Originally posted by konraDarnok Losing a full stop is going change your exposure or ISO. Less of an apples to apples comparison IMHO.
Actually it's
more of an apples-apples comparison. In FOV/DOF, 200 f/2.8 on aps-c almost exactly equals 300 f/4 on FF.
If you want to maintain shutter speed with the FF f/4 lens, you can bump up ISO that stop to do so, and your images should have about the same amount of visible noise, assuming similar sensor generations.
In other words - here's that dreaded phrase!
... 200 2.8 aps-c and 300 f4 FF will create
equivalent images.
.