I am also a bit confused with this topic.

In work of Jesacher (page 28)

www.uibk.ac.at/exphys/photonik/downloads/thesis_jesacher.pdf
The following procedure of relating experimental and computer generated pictures is outlined.

Mholo - diameter of hologram pattern displayed on SLM in pixels

Marray - side length of array used in calculations.

So we need to place a correct aperture on the Marray matrix

---copied from Jesacher---

The choice of the right diameter of the aperture is crucial. Experimentally, it can easily be determined by subsequently displaying two gratings on the SLM, the grating vectors of which differ by

**Δk (in inverse pixels)**, and measuring the corresponding spatial shift of the laser spot on the CCD. The desired aperture diameter A (in pixel units) is then determined by

A = Mholo*Marray*Δk/(2*pi*Δl)

where Δl is the focus-shift on the CCD in pixel units, MArray the side length of the data array, and MHolo the diameter of the hologram on the SLM, both measured in pixels.

end of copy---

I display a Fresnel lens (F = 50 cm) with two different gratings k*x with k = 4 or 8 and x equal to pixel number.

It shifts focus by 100 pixels.

I am confused with ¨Δk in inverse pixels¨, shall I use it as 4 or 1/4?

If I use 4 the aperture is too big to be real, and if I use 1/4 resulting intensities are still far from experimental. What do I miss?

May be you can point me out where from this relation comes from?

Thanks in advance.

Petro.