Depth of Focus

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Introduction

Depth of Focus is a term used to describe the distances where an object is deemed to be within focus. If the object is too close or too far it falls out of focus.

In this article, we will describe the basic principles of Depth of Focus, and also learn how to apply these principles for the Zivid cameras.

What do we mean by focus?

Focus is typically defined by what is called the Circle of Confusion, or CoC for short. The CoC is the radius, or area, covered by light from a point as it hits the imaging sensor after passing through the camera lens. Typically, an object is deemed out of focus when the CoC is so large that it covers more than one pixel. The net effect is that the signal power is lost from the target pixel to neighboring pixels as noise, causing both signal-to-noise degradation within the target pixel and its neighbors. When the aperture is sufficiently large, objects that are too close in front of the focus plane will be focused in front of the image sensor and blur into neighboring pixels. Similarly, objects that are too far behind the focus plane will be focused behind the image sensor and blur into neighboring pixels. See the illustration below.

The Depth of Field is defined as the distance range where an object is deemed to be in focus. The boundaries are defined by the near focus distance, , and the far focus distance, . Outside these boundaries, the CoC grows such that the light from that point begins to spread into the neighbor pixels.

The angles at which light gets refracted by the lens grows with the aperture size. For Zivid 3D cameras, this means that objects in the scene may lose focus when the aperture is too large. This, in turn, will reduce SNR and make the point cloud noisier. Therefore, the aperture should be taken into consideration when specifying the cameras working distance for optimal image quality.

How does focus affect my point cloud?

As the image gets more out of focus, artifacts such as noise and Laplacian Effect will increase. When we speak about noise, we are talking about the point precision of the point cloud. This means that the capture-to-capture variations of a given pixel will increase and that the pixel-to-pixel variation within for a given capture will increase. It is important to be aware that a slight increase in noise may still be acceptable, as it all depends on the algorithm that is going to process the point cloud afterward. For this reason, it is more often than not, acceptable to have an unfocused image.

Zivid cameras are very robust against defocus!

It is also important to note that only the regions of the image that are out of focus will be affected. By carefully considering the cameras working distance for the given application, it is possible to achieve good point clouds with optimal precision. Read more about setting up the cameras working range here: Working distance and camera positioning.

Depth of Focus for Zivid Cameras

For Zivid cameras, the following focus distances can be attained based on the table below. Note that these values are rounded to make it easier to read and memorize.

Zivid One+ Small

-number

-number

Stops

+4

+3

+2

+1

0

-1

-2

-3

-4

-5

Focus, near (mm)

480

470

460

450

430

400

380

340

300

250

Focus, far (mm)

520

530

540

570

600

650

750

950

1350

5500

Depth of Field (mm)

40

60

80

120

170

250

370

610

1050

5250

Zivid One+ Medium

-number

-number

Stops

+4

+3

+2

+1

0

-1

-2

-3

-4

-5

Focus, near (mm)

925

900

850

800

750

650

600

500

425

350

Focus, far (mm)

1100

1150

1200

1300

1500

1900

2800

13000

Inf

Inf

Depth of Field (mm)

175

250

350

500

750

1250

2200

12500

Inf

Inf

Zivid One+ Large

-number

-number

Stops

+4

+3

+2

+1

0

-1

-2

-3

-4

-5

Focus, near (mm)

1700

1600

1500

1350

1200

1000

900

700

500

400

Focus, far (mm)

2400

2600

3000

3750

5700

29000

Inf

Inf

Inf

Inf

Depth of Field (mm)

700

1000

1500

2400

4500

28000

Inf

Inf

Inf

Inf

Zivid Two

-number

-number

Stops

+4

+3

+2

+1

0

-1

-2

-3

-4

-5

Focus, near (mm)

570

530

480

425

365

305

250

195

155

115

Focus, far (mm)

905

1035

1280

1900

7630

Inf

Inf

Inf

Inf

Inf

Depth of Field (mm)

335

505

800

1475

7265

Inf

Inf

Inf

Inf

Inf

Depth of Focus Calculator

By using Zivid’s Depth of Focus Calculator, it is possible to find the recommended max aperture based on the working distance and acceptable blur radius. It is important to remember that getting sharp images in the region of interest is one of many essential things needed for optimal point cloud quality, but it is not a hard constraint. As mentioned above, Zivid 3D technology is very resilient to blur and defocus and will still be able to provide good point clouds even if the image appears out of focus, though they can be a little noisier.

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