[Brightcolours]

It's still a f4 lens in terms of light gathering capability.

Also, f8 at 600mm is plenty thin enough.

Of course it isn't. The size of the hole is the same as with a FF 600mm f8. The cone of light gathered is the same too. So how can it gather more light?

 

Due to the smaller sensor, an f4 lens on MTF gathers less light than on FF 135 format. That is where the noise advantage for FF sensors comes from too.

 

The 'F8' equvialency is only for DOF; and while you can argue it is relevant to total light gathering; it is also very misleading esp when one is concern about shutter speed not light gathering.

No, it is not misleading at all. Nothing is stopping you from using a similar exposure time.

Yes the lens gather less total light because the sensor is smaller but the amount of light that strikes a certain spot is the same

There is no "a certain spot" of any meaning where the light "striking it" is somehow the same.

There are only two meaningful "spots": The whole sensor (the image) and a photo sensitive diode "pixel". Neither will receive the same amount of light.

whether the lens is designed for 6x7 or 4/3.

The lens will give a diffrent FOV too. So, everything basically is different. On 6x7 MF it will give a different FOV, a different DOF, AND a different amount of light is gathered.

So it is a 600F4 lens equivalent with regards to shutter usage (ability to stop motion) and I think that is the relevant point.

No, it is not at all a relevant point. You are shooting with sensors with different sensitivities, yet somehow you place meaning in camera ISO settings giving the same number? Why? One can set the ISO setting to whatever one needs. Set equivaleny ISO settings, obviously. There is no limitation there.

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If you shift the argument to FF can use higher iso with less noise then there will be a long debate on just how much higher can the ISO be with with camera; after all the fuji apsc does quite well compared to the 5d Mk 3; so perhaps we shouldn't go in that direction ?

In which direction are you trying to shift the argument here?