I agree. This discussion is about 5DMkII vs GH1.

This is false. Sensor size has a strong relation to depth of field. Why do you think a 1/3" sensor and a FF sensor vary in depth of field so much?

To bring this back on topic, I had started earlier on in this thread about comparing the dof on GH1's smaller sensor size vs the 5DMkII's larger sensor size beginning here:

http://www.dvinfo.net/conf/1035594-post5.html

It's been hashed out already. All things being equal, the depth of field will be shallower on the 5DMkII with the larger sensor. And that can be good or bad depending on your goal.

p.s.

You're right that the DOF is not the same. However, you have an error in your test. You'd want to compare equivalent focal length lenses compensated for the crop factor. The field of view of a 200mm lens on the 5DMkII would be around 122mm on the 400D, since the 400D's sensor is .6111 the size of the 5DMkII sensor size.

My math:
22mm sensor width on 400D divided by 36mm 5DMkII sensor size width = .6111
200mm lens x.6111 = 122mm

No, indeed Alex is right (and I never thought of that before Alex mentionned it), you will always get the same DOF as long as your absolute aperture is the same. Picture the physic behind that, shallow DOF emerge from light coming from different direction. Only aperture has an influence on that, not sensor size.
In the industry when you have bigger sensor, you tend to have bigger aperture (easier to build it that way beause of optic manufacturing limitations), but in fact only aperture matters for DOF control.

On a technical level, looks like I'm wrong.

Depth of Field and the Small-Sensor Digital Cameras - photo.net

Digital Camera Sensor Sizes: How it Influences Your Photography

Meanwhile, it's been far easier to think in terms of sensor sizes relating to dof. My 1/3" and 1/2" sensor size cameras have a deeper dof than my 35mm sensor size cameras.

Have fun figuring it all out!

The way I see it, you can use a normal 35mm lens to create an image on the sensor, but it would mean the focal length of the lens in relation to the sensor would be say for a 20mm FL will be probably 4 times that (just a random figure plugged out of the air) for the sensor. Just like a 20mm FL lenses will be approximately 32mm on a cropped 1.6x sensor.

That 80mm FL on the 1/3" sensor might not be practical for normal shooting.

For instance on my 350D and using a 50mm lens, we are talking about 80mm equivalent. I had to move back more than 2m just to get the subject in frame. Imagine how far you will have to move away from the subject in order to frame subject on a 1/3" sensor.

I hazard to say the design of the lens on a 1/3" sensor would be different as compared to a DSLR to take into account of the smaller sensor.

Anyway I could be way wrong but it make sense to me.

Yeah, that's why if you crop an FF image you'll get deeper focus... Oh, wait a minute?!

It's the focal length and aparture. Nothing else. But a 50mm focal length is extreme tele on smaller sensors. (cropped full frame and zoomed in)