09-10-2015, 05:03 PM
You said you thought you needed more shallow DOF (you worded it "more background blur).
For sharpness, your 100mm f2.8 is fine, so for that reason there is no need to look at a 85mm f1.4.
If you still think you need more shallow DOF at times:
The 100mm f2.8 has a 100/2.8 = 35.7mm aperture
A 85mm f1.4 should have a 85 / 1.4 = 60.7mm aperture.
It will have more or less 100 / 35.7 x 60.7 = 170 -> 70 % more background blur.
It ifs upto you (the photographer) to determine what is most important, per situation. Sharpness or shallow DOF.
For sharpness, your 100mm f2.8 is fine, so for that reason there is no need to look at a 85mm f1.4.
If you still think you need more shallow DOF at times:
The 100mm f2.8 has a 100/2.8 = 35.7mm aperture
A 85mm f1.4 should have a 85 / 1.4 = 60.7mm aperture.
It will have more or less 100 / 35.7 x 60.7 = 170 -> 70 % more background blur.
It ifs upto you (the photographer) to determine what is most important, per situation. Sharpness or shallow DOF.