Lens Image Circle Calculation

[pdmoylan]

I wonder if anyone can offer a solution to calculate the increase of coverage at F32/f45 etc as a percent or in real mm terms when the f22 coverage is known. Nikon literature indicates coverage wide open and at f22. Can we deduce that the increase for each f stop is linear? If this is the case then the Nikkor 210W at F45 would have just enough image circle for 8x10 without movements.

[Dan Fromm]

Why speculate and ask others to speculate when you can ask the lens?

[Jim Galli]

Unless the design is perfectly symmetrical there usually is zero gain. Rather, Nikon stopped at f22 because diffraction loss is a trade off that their target audience would probably never consider.

[Mark Sawyer]

It has to do with where the decreasing size of the stop eliminates the mechanical vignetting by the barrel, which varies from design to design, even within a family of lenses. And keep in mind this is only for "illumination", not an acceptably sharp and useable image circle. So no, there's no set formula for all lenses.

[pdmoylan]

Thanks all. Next step is experimentation.