Quick notes on this focal length calculator:
This calculator can be useful in a variety of situations, such as Safari, Cityscapes, small object photography, etc
For example: In a Safari, you want to take a pictures of a Lion at a safe distance of around 165 ft. You’ll soon realise that you’ll need around a 1200mm on Full-Frame for just that. So you might want to buy such a lens (600mm and 2x teleconverter), or go closer instead or not filling the frame with said Lion.
Next, you want to take a picture of a small Ladybug (why not?). This insect is around 0.3in in length. On a full-frame camera, you’ll need a 3.5 magnification to fill the frame with it. Because of that, you’ll need special equipment to achieve such a high magnification. But here we have a limitation for such a tool, the working distance is not precise for this kind of work.