I've spent years shooting with the D750 and honestly, the 85mm is the sweet spot for portraits on that body. The 24-120mm is a decent walk-around lens but it can't touch the separation you get from a fast prime. For your $750 budget, you have two solid paths that will definitely impress your clients for that Seattle shoot.

• The Nikon AF-S NIKKOR 85mm f/1.8G is lightweight and tack sharp. It leaves you with extra cash for batteries or cards too.
• If you want the absolute best rendering, look for a used Sigma 85mm f/1.4 DG HSM Art for Nikon F. It is heavy but the bokeh is noticeably creamier than the Nikon 1.8. Quick tip: when you're shooting that big family session, dont stay at f/1.8 the whole time. Stop down to f/2.8 or f/4 to make sure everyone's eyes are actually in focus. The lens is sharp enough that you'll still get a great blurred background even at those apertures.

I remember when I first started out with my D750 and I bought a used lens that turned out to be a total dud. It really soured my experience for a while, so I totally get the hesitation about spending that much cash on something new. For a big family session in Seattle, you really need to be careful with your focal length. I once tried to shoot a group of twelve in a small park with a long lens and I literally had to stand across the street to get everyone in. It was super awkward trying to shout directions over traffic while people stared. If you are doing larger groups, you might actually find the 85mm a bit tight depending on the location. Here are the ones I have stuck with for reliability and safety:

• Nikon AF-S NIKKOR 85mm f/1.8G is probably the safest bet for the money. Its light and the focus is snappy enough for kids running around.
• Sigma 50mm f/1.4 DG HSM Art gives that professional look you want, but it is heavy as a brick and sometimes feels a bit unbalanced on the body.
• Nikon AF-S NIKKOR 50mm f/1.8G is the budget king if you want to save some of that 750 bucks for extra batteries or a faster SD card. I personally think the 85mm is the way to go for the separation you want, but just make sure you have enough space to back up. I bought mine new for around 450 and it has never let me down during those high-pressure shoots. Just be wary of buying used gear right before a big job... I did that once and the autofocus motor died mid-session. Stick with a new one so you have a warranty in case it acts up during the shoot.

To add to the point above: you might want to consider the working distance too. I used to love my long prime for the look, but I ended up stuck in a corner during an indoor shoot and couldnt fit everyone in the frame. It was super awkward. Are you planning to be in a tight indoor space or out in a park for this?

This thread is gold. Bookmarking for future reference 🔖

@Reply #3 - good point about the spacing. Unfortunately, I have had issues with some of those older f/1.8 primes recently. The longitudinal chromatic aberration is honestly not as good as expected when shooting wide open, especially in that flat Seattle light. It is really disappointing because you expect that creamy look but sometimes you just get purple fringing on high-contrast edges instead. Directness is key here: I think IIRC the AF motor speeds on the older G series glass are actually slower than some of the professional zooms, which might be why you are struggling. Not sure but someone told me the focus hit rate on the D750 drops significantly with certain third-party primes unless you are using the center point only. It is a bit of a gamble when you have a big client session. Just be careful with going too long because you will literally be a block away trying to shout directions at a family of ten...

^ This. Also, since you mentioned a big family session in Seattle, you really gotta think about focus reliability. I have used a ton of glass on my D750 and honestly staying native is usually the safest path if you dont want to spend your Friday night calibrating third-party lenses.

• Nikon AF-S NIKKOR 85mm f/1.8G: This is the safe bet. It is super light, focus is quick, and it is way under your budget. The bokeh is plenty creamy for professional work and you wont be exhausted after an hour of shooting.
• Sigma 85mm f/1.4 DG HSM Art: The images are stunning, for sure. But it is a heavy beast. If you are moving around a lot with a family, your wrist will feel it. Sometimes these need a bit of fine-tuning with a USB dock to hit focus perfectly on a D750, which is a bit of a gamble when you have a tight deadline.
• Tamron SP 85mm f/1.8 Di VC USD: A solid middle ground. It has weather sealing for that Seattle mist and the stabilization is nice for lower light. I would just get the Nikon 1.8G. It is reliable, sharp, and basically a standard for a reason. You dont want to risk technical drama right before a paying gig.

Would love to know this too

Wow ok that changes things. Gonna have to rethink my approach now.