How to do long exposure Nikon D750. I'm trying to do astrophotography?

How to do long exposure Nikon D750. I'm trying to do astrophotography? - 1
Added (1). What on the Nikon D750 is the option to have your lens open for 10-20-30 secs?

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Best potrait lens for APS-C (nikon)?

What lens would be the most shallow depth of field for out door portrait (where space is not an issue)

sigma 85mm 1.4, sigma 50mm 1.4 or 70-200 f2.8

I already own a nikon 35mm 1.8 and sigma17-50mm 2.8

I'm open to other lens suggestions as long as they give a shallow depth of field.

Also how does the depth of field compare when shooting 85mm 1.4 vs 200mm 2.8 when the composition is the same?

Thanks

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Potrait lens for crop sensor (nikon) camera?

I'm currently using a d5300 and would like to be able to take better potraits.

Currently i own the nikon 35mm 1.8 and the sigma 17-50 2.8

I would like to purchase a lens that could give me more shallow depth of field that the lens i currently own.

I'm leaning towards the 70-200 2.8 from tamron for verstility but also interested in the sigma 85mm 1.4 or 50mm 1.4

Would the 70-200 be good enough to give me a shallow depth of field for outdoor FULL BODY potrait? Or would i need a 85mm/50mm 1.4.

THanks

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