Originally posted by audiobomber Hi Lowell. I tried this calculation for my A*300mm f/4. The specs show magnification of .09 at a subject distance of 4m. Using 4000mm subject distance and .09 magnification, the result doesn't make sense.

I also tried calculating for the Sigma 150-500, S = 2200mm and m = 0.19, and again, the result was incorrect.

What am I doing wrong?

I have a working spread sheet if you want it, the question I have is with the A300, is the distance the distance to film or distance to front element. when you are working with close focus, then the physical length of th elens is very important. Most times, mag ratios are quoted as the distance to film plane not the true working distance.

If the A300 is considered as about 250mm distance from fil,. that puts the working distance at 3.75 meters and my spreadsheet estimates the true focal length to be about 292mm.

for the sigma lens my sheet and the numbers you give suggest 325mm at min focus.