Reconciling the Thick Lens Model with P2P Optical Bench

Started 3 months ago | Discussions thread
OP Garry2306 Regular Member • Posts: 108
Re: Reconciling the Thick Lens Model with P2P Optical Bench

bclaff wrote:

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.

You are welcome.


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.

Sure. I use 'i' for internodal as opposed to 't'


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

Sure solving the above for 't'

...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.

FWIW, actually looks like 134.01mm + 162.91mm = 296.92mm

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

Right. Here's a screenshot for those trying to follow the values (see the Positions line):

Or play with it yourself at Canon EF100mm f2.8L Macro IS USM

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.

You have discovered the obvious, you don't use f at infinity to calculate t at MFD, for that you need f at MFD.

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

No adjustment is necessary just realize that f (almost always) changes as you focus.

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

Right. It's not the bellows factor. Lenses are complicated and focal length as you focus closer than infinity (or even exact values at infinity) are not obvious.
One real practical use of the Optical Bench is to get focal length and pupil magnification particularly at higher magnifications.

(Of course focal length can be measured for interchangeable lenses with extension tubes.)

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 think your only "hang up" is trying to use focal length at infinity in non-infinity scenarios.


Thanks for taking the time to write that.

I fear I need to stop looking for a ‘better way’ than my current approach, as I just don’t have better numbers. 
For instance, the hub ‘only’ gives data for, say, the EF-M 11-22 at infinity. 
So, other than measuring things myself, which is not going to happen, I’ll pragmatically carry on using the split/thick lens model, based on the manufacturer’s MFD, infinity focal length and mag at MFD. That is assume at a given focal length, that the hiatus remains fixed.

It’s not right, but it’s a better model when focus bracketing than using a thin lens model.

Also, by choosing a sensible overlap CoC and fixed pupil mag, I’ll ensure no ‘focus gaps’

The final realism is that the camera, Canon, isn’t that ‘accurate’ at reporting focus position.



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