[Brightcolours]

Brightcolours, a f/2.8 is slower than f/1.8 no matter which sensor size.

f2.8 is only slower than f1.8 on the SAME sensor. Or film. The sensor/film defines the speed. The f-value only describes the hole size. If you change the sensor or film, the speed will change. 

That was the reason I wrote "you can't just multiply the aperture by the crop factor" - only if it comes to DOF, but not to a gain of 1.5 f-stops faster shutter speeds, not to a gain of a relatively narrow DOF in wide-angle (for APS-C). To compare that shutter speed, it has to be a 28-55/1.8 for FF!

No, you are wrong there. if you set the cameras to an equivalent ISO value, the exposure times will also be the same. If you think of ISO still as "film sensitivity", you must think and read a bit more on the subject. ISO says nothing about sensitivity in the digital age.

And that sounds much less cheap than a 28-55/2.8 - each manufacturer can do that.

 

I admit, 1.5 f-stops are easily done by contemporary VR/OS devices, one can handhold up to 4 f-stops - but one doesn't get short shutter speeds by VR//OS.

Just change the ISO setting to an equivalent ISO setting, If you use ISO 200 on APS-C, use the equivalent ISO setting of 200 x 1.5 x 1.5 (square) = ISO 450. If you have a sensors with the same amount of sensels (pixels if you want), at ISO 200 on 1.5x crop and on FF with ISO 450 each sensel will register the same amount of light during the same exposure time.

It looks long and thin in the pictures, but given the filter size 72mm, it's a huge piece of gear.