Hhmm...
When my reality is different from my understanding, I've learned to bet that the problem is with my understanding. I would be interested to hear what differences you perceive.
That is a good article, but some time and possibly other experience is required to understand everything that's in it.I found this article very good: http://extreme-macro.co.uk/microscope-objectives/
With microscope objectives, many configurations will automatically scale the effective F-number to match the size of sensor. In particular, this happens whenever you tweak the relay optics (such as tube lens focal length) so as to maintain constant FOV by changing the magnification. In addition, with microscope objectives the sensor is often good enough to capture all detail that's in the optical image, so diffraction is the limiting factor. In these cases, DOF depends entirely on the objective, not at all on the sensor size.
To explain...I wonder why f/8 on Canon relates to f/16 on Nikon?
With modern Nikon systems, the setting on the camera is expressed as effective f-number, already corrected for magnification. With Canon systems, the setting on the camera is expressed as nominal f-number, which must be corrected for magnification. The usual correction is to multiply by a factor of (m+1), although this may not be exactly correct with some lenses. So, "f/8" at 1:1 on Canon is likely to be around f/16 effective, while on Nikon "f/16" already means f/16 effective. I suspect that this difference is responsible for many discussions about why some lenses give so much DOF. Especially in Nikon communities, the Canon MP-E 65 is famous for giving lots more DOF than people would expect based on their own equipment. The difference is more easily understood by realizing that f-numbers on the MP-E 65 are expressed using the industry standard of "at infinity focus", while actual magnifications are limited to the range of 1X to 5X. As a result, in practice a setting of "f/11" on the MP-E actually means an effective f-number from f/22 to f/66, depending on magnification.
In general the advantage of full frame is more flexibility. By stopping down farther and cranking up the ISO to compensate, you can take the same image with a full frame camera that you can with a crop sensor. The reverse may not be true -- for example while you can buy an f/1.4 lens for full frame, you cannot buy the corresponding f/0.875 lens for 1.6 crop. So, if you want seriously blurred backgrounds for portraits, full frame is a better bet. Similarly if you want to work in low light with moving subjects, and you do not care about DOF, then full frame is better because the larger hole in the longer FL lens allows to collect more light in the same time.I was thinking to buy full frame to get better DOF
For most macro/micro applications, there is little difference in image quality between full frame and crop sensor, unless you're in a position to obsess over pixel noise, and then full frame will win if you can provide enough light to fill up the sensor.
--Rik