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CHOOSING A CAMERA FOR CRACK MONITORING

Articles 4-01-2022, 23:10 169    print

The PhotoMicrometer software performs photogrammetric calibration of each photograph when processing the results of crack monitoring. This allows you to use almost any camera for monitoring and does not require its preliminary photogrammetric calibration.
However, images taken with different cameras differ both in image quality and photogrammetric characteristics. And these features, of course, must be taken into account when choosing a camera to perform monitoring, since they affect both the convenience of operation and the accuracy of the results.

The camera requirements for monitoring cracks and deformation joints can be divided into 4 groups:
1. Photograph quality.
2. Zoom of the lens.
3. Stability of photogrammetric parameters (interior orientation parameters).
4. Completeness of information about the focal length of the camera in the EXIF protocol of the photograph file.

Photograph quality
Image quality (resolution, noise level), as a rule, depends on the size of the digital camera sensor (the larger it is, the better). In this regard, the best results are shown by SLR and mirrorless cameras with interchangeable lenses, having sensors ranging in size from 1” to a full-format frame or more. Any of these cameras can provide high image quality.

Zoom of the lens (focal length).
For photographing from short distances (up to 1 meter), any camera that allows you to get a high-quality image of the marker is suitable.
When photographing at long distances (from 1 to 40 meters), it is necessary to use cameras with telephoto lenses. It is convenient to use lenses with variable focal length (Zoom lenses). For photographing both SLR (mirrorless) cameras with interchangeable lenses and telecompact cameras can be used – cameras with small matrices, but with powerful built-in telephoto lenses that allow you to get clear and detailed images of the marker at distances of up to 40 meters or more.

Stability of photogrammetric parameters.
By this term we mean first of all the presence or absence of a stabilizer in the optical system of the camera. Of course, it is better to use cameras and lenses without built-in optical stabilizer. All other things being equal, this makes it possible to increase the accuracy and reliability of the solution. However, the algorithms implemented in the PhotoMicrometer software make it possible to successfully process even the images obtained using optical stabilizer.

Completeness of information about the focal length of the camera in the EXIF-protocol of the photograph file.
Along with the image, modern cameras save additional technical information to the image file, which is contained in the so-called EXIF-protocol of the image. From the data contained in this protocol, in addition to the date and time of photographing, the PhotoMicrometer software uses such important information as the focal length of the camera and the physical size of the photosensitive matrix. This information will be missing in the EXIF protocol of the file if non-standard or old mechanical lenses without electronic control are used for photographing. Also, this information is missing or is given with errors in some smartphone models. The PhotoMicro-meter software allows you to process images taken by this equipment, but information about the focal length and the size of the matrix will need to be entered manually.

Thus, there are no absolutely universal solutions for performing "crack monitoring by photos", and the choice of a camera for monitoring is always a compromise between price, characteristics and ease of operation.

The most important parameter to consider when choosing a camera for monitoring is the estimated photographing distances. The dependence of the photographing distance and the equivalent (in terms of the 36x24 mm frame format) focal length of the camera is presented in the table:

Photographing distance (m)

0.2

1

5

10

20

30

40

Equivalent focal length of the camera (mm)

14 – 55

40 - 200

≥ 200

≥ 400

≥ 800

≥ 1200

≥ 1600



This table is calculated for a camera with a resolution of 15 megapixels. For cameras with a different resolution, the focal length values shown in the table will differ. For example, for a camera with a resolution of 50 megapixels, the corresponding focal length values will be approximately 2 times less.
In this regard, the most multipurpose and not expensive solution are telecompacts: Nikon Coolpix P1000, Nikon Coolpix P900, Nikon Coolpix B600, Canon PowerShot SX70HS and others. These cameras allow monitoring at distances up to 40 meters, and the first 2 models - even more. These cameras are also suitable for photographing from short distances.

If the estimated photographing distances are in the range from 0.1 to 30 meters, then the best quality results will be provided by SLR or mirrorless cameras with interchangeable lenses (Sapop EOS, Nikon D, Sony Alpha, etc.). At photographing distances of about 20 meters, the equivalent focal length of such a camera should be more than 600 mm. As already mentioned, it is convenient to use lenses with variable focal length.
At distances up to 20 meters, a completely universal solution can be the use of a camera with 1” matrices and powerful built-in varies-lenses: Sony Cyber-shot DSC-RX10 (models M3, M4) or Panasonic Lumix DMC-FZ2500.

For photographing from short distances, as already mentioned, any camera that allows you to get a high-quality picture of the marker is suitable.

Regardless of the selected camera model, when monitoring cracks on buildings and structures, it is important:
- When photographing from distances of more than 5 meters, use a tripod.
- When performing all monitoring cycles, the same camera should be used - this will reduce the impact of possible residual systematic errors and increase the reliability of monitoring results.

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