I’ve started designing custom film cameras, and needed tools to understand what kind of tolerances and imperfections can I get away with when guessing focusing distances. And more importantly, what kind of image would a lens create, and how the field of view, depth of field, focal length and aperture would correspond across different film formats. I haven’t really found good tools online, so I created some myself. Hope they can help someone:

Circle of Confusion Calculator
This one shows what would be in focus and how front/back blur behaves across formats. It was really eye-opening for me to play with it and understand the relationships between the different factors and how they shape the image. The Circle of Confusion tells us what a point light source turns into after going through the lens. It is basically the brush with which you paint light. And at different film formats (at similar image viewing distances), similar sized CoC might appear to paint a sharp or a blurry picture. Play with the tool to find out more!

Background Blur Accumulation Calculator
This one shows background blur accumulation at different format and apertures, to see what lens you would need in e.g. 35mm to simulate a LF look. In this case specifically an abstract measure of “how much blur will there be for 2 meters behind the focus point”. It was created specifically for environmental portraits and uses vertical angle of view (instead of, as often used, the diagonal) to get a better feeling of background separation in portraits. It tries to help you with the questions of “Where should I place the person?” and What lens and what aperture should I use for better background separation?"

Helicoid Extension Calculator
This one shows your helicoid/focusing rail extension across focal lengths and focus distances.

Distance Scale Creator
This one is for creating a distance scale and printing it out at home.

All of them are interactive, so play around and see how it all relates. Let me know if somethings is not working as expected!