For depth of field equivalence, use the ratio of format sizes (commonly the crop factor, often based on diagonal) when the compared formats have similar aspect ratios and you’re assuming the same framing, print size, and viewing conditions.

A practical rule is:

equivalent f-number = reference f-number ÷ crop factor

where crop factor = reference diagonal ÷ target diagonal.

So if 8×10 at f/64 is your reference, a smaller format needs a proportionally smaller f-number to give a similar depth-of-field appearance in the same final print. Example: 4×5 is about a 2× crop from 8×10, so the equivalent is about f/32.

Important caveats:

• This is about depth of field in the final viewed image, not exposure. Exposure per unit area at a given f-number does not change with sensor size.
• Depth of field is fundamentally set by focal length, aperture, focus distance, format size, print/display size, viewing distance, and viewer acuity.
• If you use the same focal length and distance on different formats, depth of field does not magically change because of sensor size alone; equivalence assumes you change focal length or camera position to keep framing the same.

So yes: for your purpose, diagonal ratio/crop factor is the usual shortcut.