Reconciling the Thick Lens Model with P2P Optical Bench

Started 3 months ago | Discussions thread
OP Garry2306 Regular Member • Posts: 108
Re: DoF estimates

bclaff wrote:

Bernard Delley wrote:


The function f(s) shows a lot of variation. For example the 70-300mm zoom shown in my first post goes down from 300mm to 174mm at MFD.

I assume specifically the Nikon AF-S Nikkor 70-300mm f/4.5-5.6G VR in your gear list?


As I showed in a thread with illustrations gone now estimating DoF becomes more simple when calculating it from the magnification ratio, rather than from the distance. ...

Totally agree. Magnification is how I derive the equations and use DOF in the field.
It's not only focal length independent but I find estimating magnification much easier than estimating distance.



I understand what you both are saying, however, my use case is rather unique.

That is I’m using the Canon EXIF reported sensor to object distance, in fact the lower value as I wish to ensure a positive DoF overlap when focus bracketing with my in camera Lua script.

My most stressful use case is when I focus bracket from the MFD of the lens out to a multiple of the hyperfocal, ie an implied infinity defocus blur.

Life is further complicated as Canon reports a fixed focal length value with focus.

It’s easy to ensure that my split/thick lens model is correct at the MFD, as I can work out the hiatus at that location, ie from a measure magnification. Thus like a stopped watch, that is precisely correct twice a day, I know my DoF calculations are ‘right’ at the MFD, plus I can even throw in a pupil mag correction term at this position.

It all goes ‘wrong’ once I start focusing away from the MFD. That is I have no chance or desire to functionalise focal length, hiatus and pupil mag.

But I’m saved by the following logic.

I’m not that interested in the DoF at a single location, as I need to ensure I don’t have a focus gap as I focus bracket, and believe this is best achieved by choosing a suitable defocus (overlap) blur criterion, using the Canon lower value for focus distance and assuming the hiatus at the MFD is fixed through the focus range. Plus noting my focus bracketing algorithm is not fit for macro use, and best for WA to normal lenses, ie not long focal tele lenses.

My approach may/will result in a few extra focus brackets, but it has the value of not creating focus gaps. That is I will predict less DoF than if I knew the lens internals at a particular focus, eg focal length, hiatus and pupil mag etc

Bottom line: I think I will give up trying to refine my lens model, and I’ll stick with the logic above, which seems to work robustly. I would like to say, however, both of you have contributed to my education, and for that I thank you both.



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