Use of Macro setting when calibrating a lens

[Nathan Craig]

Most of the images I process in PhotoScan are taken from a kite and the lens is focused at infinity. To calibrate a lens using PHotoScan Lens, I must move the camera close to the computer monitor. So close in fact, that I must use the camera's macro function. Should I expect the use of a different mode to have a significant impact on lens calibration?

Thanks,
Nathan

[Alexey Pasumansky]

Hello Nathan,

Wide angle lens calibration using AgiSoft Lens can be indeed problematic as the camera needs to be placed very close to the monitor. As the camera calibration parameters depend also on the focusing distance, it may be very hard to get sharp pictures of the screen. Focusing the camera on the screen in undesirable, as it will affect the calibration.

You may find the following recommendation useful:
1. Try to use larger screen when possible.
2. Use higher aperture setting. This may also require using a tripod and higher exposure setting.

Please note, that even this recommendation may be insufficient to capture the calibration photos using infinite focus setting. In this case only approximate calibration values will be obtained, but they should be sufficient for a proper operation of PhotoScan. Just make sure that "Fix calibration" flag is not set during photo alignment, so that PhotoScan can refine the obtained calibration.

[Nathan Craig]

Hi Alexey,

Thank you for the information. If I understand correctly, if the images for a model are shot at infinity it is desirable to have the calibration images also shot at infinity. The focus point influences the calibration. To attempt to achieve focus at infinity using Agisoft Lens, I should strive to use a large monitor, high depth of field, and a tripod. It may be challenging to find a monitor large enough to fill the field of view when the camera is at infinity.

Can Agisoft Lens perform a calibration from some other regular grid? For example, a large printed array of black and white squares or a building with very regular squares. With other lens calibration systems, I've used large printed sheets or images of buildings with regular squares. Might such images work with Agisoft Lens?

Thank you again,
Nathan

[Alexey Pasumansky]

Hello Nathan,

You have understand right. And you also can use large printed calibration grid but you have to make sure that it is flat and squares are really squares (with the same side length and orthogonal sides).

[Nathan Craig]

Dear Alexey,

Thank you for the clarification. This is quite useful information.

Regards,
Nathan

[JC]

Could we project the grid onto a projectore screen ~1.5m x x1.5m?

[dcm39]

I guess an easy approach would be to print an A0 poster sized sheet with the same grid. Would it be possible to include a pdf of such a grid with the lens software?

Would the idea of a projector onto a flat wall work also?