In recent lab course we examined diffraction of light. We used a laser with $\lambda=510$nm. The light passed the diffraction grating then was mapped by a lens (distance grating- lens=$2f$) onto the intermediate image plane where we adjusted an aperture. Then the light passed another lens (distance aperture- lens=$2f$) with same focal length and the diffraction pattern was recorded by a CCD camera.
So far so easy.
My problem is that the diffraction pattern wasn't captured in the focal plane, i.e. at distance $f$ from the second lens but at a distance of almost $1.5f$. I shall explicitly explain why this happend but I have no idea.
We moved the camera along the optical axis but only got a sharp pattern for the mentioned distance, so I thought of the resolution, meaning that I calculated the distance where beams of a certain diffraction order would cover a certain amount of pixels but came to no result consistent with what we measured.
Does anyone have an idea?\

enter image description here

Edit: What I meantioned as grating was in fact an element consisting of several gratings with different spacings aligned in a row. So the aperture was used just to select anticipated areas, i.e. a certain grating.

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    $\begingroup$ Can you sketch out the experiment? $\endgroup$ Jun 28, 2022 at 16:28
  • $\begingroup$ I did. The experiment was meant to be an analogy and preparation for a part using the TEM and it was really just what I described. $\endgroup$
    – Bababa
    Jun 28, 2022 at 16:45
  • $\begingroup$ Assuming your illumination is collimated, have you tried to place the grating and aperture at distances $f$ (not $2f$), i.e., in the front and back focal planes of $L_1, L_2$, resp.? $\endgroup$
    – hyportnex
    Jun 28, 2022 at 18:31
  • $\begingroup$ @hyportnex I've thought of different constellations. But the task explicitly says that I have to put the aperture in the intermediate image plane and to have a magnification of 1, hence $d_{object}=2f$. $\endgroup$
    – Bababa
    Jun 28, 2022 at 18:51
  • $\begingroup$ I thought you wanted to do this en.wikipedia.org/wiki/Fourier_optics#4F_Correlator $\endgroup$
    – hyportnex
    Jun 28, 2022 at 19:10

1 Answer 1


I'm not sure why you started by collecting images at the end of the setup. First thing: is your original source well collimated? Next, why are you imaging the grating plane? Your "A plane" will just have an image of the grating itself.

You should be treating the grating as a source at infinity. Use something - white paper -- to see where the diffraction pattern is cleanest after the first lens. My guess is closer to a distance of "f" . But then, if you put an aperture there, you will remove all the higher spatial frequencies, leaving a "cleaner" wavefront exiting. And again, your second lens is imaging the aperture plate, not the wavefront.

  • $\begingroup$ What I meantioned as grating was in fact an element consisting of several gratings with different spacings aligned in a row. We used the aperture in the intermediate image plane to select only one of these gratings such that we can record a clean diffraction pattern with the camera. The aperture wasn't a mask in the back focal plane of the first lens filtering spatial frequencies. Hope I understood you correctly! $\endgroup$
    – Bababa
    Jun 28, 2022 at 17:45

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