DOF will be smaller because of the bigger magnification.
You can see it:
Take two photos - one with TC and one w/o TC.
If you crop the image taken w/o TC to the same framing of the other one, you get two identical results.
DOF is described perfectly at http://toothwalker.org/optics/dof.html
but that includes enlarging.
i guess you mean croping and enlarging to the same size as the other image?
i don´t want to imagine what that does with the COC.... and stuff.
and how can i get the same results when you first say the DOF is different?
because when i enlarge the image the parts that appear sharp will then become unsharp?
because i enlarge the COC?
i want to know if i can calculate the DOF of a lens 100mm f2 + 1.4x TC the same way i would calculate a 140mm f2.8 lens.
if what neuroanatomist wrote is correct, and i can ignore the other stuff i have read (that lenses with the same physical aperture diameter have always the same DOF at the same focus distance).
Yes, I mean cropping and enlarging to the same physical size on the display / paper / ...
Difference can be seen on cropped and uncropped image (TC image is for getting adequate magnification).
I can not help you with math to calculate the DOF, because it depends on the final magnification ~ crop, lens sharpness, viewing distance, and many other factors, thus such number does mean nothing to me... (I need to know my lenses and predict the real result with different apertures; I do not want to convince anybody about the sharpness of the photo because DOF calculator says: it is in the DOF...)
There is another thread discussing DOF and background blur deeply: http://www.canonrumors.com/forum/index.php?topic=11109