All of the above... That was easy
The longer lens, bigger aperture and sensor will give you more separation... Then the min focus distance also comes into play too... Robin Wong uses close focus a lot to drop the background with m43.
Usually 35-50 is better with body portrait and then you can go up for the others as tele lenses squeezes the subject with tighter view vs 35mm will give you a bigger nose, but sometimes it doesn't matter
25mm f2.8
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50mm f2
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100mm f2
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Here are some people shots, mostly not posed:
m43 25mm 1.8 ~ 50mm 3.5 equiv
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5D + 50mm 1.4
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55mm 1.8
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m43 45mm 1.8 equiv 90mm 3.5
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m43 42.5mm 1.2 ~ 85mm 2.5
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85mm 1.4 where the dof gets tight with 2-3 people shots
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135mm f2
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m43 75mm 1.8 ~ 150mm 3.5
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apsc 135mm ~ 216mm 3.2
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Tighter shots
apsc 85mm 1.8 ~ 136mm 2.8
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135mm apsc ~ 216mm f3.2
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It was a very long post, but it is best to illustrate it w/ photos. I see some pros use 70-200 2.8 full frame as it gives you range and enough dof to work with, but I prefer the smaller primes... I replaced 85mm/135mm with 100mm when I started to use mirrorless, but 135mm f2 has a lot going on. Sony 135mm 1.8 is even heavier at 2.2lbs~1kg... Oly 75mm is the only one close to FF equivalent... So pick your poison
Here is a pro specialized in a studio as a headshot photographer:
Video: Behind the Scenes with Peter Hurley, the Headshot King
For a decent headshot, I generally like a minimum of 85mm (I prefer 100 or higher though) for full frame DSLR’s and 100mm and above for medium format cameras. Generally 100mm is what I roll with on a full frame DSLR, so have something that can hit that range if you are on a zoom or a prime in that neighborhood.