[Brightcolours]

f/4 on MFT meters the same as f/4 on FF. Similarly an f/4 lens on APS-C or medium format would meter the same. The sensor size is irrelevant.

Sensor size is NOT irrelevant. What is irrelevant is what you are saying (that "f/4" meters the same").

A lens does not meter. Would you give ISO 100 film and ISO 640 film the same exposure duration? No?

Why is that?

Maybe because they have different sensitivities?

Ok. So, why are you adamant in exposing sensors with different sensitivities the same? Sensors with smaller pixel/sensel/photo sensitive diode pitch have different sensitivities).

 

Point is: ISO is NOT what it used to be with film. One does not set a meter to meter for a certain film sensitivity anymore. Now one simply changes the ISO setting for whatever a certain image requires, and ISO is not about sensor sensitivity.

So don't bring in "metering at f4" as argument, it basically is a non-argument. Set the cameras to equivalent ISO settings and "f4" will NOT give similar exposure results. Nor similar DOF results.

Relative to the sensor size, the amount of light hitting the MFT sensor is the same as a f/4 would be on FF.

You are saying: the light hitting the MFT sensor is LESS than the light hitting the FF sensor. That is because the MFT sensor is so much smaller (4x less light). Also: suppose the sensors have similar amount of pixels/sensels/photo sensitive diodes. Then the MFT sensels collect 4x less light energy, due to their smaller size. So, both on image level, and on pixel level, the MFT sensor will collect less light at f4.

Consequently, the exposure time is similar.

No, exposure is not similar at all. See above. (Less light per image. Less light per pixel. Difference in sensor sensitivity. You just do not realize that because you misunderstand what "ISO" means in the digital age.)

This is what I meant to say.

 

What really matters is that a 300mm f/4 MFT lens acts as a 600mm f/4 FF equivalent lens in terms of exposure

No, it does not. Get that ISO 100 on MFT does NOT mean the same as ISO 100 on FF. The amplification to the signal will NOT be the same. Just like with film, where a bigger grain gives a higher sensitivity, a bigger sensel will collect more light too. So, would you say that ISO 100 film in a 4/3rds size film camera and an ISO 400 film in a FF film camera get equal exposure, with f/4?

I think you will not. Yet that is what you are saying about MFT vs FF sensors. 

 

With film, ISO says something about sensitivity. With digital, it does not. So, stop looking at ISO as a set thing. That is not valid anymore. With digital, ISO is just about getting more or less similar results REGARDLESS of sensitivity, by applying different amounts of amplification and/or sensor voltage for different sensors.

and f/8 in terms of DOF. That makes it for a hand hold-able system which is not the case when considering FF.

That is nonsense.... Why can you handhold 300mm f4 at ISO 400 and not 600mm f8 at ISO 800? Makes no sense.... One REALLY can change ISO settings to whatever one needs. Or even have the camera do it itself with Auto ISO setting.

 

There is no reason what so ever to set different cameras at the same ISO setting number.