Strobe for head&shoulder portraits @ f/2 with beauty dish 40-50cm. Which power?

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Paolus Junior Member • Posts: 35
Strobe for head&shoulder portraits @ f/2 with beauty dish 40-50cm. Which power?

I want to start a new project with head and shoulder portraits in studio, with a special look.

I have a Nikon D750 FF DSRL and want to buy a used Nikkor 135 mm f/2 AF DC for the project.
The reason why I ask your help is that I need to buy my lighting equipment and I am a bit lost at the moment.

A Guide Number "GN Calculator" found online tells that at ISO 100 and f/2 (my choice) and manual varipower at 1/128, the GN of a strobe should be 18 (in meters) to shoot a subject at 0,8 m (2,5 feet) if I am not using a light modifier (like a softbox or other).
The so short distance from the subject as 0,8 m is because I decided to go for a beauty dish of 40 or 50 cm (16-22") as my modifier, and I read it has to be set at one and a half distance its diameter 60-80cm from the subject to obtain its special effect of light falloff. I like a white one with less harsh light and softer transition to shadows, but I am open to suggestions on silver color (no softbox). I decided to go for a strobe and not a speedlight for the useful use of the modelling light.

The question: Which strobe could I buy to shoot at
ISO 100,
f/2,
1 meter max distance but may be half a meter (say 2 feet)
considering a beauty dish (which I don't know how much light it takes away)
??

Is the manual mode the only mode possible and so I should look for such a little (inexistent?) strobe or there is something that I completely miss?

I can't believe nobody uses a professional strobe in a studio with wide open lens and close subject...

By the way, not many strobes reach 1/128 of power reduction. For example the Godox SK 300 II has a GN of 58 and only a minimum power of 1/16 so if used at f/2 means, as per the Guide Number calculator, I should put it out of my room at 7,2 meters from the subject...! It can't be so!!

Thank you very much for your help and sharing experience.

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