When you are looking for lenses for your BMPCC4k most prefer fast lenses for lowlight and bokeh but how different is the f1.8 and f2.8? I will be specifically discussing aps-c ef lenses which increase the fov and light with a speedbooster. For instance, a 18mm ef lens becomes a 24.3mm and f1.8 becomes f1.27 and f2.8 becomes f1.98 with a .71x speedbooster. The lenses used for this test are: Sigma 18-35mm f1.8 ($700) and Sigma 17-50mm f2.8 ($293 currently on BH Photo).
This unscientific test had the lenses set at 18mm. How much different will they look? Since f1.8 lenses tend to cost a little more than double (Sigma Art series) the proof will be in the pixels. So is it worth it?
As you can see with the lens wide open the DOF is shallow with the f1.8 and has more light with bokeh. The f2.8 has a wider DOF and little to no bokeh and the whole shot is very dark. You may try to save money by going for the f2.8 and if that fits your needs then good for you.
The lenses were set wide open at 18mm because there tends to be a wider DOF but the fast f1.8 lens creates a great natural looking bokeh that will only increase at 35mm.
So the verdict is, if you are looking for a lens that is fast and creates a great shallow DOF that produces bokeh in lowlight then look for a sub f2.0. Although considering many lenses that are f2.8 are considered fast may depend on your needs.
Keep the magic alive,
MT
editor/founder