Jack, just want to make sure we are on the same page. I'm looking into the 80mm 1.9.... What you say the 1.9 has about twice the DoF I am assuming you mean that at 1.9 (as you state) has more DOF in focus, i.e., not soft backgrounds, as the f3.5 at 3.5?
Thank you.
Let me put it this way to be more clear...
At 10 feet subject distance, the 80 at f1.9 will have about 4" total DoF. At 10 feet subject distance, the 150 at f2.8 will have about 2" total DoF. So technically, the 150 has much shallower DoF.
However, to get similar image magnification to the 80 in the above example, we really need to back the 150 off to 18-1/2 feet, and here the 150's DoF at f2.8 is closer to 7" total. So for a given image magnification, the 80 at f1.9 excels. Compare the 110 f2 and the 80/1.9 wide open at comparable image magnifications and they are essentially identical for DoF.
However #2... But then there is perspective compression; longer focal lengths at comparable image magnifications generate flatter perspectives. Stated another way, you moved further away from the subject with the longer lens to maintain image magnification and the new shooting position alters perspective. Not really pertinent to DoF calculations, but it is relevant to how the final image appears, and here the 80 and 150 at similar image magnifications will generate notably different 'looks' to the final image.
Clear as mud now, right? Sorry... Suffice it to say, I own *both* the 150/2.8 and the 80/1.9 (as well as the 80/2.
simply because of the look from the 1.9 lens wide open -- IMO it is a bargain for the look it generates.
Cheers,