# Reconciling the Thick Lens Model with P2P Optical Bench

Started 4 months ago | Discussions thread
Re: Reconciling the Thick Lens Model with P2P Optical Bench
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Garry2306 wrote:

As we we know, short of being the lens designer, with full access to optical CAD models etc, The PhotonsToPhotos Optical Bench (OB) is the 'best' insight we have to understanding our lenses: thank you Bill.

At the other extreme we have the Thin Lens model.

Slightly better than the Thin Lens Model is the Split or Thick Lens Model, where we insert a 'pseudo hiatus', which in this post I'll call t.

The Thick Lens Model gives us the following equation: x = (1+1/m)f + t + (1+m)f

Where x is the object to sensor plane distance, m the magnification, and f the focal length.

In the Thick Lens Model the front principal (H) is positioned at (1+1/m)*f from the object and the rear principal (H') at (1+m)f from the sensor.

The hiatus (t) being given by x - (f(1+m)^2)/m

<snip>

Bottom line: Should I 'just' accept the thick lens model and stop trying to model the OB with it? Or am I missing a trick in the thick lens model and not correctly using f

I have not worked through your calculation in detail, but you seem to be looking for a single model to predict macro performance as focus is adjusted.

The "thick lens" model follows from Gaussian optics results for paraxial imaging, which enable one to ray trace and calculate image and object positions if one knows only the positions of four cardinal points of the lens. Wikipedia: Cardinal points.

This assumes that the optical arrangement is fixed.  It works perfectly if you mount a lens in a bellows and use the bellows to focus images at different object distances.

All bets are off if you re-arrange the glassware between comparisons.  The Canon 100 mm f/2.8 lens relies on internal focussing.  When you focus the lens, effective focal length, principal plane locations and principal plane separation (hiatus) will all change.

For each focus position, Bill's optical bench gives you the effective focal length and principal plane locations, so a new thick lens model for each focus setting.

For an arbitrary lens design, I am not aware of a reliable way to predict how these model parameters will change with focus, other than measurement or modelling of the configuration.  It depends entirely on how each individual lens is constructed.

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Alan Robinson

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