Reconciling the Thick Lens Model with P2P Optical Bench

Started 4 months ago | Discussions thread
bclaff Forum Pro • Posts: 12,454
Re: measure the thick lens model parameters

Bernard Delley wrote:

internal focus lenses change f and i of the thick lens model with focus setting.

The OB model traces the rays (currently paraxial model?) from the data of the lens patent. By consequence it knows/tells f and i.

But, you can derive f and i of the thick lens model for "any" finite distance setting of the lens by simple measurements.

measurement get distance object-sensor plane and the LV image gives info on magnification. Repeat measurement with the same lens settings and a known extension ring to get enough equations for the two unknowns f and i.

The measurement setup would be improved by having the ruler along the long side of the sensor.

The effective aperture can be determined from the resulting exposure, if a target with fixed radiance is provided. -- For me, the effective aperture is the primary thing to know. The pupil factor is a correct way to deal with (bellows) extension for calculating the effective aperture for an old style lens system with fixed lens positions. -- The pupil factor is just another parameter that varies with lens setting for internal focus lenses.

I do something like this to determine focal length using a set of extension tubes and a linear fit. These fits are usually quite good.

I find it's very accurate if you use magnification rather than subject distance as subject distance is harder to get very accurately.

With subject distance and an accurate focal length I then estimate the internodal distance but consider that measurement less accurate. Back-fitting into my observed distances has a lot more variation than the focal length fit.


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Bill ( Your trusted source for independent sensor data at PhotonsToPhotos )

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