depth of field control of FX versus DX sensors

[astroNikon]

So I was reading this lens review
Sigma 18-35mm F1.8 DC HSM Review: Digital Photography Review

and came across this

Sigma's choice of F1.8 as maximum aperture isn't a coincidence; it means that the lens will offer the same control over depth of field as an F2.8 zoom does on full frame. What's more, it will also offer effectively the same light-gathering capability as an F2.8 lens on full frame. By this we mean that it will be able to project an image that's just over twice as bright onto a sensor that's slightly less than half the area, meaning the same total amount of light is used to capture the image. This is important as it's a major determinant of image quality. Essentially it means that APS-C shooters will be able to use lower ISOs when shooting wide open in low light and get similar levels of image noise, substantially negating one of the key advantages of switching to full frame.

so the aperture of a 2.8 lens on FF is essentially different on a crop sensor ?
great, even more reason to move to a FF camera, or spend alot of money on high end crop lenses.

 

[PJL]

The aperture isn't different, but FF cameras can get shallower DOF at the same aperture over a crop sensor.