Depth of Focus

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. This causes the target and its neighbors to have a lower signal to noise ratio. When the aperture is sufficiently large, objects that are too close 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, \(d_{near}\), and the far focus distance, \(d_{far}\). Outside these boundaries, the CoC grows such that the light from that point begins to spread into the neighbor pixels.

The angles at which the lens refracts the light rays grow with the aperture size. For Zivid 3D cameras, this means that objects in the scene lose focus when the aperture is too large. At the same time, this reduces the SNR and makes the point cloud noisier. Therefore, it is essential to carefully select the apertures suitable for the camera’s 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 Contrast Distortion 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. 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.

Note

Zivid cameras are very robust against defocus!

It is also important to note that only the image regions that are out of focus will be affected. Consider the working distance of the camera for the given application to achieve good point clouds with optimal precision.

Depth of Focus for Zivid Cameras

Zivid One+ Small

Zivid One+ Small
\(f\) -number	\(f/1.4\)	\(f/2\)	\(f/2.8\)	\(f/4\)	\(f/5.6\)	\(f/8\)	\(f/11\)	\(f/16\)	\(f/22\)	\(f/32\)
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

Zivid One+ Medium
\(f\) -number	\(f/1.4\)	\(f/2\)	\(f/2.8\)	\(f/4\)	\(f/5.6\)	\(f/8\)	\(f/11\)	\(f/16\)	\(f/22\)	\(f/32\)
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 [1]	Inf [1]
Depth of Field (mm)	175	250	350	500	750	1250	2200	12500	Inf [1]	Inf [1]

Zivid One+ Large

Zivid One+ Large
\(f\) -number	\(f/1.4\)	\(f/2\)	\(f/2.8\)	\(f/4\)	\(f/5.6\)	\(f/8\)	\(f/11\)	\(f/16\)	\(f/22\)	\(f/32\)
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 [1]	Inf [1]	Inf [1]	Inf [1]
Depth of Field (mm)	700	1000	1500	2400	4500	28000	Inf [1]	Inf [1]	Inf [1]	Inf [1]

Zivid Two M70

Zivid Two M70
\(f\) -number	\(f/1.4\)	\(f/2\)	\(f/2.8\)	\(f/4\)	\(f/5.6\)	\(f/8\)	\(f/11\)	\(f/16\)	\(f/22\)	\(f/32\)
Stops	+4	+3	+2	+1	0	-1	-2	-3	-4	-5
Focus, near (mm)	570	530	480	420	370	300	250	190	150	110
Focus, far (mm)	900	1040	1280	1990	7630	Inf [1]	Inf [1]	Inf [1]	Inf [1]	Inf [1]
Depth of Field (mm)	330	510	800	1570	7260	Inf [1]	Inf [1]	Inf [1]	Inf [1]	Inf [1]

Zivid Two L100

Zivid Two L100
\(f\) -number	\(f/1.4\)	\(f/2\)	\(f/2.8\)	\(f/4\)	\(f/5.6\)	\(f/8\)	\(f/11\)	\(f/16\)	\(f/22\)	\(f/32\)
Stops	+4	+3	+2	+1	0	-1	-2	-3	-4	-5
Focus, near (mm)	755	680	605	520	430	350	280	210	160	120
Focus, far (mm)	1480	1870	2870	14290	Inf [1]	Inf [1]	Inf [1]	Inf [1]	Inf [1]	Inf [1]
Depth of Field (mm)	730	1190	2260	13770	Inf [1]	Inf [1]	Inf [1]	Inf [1]	Inf [1]	Inf [1]

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. Keep in mind, getting sharp images in the region of interest is one of many 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. It 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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The camera has its hyperfocal distance at the near focus value. Hyperfocal distance is a distance beyond which all objects can be brought into an acceptable focus.