Unclear output "Necessary correction at camera phakic"

[crnh]

@ljpors It is currently unclear what the output value "Necessary correction at camera phakic" in the example notebooks and the online demo means.
I assume it has to do with the focus setting of the camera in combination with the eye's refraction, but I am not sure about this. Can you explain the meaning of this value? And is it really necessary to show it in the online demo? It seems to always be equal to the refraction.

[ljpors]

In PAROS, the camera is modelled as two lenses: a condenser lens (with a stable lens power) and a focusing lens, the lens power of which is fitted such that the image produced by the eye-camera lens system is in focus.

The power of the focusing lens is indeed (almost) equal to the refraction of the eye.

The reason why we show it, is that we, through showing it, have defined the whole camera setup. As the camera model is based on a couple of assumptions, that is a nice thing to do in my opinion.

We may alter the explanation by putting 'Camera focusing lens power' instead.
The addition 'phakic' is not necessary in my opinion, as that pertains to the eye and this statistic pertains to the camera.

[crnh]

Thanks for the explanation. 'Camera focusing lens power' is indeed a clearer description, at least for the demo. External users found the current description quite confusing.

[crnh]

I had a closer look at the central magnification calculation example, and the code doesn't seem to align with your explanation.
I agree that it would make sense to report the camera focus lens power, but the parameter returned by calculate_magnification and printed at 'Necessary correction at camera phakic' is actually the glasses_power:

PAROS/PAROS/fundscale.py

Line 642 in d29b3b7

glasses_curvature, glasses_power = eye.calculate_refraction(display=False)

The value of 'Necessary correction at camera phakic' is thus not 'almost' equal to the refraction, it should be exactly equal to the refraction. If it is not, there is a discrepancy between the eye model and the specified spherical equivalent of refraction.
Furthermore, the power of the focus lens does depend on the eye's refraction, but it is not almost equal to the power of the corrective lens. For example, for the Navarro eye the focus lens has a power of approximately 4D.

This raises a new question: which value(s) do we want to report? The focus lens power, the fitted SE (so it can be compared with the specified SE) or both?

[ljpors]

Thanks for looking into that.

You're right, it is the glasses power that we're printing there. That's strange, as if I remember correctly we did mean to print the camera focusing power here. Let's discuss after all the holidays what we want to choose here.

I do not understand however, that the focusing lens can have a non-zero lens power for the Navarro lens. Let's discuss that in-person later as well.

[crnh]

My bad, I mistook the unit of the focus lens radius for millimeters, resulting in a 1000x larger power. In fact, the unit is meters, so the power is close to 0. Nevertheless, for a myopic eye with an SE of -4 D, the focus lens has a power of -1.16 D, so still nowhere near -4 D.