Originally posted by HavelockV That is plain nonsense. "Absolute aperture" in millimeters would be a static value based on the actual physical reality of a physical lens diameter and the physical size of the aperture.
Put a converter on your lens and the absolute aperture doesnt change as your physical object doesnt change, but the photographic result incl. DoF changes a lot. The resulting F-Stop changes but the absolute aperture not.
Sorry, you're right about the DOF. Since DOF is not a precise value, but a matter of perception, viewing at a higher magnification will change it. Cropping and then viewing at the same size as before will have the same effect. I should have qualified that part.
The rest holds true though. Total light gathered for a given object in the scene, as long as it fits in the frame, is this same no matter how many teleconverters you put on (minus transmission losses). Put a 1.4x converter on, and the object takes up 2 times more area and has 2 times less light per sensor area which is what you use for exposure. Light per
object area is the same. If your field of view is the same, but the absolute aperture is greater, then the total light gained per scene increases. This is the fundamental reason of why "larger sensors have better low light performance", something that everyone seems to take for granted, but few understand why. No, it has nothing to do with "pixel size".
Same goes for the ability to distinguish two points of light right next to each other (i.e. "detail"). This is limited by absolute aperture due to diffraction. If you have an extremely high pixel density, and can keep cropping without running into a pixel limit, you will see that at 200mm f/2.8 is capable of resolving miniscule detail that a 400mm f/5.6 can (assuming these are good lenses that are diffraction limited rather than abberation limited in the center), but at 100mm f/2.8 you cannot, no matter how much you crop or how many teleconverters you add. Teleconverters are only useful when you are limited by the number of pixels (how much you can crop) before you run into diffraction limitations.