That’s great @dfrazer. I love how you said “simple trigonometry”. I have forgotten most of the trigonometry I ever learned, but I would have fun studying out the formulas if I had the time. I think that realistically I would use your field trick. I assume that it is for crop frame?
Another thing: I noticed in the link that you sent for the depth-of-field-and-equivalent-lens-calculator, that “crop-frame” is not an option under the sensor size. Which of those options would most closely resemble my Nikon D3100?
Thank you for your help.