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How focal length affect 3D modeling results


For oblique photography,there are four scenes that are very hard to build 3D models:


The reflective surface which cannot reflect the real texture information of the object.For example, water surface, glass, large area single texture surface buildings.


Slow-moving objects. For example, cars at intersections


Scenes where the feature-points cannot be matched or the matching feature-points have large errors, such as trees and bushes.


Hollow complex buildings. Such as guardrails, base stations, towers, wires, etc.

For type 1 and 2 scenes, no matter how to improve the quality of the original data, the 3D model  will not improve anyway.


For type 3 and type 4 scenes, in actual operations, you can improve the 3D model quality by improving the resolution, but it is still very easy to have voids and holes in the model , and its work efficiency will be very low.


In addition to the above special scenes, in the 3D modeling process, what we pay more attention to is the 3D model quality of the buildings. Because of the problems related to the setting flight parameters, the light conditions, data acquisition equipment, 3D modeling software, etc., it is also easy to cause the building to show: ghosting, drawing, melting, dislocation, deformation,  adhesion,etc.


Of course, the above-mentioned problems can also be improved by 3D model-modify. However, if you want to carry out large-scale model modification work, the cost of money and time will be very huge.


3D model before modification


3D model after modification

As a R & D manufacturer of oblique cameras, Rainpoo thinks from the perspective of data collection:

How to design an oblique camera to successfully improve the quality of the 3D model without increasing the overlap of the flight route or the number of photos?

2、What is focal length

The focal length of the lens is a very important parameter.It determines the size of the subject on the imaging medium, which is equivalent to the scale of the object and the image. When using a digital still camera (DSC), the sensor mainly are CCD and CMOS . When a DSC is used in aerial -survey, the focal length determines the ground sampling distance (GSD).

When shooting the same target object at the same distance, use a lens with a long focal length, the image of this object is large, and the lens with a short focal length is small.

The focal length determines the size of object in image, the viewing angle, the depth of field and the perspective of the picture. Depending on the application, the focal length can be very different, ranging from a few mm to a few meters. Generally,for aerial photography,we choose ,we choose the focal length in the range of 20mm ~ 100mm.

3、What is FOV

In the optical lens, the angle formed by the center point of the lens as the apex and the maximum range of the image of object that can pass through the lens is called the angle of view.  The larger the FOV, the smaller the optical magnification. In terms, if the target object is not within the FOV the light reflected or emitted by the object will not enter the lens and the image will not be formed.

4、Focal length&FOV

For focal length of oblique camera,there are two common misunderstandings:


1) The longer the focal length, the higher the flight height of drones, and the larger the area that image can cover;

2) The longer the focal length, the larger the coverage area and the higher the working efficiency;

The reason for the above two misunderstandings is that the connection between focal length and FOV is not recognized. The connection between the two is: the longer the focal length, the smaller the FOV; the shorter the focal length, the larger the FOV.

Therefore, when the physical size of the frame, the frame resolution, and the data resolution are the same, the change in focal length will only change the height of the flight, and the area covered by the image is unchanged.

5、Focal length&Working Efficiency

After understanding the connection between the focal length and FOV, you may think that the length of the focal length has no effect on the flight efficiency.For Ortho-photogrammetry, it is relatively correct (strictly speaking, the longer the focal length, the higher the flight height, the more energy it consumes, the shorter the flight time and the lower the working efficiency).

For oblique photography, the longer the focal length, the lower the working efficiency.

The oblique lens of the camera is generally placed at a angle of 45 °,in order to ensure that the image data of the edge facade of the target area is collected, the flight-route needs to be expanded.

Because the lens is obliqued at 45°, an isosceles right triangle will be formed. Assuming that the drone flight attitude is not taken into consideration, the main optical axis of the oblique lens is just taken to the edge of the measurement area as a route planning requirement, then the drone route expands distance EQUAL to the flight height of the drone.

So if the route coverage area is unchanged, the real working area of the short focal length lens is larger than that of the long lens.