 3.674 we had with the first setup using a 25mm lens and 5-micron pixel.
Going from f:/2 to f:/8 is 4 f-stops, which represents a difference of 16x less light transmittance.
For normal photography that is not a huge issue. You would simply lengthen your shutter duration from 100 microseconds to 1.6 milliseconds, which is quite a short shutter duration for everyday use.
But if you are trying to image a fast-moving object such as the fan shown here, you will not get acceptable results with a long shutter duration:
So for high-speed imaging, accommodating larger pixels means either having a lot more sensitivity to begin with, or a lot more light!
For more information regarding depth of field and lens selection, Register for our free Fastec Tutorials and Tools.
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