Obtained from Researchgate comments

The camera constant/length is sometimes useful but not mandatory to have for quantitative analysis of electron diffraction in the TEM.

For small scattering angles, Bragg's law becomes from nλ=2dsinθ (for n=1) to λ=~2dθ tan2θ can also be approximated as ~2θ (again, small angles) which can be described as R/L, where R is the distance of the diffracted beam from the central, non-diffracted beam on the image plane (i.e. phosphorescent screen, plate camera etc.) and L is the distance between the image plane and the sample (see link below).

Altogether 2θ = ~λ/d = R/L Rearranging we'd get Rd = λL, or d = λL/R

This expression is useful as it directly (and simply) relates the d-spacing and the distance between diffraction spots. λL is constant and it is called the camera length. To analyze and index a diffraction pattern you'd normally not need this constant because you can look for relative ratios between R's and compare them with relative ratios between known d's (and then check if the relative angles between them make sense with respect to symmetry per reflection). Once the phase is identified you can straightforwardly calculate λL from the same equation.

If theoretically you only have reflections only from a single d-spacing then you'd have to use the known λL to relate d to R.

Note that L and therefore the camera length change when you change the optics and imaging device in the microscope (i.e. the screen and the camera are not usually at the same height, diffraction lenses can be in different conditions etc.)

Further reading: http://www.ammrf.org.au/myscope/tem/background/concepts/imagegeneration/diffraction/cameralength/ http://www.doitpoms.ac.uk/tlplib/diffraction-patterns/printall.php

R is obtained from the image in pixels, while d are the theorical values for aluminium.

Constant λL = 652 ± 0.07% pix*Å. In order to retrieve the d-spacing from an unknown sample, get R values and compute d = λL/R.