# Reconciling the Thick Lens Model with P2P Optical Bench

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Reconciling the Thick Lens Model with P2P Optical Bench

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

Now the problem/challenge/confusion...at least in my mind.

How to reconcile the Thick Lens Model to the data in the OB?

As an example, let's take the Canon 100mm 2.8 L Macro and use the OH data at the maximum mag, ie minimum focus. The OB gives us the following: m = 1, x = 134.76 + 162.91 = 297.67 (pretty close to the Canon's stated MFD of 300mm.

The OH gives the hiatus at the max mag, ie min focus, as 14.92-13.25 = 1.67.

If we use the Thick Lens Model to estimate t, using the OB value for x (ie 297.91), we get a value of t of 297.91-(100*(1+1)^2)/1 = -102.09

Clearly a large difference between the two models: OB vs Thick Lens

However, if I use the OB value for f at the max mag, ie 74, we get the 'correct' value for t, is the Thick Lens Model and the OB are essentially the same.

So, my question is this: how do I 'adjust' the standard thick lens model to better represent the lens as captured in the OB?

How do I adjust f from 100 to 74? Is there a simple factor to be used, eg like the bellows factor?

BTW it's clearly not the bellows factor (1+m/p) as this would give (1+1/0.28) = 4.57

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

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