Originally posted by nobody And do not forget that using TC makes DOF noticably more shallow so this is the reason to to use smaller aperture eg 11 instead of 8 and this may lead to IQ degradation due to diffraction.
Putting a 1.4 TC on the 100 2.8 makes it a 140 ƒ4...
Pentax K-30
According to DoF Master
Online Depth of Field Calculator
100mm , ƒ2.8, @ 10 feet, total DoF .17 feet.
100mm, ƒ4 ---,@ 10 feet, total DoF .47 feet
140mm , ƒ4... , @ 10 feet, total DoF .24 feet
So wide open, more DoF wide open on the 140 ƒ4. From the same position you actually get more DoF wide open with the TC.
Diffraction tends to become an issue at ƒ8 and goes downhill from there.
100mm 15 feet DoF = total DoG 2.16 feet
Back up to 21 feet to keep the framing roughly the same
140mm, 21 feet, ƒ8 - total DoF = 4.28 feet.
You can actually double your DoF without changing your framing by using the TC and backing up, at the same ƒ-stop.
These kinds of things are usually a lot more complex than most people expect.