I think some confusion came in when in 1 paragraph I mentioned two options, for lighting, not two for AF Assist. Flash and Assist are not related other than when the SB700 is mounted in the shoe, its IR 3 LEDs are used instead of the visible light camera Assist Light which is a bright white light.
The advantage of using AF Assist is simply for faster and more accurate AF with much less if any hunting in low light. Using a faster maximum aperture lens does about the same thing, allows more light hit the focus sensor. A camera meters and uses the AF and VF at maximum light gathering, the widest aperture with the lens iris fully open, then, when the shutter is triggered, the aperture closed down to reflect the calculated exposure. So a f/1.8 lens, even is taking the image at f/5.6, meters and focuses at 1.8. So a fast lens focuses with hunting, in lower light than a slower lens,at f/3.5 as maximum aperture. So if you got the 85 1.8, everything related to taking the shot will be better than a f/2.8 lens regardless of what aperture is used for the actual exposure. So getting that 85 1.8 will improve everything; metering, AF, manual focus, VF etc. even if you shot at f/8

Stan
St Petersburg Russia

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