• VR
  • 2018.08.07

The hint of VR resolution and select of the lens

This article is an explanation of VR resolution and hints on lens selection.

The resolution required for 360 VR and 3D 180 VR is 3840 x 1920 ( equirectangular format).

Therefore, the cameras inevitably used for VR shooting are limited to cameras that can shoot 4K movies.

In addition, it is necessary to select the cameras and the lenses considering the size of the sensor and the size of the image circle of the lens.

It is easy to understand, but it is confusing if you do not understand, so I will explain the resolution of VR and how to select the correct lens.

Sensor size and image circle size

Sensor size

◾️Sensor size and image quality
With the same resolution, the larger the sensor size, the better the image quality.

For example, even an action camera can shoot 4K, but the image quality is quite different from the 1 / 2.3 “size sensor used in the action camera and 4K, which can be shot with full size sensor.

In order to place 4K pixels on a small sensor, you need to reduce the size of 1px, so the pixel size is quite different from the full size 4K sensor.

The size of the image circle

Each lens is designed with image circle size according to sensor size.
For example Entnaiya Fisheye, the size of the image circle size is different depending on the difference in focal length.

When using a fisheye lens for VR movie shooting, you will finally select the lenses to use from the required resolution and equipment.

The number of cameras (lenses) and viewing angle necessary for 360 VR

Reason why the fisheye lens is used for VR shooting

The biggest merit of shooting VR movies with a wide-angle fisheye lens is that you can shoot with a small number of units.

As the number of cameras decreases, the number of stitch lines decreases, making the stitch work after shooting very easy.

or example, if you use a 250 degree wide angle lens, you can shoot 360 degrees with two cameras back to back, but if you do not use a fisheye lens, more cameras are needed to capture 360 degrees .

GoPro which was frequently used in VR shooting has a lens of about 170 degrees in the diagonal direction, it becomes a super wide-angle camera if it is considered with a general camera, but in the case of VR movie shooting, at least six cameras You will need.

Necessary angle of view of 190 ° or more in vertical direction

It is very important to have a field angle of 190 ° or more in 360 VR shooting.

To cover top and bottom 180 °, you can not stitch without 190 ° angle of view.
There are holes in the top and bottom.

In order to cover the holes you need to set the camera to be up or down.

n order to cover the holes you need to set the camera to be up or down.
In other words, since it is necessary to shoot in multiple rows, setup becomes very complicated and the stitch line becomes complicated.

Horizontal angle of view is recommended to be 400 ° or more for all camera angles

In the case of a horizontal field of view, it is desirable that the total viewing angle of all cameras is 400 ° or more.

For example, if the vertical field of view is 190 degrees or more and the horizontal field of view is 200 degrees or more, two cameras can cover all 360 degrees.
When shooting with three cameras it is possible to cover all fields of view 190 ° or more and horizontal field of view 135 ° or more can cover all 360 °, but it is safer to consider about 140 ° with a little margin.

As the overlapping area increases, the parallax can be corrected so that the stitching becomes easier.

The resolution of VR depends on the number of pixels of 180 °

1Number of pixels in the range of 180 °

If you use a lens with a small image circle for a large sensor, you will be able to shoot 360 VR movies with fewer cameras (lenses), because the image circle will be all included, but a resolution problem will come out.

The resolution of the VR movie depends on the number of pixels of 180 °.
In other words, using a lens with a small image circle will result in a lower resolution because fewer pixels are available in the 180 ° range.

Because it is difficult to understand by words, I will explain with a diagram.

◾️Number of pixels in the sensor
For example, Super 35 mm which is a general sensor size for video.
The sensor size is about 23.5 mm x 13.22 mm (it varies slightly depending on the camera.)
The resolution can be output at 3840px x 2160px with 4K shooting.
In other words, about 163 pixels are arranged for 1 mm of sensor.

◾️Image circle size
The specifications of the lenso HAL 250 3.6.
180° = Diameter 11 mm
250° = Diameter 14.25 mm

◾️Diameter 14.25 mm
163 px x 11 mm = 1793 px

The ratio of equirectangular format which is the format of VR movie is horizontal 2: vertical 1.
Since it is horizontal 360 °: vertical 180 °, the guidance of the obtained resolution is horizontal 3786 px by vertical 1793 px .

It is not enough for 440’s 3840 x 1920, but it also gives errors in the position of the crop and software judgment.
In the case of VR movies, since it is originally stretched from the image of the circumference and converted to equirectangular, I think that it is an allowable range if it is in the range of about 5%.

How to select the most suitable lens

When selecting the optimum lens, you have to think about how the 180 ° image circle fits into the sensor.

When giving priority to resolution, it is important to select a lens with a field angle in the range that can be stitched so that the 180 ° line fits into the sensor as much as possible.

The case of Super 35 sensor (23.5 mm x 13.22 mm) is as follows.

Image circle size of the lenses
HAL 250 2.3 φ7mm@180/φ7.3444mm@190
HAL 250 3.0 φ9.2mm@180/φ9.68mm@190
HAL 250 3.6 φ11mm@180/φ11.66mm@190
HAL 250 4.3 φ13.1mm@180/φ13.8mm@190
HAL 250 6.0 φ18.2mm@180/φ19.16mm@190
HAL 200 3.6 φ11mm@180/φ11.52mm@190
HAL 200 6.0 φ18.2mm@180/φ19.08mm@190
* The size (diameter) of the image circle at each angle of view can be calculated from the image height of the spec table (radius of the image circle).

◾️In the case of two cameras
The lens that seems to fit just 13.22 mm in height will be HAL 250 3.6 (φ 11 mm @ 180 / φ 11.66 mm @ 190).
In other words, in the case of a camera equipped with a Super 35 sensor, you can shoot 360 ° with two units using HAL 250 3.6. The resolution can also be obtained about 4K.

Although HAL250 4.3 is also regrettable, 190 ° line has exceeded the sensor, so there is a possibility that holes may be emptied by stitching.

◾️If more resolution is needed
If further resolution is required, use the sensor vertically.
A general sensor is longer in the horizontal direction than in the vertical direction and has many pixels. If you use the sensor vertically, you will be able to increase the resolution at 180 °.

If the sensor is vertical, 23.5 mm of the original width will be vertical. HAL 250 6.0 φ 18.2 mm @ 180 / φ 19.16 mm @ 190 etc. will be available.

If it is 163px x 18.2mm it will be 2967px. Approximately 6K of equirectangular can be output.

However, since the horizontal field of view in that case will only enter about 120 °, you will have to think about a set using four cameras.

Horizontal 16: 9 video format is actually very disadvantageous for VR.
A camera that can shoot at an aspect ratio of 4: 3 with anamorphic like the Panasonic GH 5 series can be said to be a VR oriented camera because it will increase vertical resolution and reduce waste of left and right blanks.

Summary

How the 180 ° image circle enters the sensor is an important point in 360 VR.
Be sure to check when selecting equipment and lenses.

Points

◾️The resolution of VR is determined by the number of pixels included in 180 °.
◾️Vertical viewing angle is 190 ° or more.
◾️The total horizontal viewing angle is recommended to be 400 ° or more.
◾️If higher resolution is required, use vertical sensor to increase vertical resolution.

Others

◾️Sensor size may vary depending on camera and manufacturer.
◾️Crop factor may be different depending on camera and mode.