Lens geometry

Hemispherical lenses

The lens geometry affects the way the hemisphere viewed by the camera is reproduced as a circle on the hemispherical picture. The horizon is a circle and the zenith is its centre. Within the circle, any pixel has a relative radius corresponding to its distance to the centre divided by the radius of the horizon circle. Between the zenith (zenith angle Θ = 0) and the horizon (Θ = π/2 rad), the relative radius and the zenith angle are related, but this relation is not exactly linear and depends on the type of lens used to take the picture.

Wide-angle lenses

If the viewing angle of the lens is below 90° from the centre (Θ_max < π/2 rad), then the horizon is not visible. Depending on the focal distance of the lens (and on the size of the sensor on digital cameras), only parts of the circle representing the viewing angle may be visible, or the picture frame may even be filled, with no visible circle. In the latter case, it is possible to use the edges or the corners of the picture to set the viewing angle. Any point beyond the chosen viewing angle or beyond the edge of the picture frame will obviously be omitted from the analyses.

Settings

In Hemisfer, a polynom up to the 5th grade is used to calculate the zenith angle Θ as a function of the relative radius R:

R = a₁ Θ + a₂ Θ² + a₃ Θ³ + a₄ Θ⁴ + a₅ Θ⁵

Note that this function has 2 fixed points (0,0) and (Θ_max,1). For this reason, the last parameter is always fixed by the previous ones.

The parameters of different lenses are stored in a file named LENSES.CSV. The location of the LENSES.CSV file depends on the permissions that users have on the installation folder of the software. In a standard installation, users do not have write permisssions there and the LENSES.CSV file is thus put in the AppData\Roaming\Hemisfer folder within the user folder. In this case, installing a new version of Hemisfer will not affect the current LENSES.CSV file. However, if the programme folder of Hemisfer is writeable (typically on an external drive for a portable use) then the LENSES.CSV file will be located directly there. If this LENSES.CSV file is not found or if the definition of the current lens is not found in it, then Hemisfer will search it in the current working folder from which photographs and parameter files are being loaded. It is also possible to load a specific lenses file by clicking on the load lenses button. To avoid discrepancies between different versions, however, it is recommended to work with the lens file in its standard location.
Delta-T's HemiView LENSES.CSV files have a similar structure and can thus also be read by Hemisfer. Beware of possible format changes that may be introduced if you edit a LENSES.CSV file with a spreadsheet software: in some cases, it may become unreadable for Hemisfer.

If the lens is in your lens file, then you can chose it from the pull-down list. If not, then you can define a new lens by clicking on the new lens button. To delete a lens from the list, click on the delete lens button. These changes are made permanent only after clicking on the save lenses button.

Designing a new lens function

If you don't have parameters for you lens, then you have to measure them yourself. Use a setting like this to take a picture of angle markers:

On the picture, measure the position of the markers from the centre and express them as a fraction R of the picture radius. Calculate then a polynomial regression of R against Θ, like in the equation above. If necessary, refer to the documentation of your statistical software to do this. It is barely ever necessary to use all 5 degrees of the polynom. Decide according to the precision you wish.