BebuLamar

The OP said he doesn't have a flash that doesn't mean a flash is not allowed. If so the OP should rent or borrow a flash if that would help.

davidrb Loc: Half way there on the 45th Parallel

A good (sturdy) tripod is a must for you. Insure no movement of your camera is possible. Use the camera's timer and plan you shots thoroughly. Know your camera's limitations and do not exceed them, especially ISO. Consider using the highest ISO possible but be careful as shadows will create unwanted noise. Remember, you are copying another piece of work. It's shadows become your shadows. Practice shooting in this environment to insure you are ready for your big shoot. This is NOT difficult to do. I've used my 1-DX at midnight with no flash and got great results.

R.G. Loc: Scotland

For people you need to keep the shutter speed up. For exhibits you can use slow shutter speeds but you'll need a tripod.

If tripods aren't allowed in the gallery (sometimes the case), ask about monopods. You won't be able to use shutter speeds that are as slow as when you use a tripod but they will be slower than when shooting hand-held.

Rick from NY Loc: Sarasota FL

A polarizer that robs you of 2 stops of light is the worst thing you can do. And a tripod won’t help at all unless you are shooting the artwork itself.

As someone mentioned, slow shutter speeds will not work shooting people and shooting wide open aperture will undoubtedly give you unsatisfactory results since your depth of field will be minimized.

The answer to your question is really quite simple. Jack up the iso to the point where you have sufficient shutter speed and dof and just shoot handheld. Yes, you will have grain, but a grainy photo is better than a blurred photo.

Grain is not your enemy when you have no other options.

DirtFarmer Loc: Escaped from the NYC area, back to MA

A gray card isn't necessary. A piece of white paper or a 3x5 card will do fine for color balance. The gray card is meant for exposure determination as well as white balance.

When I don't have a gray card in the image I sometimes use a tooth or the white of an eye for reference. A white patch on clothing works fine.

gvarner Loc: Central Oregon Coast

Use Auto ISO and a shutter speed of 1/60 or faster. High ISO will give you some grain but at least you’ll have the photos. You can clean up some of the grain in PP.

ELNikkor

Good that you have a D750 and a 1.4 lens. Don't know how well-lit the gallery is, but at ISO 6400 and f1.4, you should be able to get plenty of good shots. I can shoot down to 1/30 of a second with my D750 and the 24-120 f4 VR lens with no blur, and since there probably won't be any action shots, you should be fine shooting at slow speeds. In less than a minute using the rear screen, you can figure out a suitable ISO/shutter speed. (You might even be able to shoot 1/60 at ISO 1600.) A bit of under-exposure is fine for a dimly lit room, as it makes the photo seem more natural. Leave the white balance on "Auto 1", unless you want the slight orangey effect of incandescent, for that set WB on "Auto 2". Good luck and let us know how it goes!

ELNikkor

(He has a flash on the camera)

TriX Loc: Raleigh, NC

My starting point (subject to revision as you view the results/histogram) and assuming moving people would be 1/125, f2.8-f4 and auto ISO with the upper limit set at 12,800. I would make every effort to use the full DR of the camera and not underexpose (ETTR). You may not want a smaller aperture (higher f stop) as you may want to isolate specific subjects from the crowd. The D750 is good in low light/high ISO, so don’t be afraid of high ISOs - the noise can be somewhat ameliorated in post.

ELNikkor

Right!

dbrugger25 Loc: Raleigh, NC

I would keep the camera on a tripod and keep the camera level so the paintings look rectangular. You can adjust the height of the tripod as necessary. Consider placing the tripod legs on furnature moving wheels that can be bought at places like Home Depot or Lowes. I have been at crowded events where carrying the tripod setup is awkward and it is easy to bump into people and things. When on wheels, the tripod is wasy to move and adjust. The suggestions about ISO, shutter speed and f: stop settings are valid. A remote release might be a good idea. Consider bringing a wide angle lens since you don't know what to expect.

CPR Loc: Nature Coast of Florida

My guess would be your goal should be to capture photos of all the attendees in poses that tell the story. You're not there to make beautiful photos of the art. Set the camera for good DOF and go with Auto ISO. For the art you could use a tripod but most likely handheld would work to produce the product.
I would also ask the person you're standing in for what they would do?

Abo

That's a double edged sword... a polarizer cuts down the light even more...

If you are shooting the exhibits, I have to agree with Dirt Farmer... take a tripod.

Rboo,

having spoken of double edged swords, there is a technique I'm prepared
to share:

A problem you may face, is a constant flow of people between your camera
and an exhibit you want to photograph.

Screw a 4 or 5 stop neutral density filter on to your lens (with a polariser as well if you need/want).
That step cuts down light drastically enough to be able to do a 10 plus second exposure. (and you can use a low ISO to mitigate noise)
With your camera on a tripod, those that walk through your frame are not
exposed long enough to be visible.

If you need to shoot the viewers, and if you can get away with it,
in a firm and commanding voice, exhort "please be still for the camera ladies and gentlemen"
if your subjects comply, activate the shutter... In that case, of course you do not use the filters lol

The below is a 10 second exposure taken around the "blue hour" in the Melbourne CBD;
hundreds of cars and dozens of pedestrians past through the frame while the shutter was open... all invisible.
I used a 5 stop ND Filter:

chasgroh Loc: Buena Park, CA

I belong to the Huntington Beach Arts Council and shoot inside shows regularly. I don't (and wouldn't) use a tripod, usually shoot with an f4 lens, and move pretty fast. You should get plenty of shutter with that f1.4 because, usually, the art, and hence the space, will be pretty well lit. I also don't take any particular WB precautions and depend on post production to take care of that if there are issues (my Nikons handle that aspect decently...LED lites are common backed by flourescent). Sometimes I'll take a phone shot and see what it's getting, color-wise. All that said, the most irritating thing is reflections produced by glass-covered art. Ugh (that is why I went to non-glare plexi for my pieces). So, you may have to find the angle(s) of incidence and shoot accordingly (filters...nah). I did the Nelson-Adkins Museum of Art a few years back with my then brand-new Z6...I literally ran through the place and the results were outstanding (using the back screen to focus and shoot...SO handy) now IBIS had a say in the results, that's for sure, but with your 1.4 you should be fine. Shoot RAW.

So, I just had a thought...my routine is for shooting hanging, flat, art. Of course if you're shooting people, candid, I'm still convinced you'll have plenty of shutter and be able to shoot away...I've shot plenty of 1/80th and been just fine, but don't be afraid to pop your ISO up enough to get better "people" shutter, that 750 should work great.

k2edm Loc: FN32AD

yes, and it seems that the iso on modern digital cameras almost enables you to take photos in the dark, the days of super XX, tri-x & hi speed ektchrome (400) are behind us.