# $\chi^2$ calculations¶

## Image plane optimization¶

With option *forme < 0*, Lenstool performs image plane optimization. These calculations are done in parallel with OpenMP. However, allowing a large number of CPUs not necessarily increases the efficiency of the calculations. Managing the multithreading might take more time than doing the calculations themselves.

Here is a little benchmark (thanks to Benjamin Clement), that tells us how Lenstool scales up with the number of images per system.

For instance, with 2 systems of 2 images each, Lenstool effectively uses 2 CPUs. Here are some other examples:

- Sum of images per system = Total number of images ==> CPUs effectively used
- 2+2 = 4 => 2
- 2+3 = 6 => 2
- 3+3 = 8 => 3
- 4+3 = 10 => 3
- 4+3+2 = 12 => 4