Teleconverters and extension rings...

[ffureel]

Hi all.

I am trying to figure this out but struggling. Ok.

Lets say we have a 50mm f/2 lens.

Question 1/

If I use an extension tube, is the only effect that it has is in reducing the minimum focus distance? An example, If I take a portrait shot with the lens as standard, at minimum focusing distance and wide aperture. If I added an extension tube and focussed the lens further back to take essentially the same photo, would the extension tube photo have shallower depth of field? Or would the focusing further back on the lens negate the closer focusing of the extension tube?

Question 2/

2 x teleconverter. If i added one of these to my 50mm lens I know you have to multiply the focal length by 2 and also the effective exposure. So the lens would become a 100mm f/4 lens roughly. Now the f/4 bit is where I am struggling. I know the exposure is effected to an equivalent of f/4 but what about the depth of field? Would the depth of field be like a 100mm f/2 lens or like an f/4 lens?

Another question. Is it possible to reduce depth of field with a lens? Say we are at widest apertures. the camera is not allowed to move, change focus, subject does not move etc... Is it only possible by using a lens with a wider aperture?

Thankyou.