Symmar 210 convertible 370 coverage.

Computing coverage is reasonably straightforward but one of those things which is simpler to just try and see. You can use the lens equations to get the image distance - 1/subject distance (8 ft in your example) + 1/image distance = 1/f where f is the focal length). Now if you know the coverage at infinity i.e., when image distance is 1 focal length, you can use the laws of similar triangles to derive the coverage at this new focus distance. In other words, coverage at infinity/image distance at infinity (i.e., one focal length) should be equal to coverage at 8 ft/image distance at 8 ft. You've calculated image distance at 8 ft from the lens equation. Plug and chug....

I think at the 210 end, you can get away with 8x10 coverage as long as you focus close and don't have much movement demands - corner might be problematic. Given that 300+mm is considered normal on 8x10, 370mm may still prove short for portraiture. Also, resolution at 370 is not as good as at 210 but this might actually be a benefit for portraiture.

Lastly, try using the front element of your lens i.e., removing the rear element. By my rough calculations thats roughly a 485mm f/16 lens you end up with. I have used this lens in this fashion. Resolution can be considered appalling but mine gives me very nice soft focus effects, a little difficult to control but sometimes pleasing....

Hope this helps. DJ