Sunday, December 13, 2020

LensSim 1.2

As I have discussed in previous posts, LensSim is not there to model the actual insides of your lens, as it is impossible to know what the lens designer has optically laid out internally. We also have no idea what lens elements are moving and thus how, for example, focus is managed.

What we can say is that representing a modern lens by a highly simplified 'thin lens' model is the worst we can do.

The DOFIS model, on which LensSim is based, is about the best we can do, in ignorance of the lens design and by carrying out lens modelling in ray tracing CAD packages.

Although not perfect, I've now added a representation of the entrance and exit pupils into LensSim, using the following estimate for the entrance pupil radius:

Where f is the focal length, m the magnification, p the pupil magnification, and N the f-number at infinity, ie printed on the lens.

The exit pupil radius is assumed to be p times the entrance pupil radius.

In LensSim things now look like this:

Here we see a representation of the entrance and exit pupils, positioned (rotated 90 degrees out of their optical plane) at their location on the lens axis.

Bottom line: LensSim remains a tool to help understand your lens and identify some key properties that are useful to know, ie front principal location, entrance pupil location and the FoV changes due to lens breathing. But remember, LenSim is not modelling the actual design. All we can say is it is a better representation than a classic thin lens model.

As usual I welcome any feedback on this post.

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