That is one of the most difficult assignments possible. I worked at that for years...

The Nikon flagship DSLRs starting with the D3S are the best bodies to use, each being slightly better than the previous model and all being significantly better than any other camera. Today a D5 is the best if your budget can deal with it, and a used D3S is the least you want.

Keep in mind that the best DX body will never be able to close the gap. Don't even think about it.

There were significant improvements with each model up to the D3S, but only small improvements at each step along the way since. If money is not tight the D5 is worth it, but the image quality difference is not as great as the cost difference.

I am retired, so the D5 is definitely out of my price range now. I am the "official event photographer" for the high school, and I know they would not pony up the money for that body. I am drooling over a D3s because I have a friend with one of those and I have been envying his images every time he shoots a concert or a play. His low light images have almost 0.0 noise and the sharpness and color rendition is just like being there. I keep hoping that a UHH member will want to sell his D3s. I regret not buying one of those when I was working and had more disposable income, so I keep my fingers crossed that one with a low shutter count will pop up.

ANY recent APS-C DSLR or mirrorless camera will simply blow away ANY film you can buy to use in your old SLRs. Below is a test shot I did at ISO 16000 with one of my 20MP, APS-C format Canon 7D Mark II cameras:



Sure, if you look closely at the enlarged detail you'll see some noise, but that's a lot larger than I'd ever print this image and IMO the "noise" is pretty well controlled. This image was done without flash, by the light of a single 60 watt CFL bulb and a small window, each roughly 8 to 10 feet from the subject. Lens was a 100-400mm zoom at 158mm and f/5, handheld with image stabilization that made possible 1/125 shutter speed at ISO 16000. Image was shot RAW with care not to underexpose (which always amplifies the appearance of noise), and was post-processed in Lightroom 6 with default level of noise reduction.

I regularly use my Canon 7DIIs at ISO 3200 and 6400, without concerns. Sometimes, when necessary, I'll use them even higher at 8000, 12800 and 16000 (can be pushed higher, but I don't due to noise). With the highest ISOs I usually do extra noise reduction in Photoshop (with a Noiseware plug-in). But, again I deliberately DID NOT, do any special noise reduction for the above test shot. When I first got the cameras, I did that test in order to see "worst case" at ISO 16000.

Besides, try pushing ANY film to ISO 16000!

"Full frame" cameras with modest resolution and bigger, less "crowded" sensors are even better for ultra high ISO. Since you've probably already got Nikon mount lenses, I might recommend 24MP Nikon D750 or a 36MP Nikon D810 (depending upon your budget). Your lenses also can be used via simple, inexpensive adapters on Canon DSLRs such as their full frame 26MP 6D Mark II or 30MP 5D Mark IV (or earlier versions in either of those series, which might now only be available used or refurbished). If you wanted a little bit smaller "mirrorless" camera instead, Nikon and Canon don't offer one that's full frame yet, but Sony makes some full frame mirrorless that can be used with vintage Nikon F-mount lenses via simple adapters. Their A7-series models are more affordable and a modest 24MP, while their A7R-series are higher resolution models that tend to be more expensive.

Pentax also makes excellent full frame cameras now, but AFAIK you wouldn't be able to adapt and use your existing lenses on them.

Nikon makes a number of APS-C cameras (D3000-series entry level models, D5000-series step up models, D7000-series and D500 advanced amateur and pro-oriented models). Those can also be used with your current lenses, except they will be subject to a 1.5X "lens factor" due to the smaller size sensor. Lenses will "act longer" than you're accustomed to.... For example, a 50mm standard lens on your film camera becomes a neat, short telephoto and "acts like 75mm" on these crop sensor cameras. This is great for telephotos, but not so great for wide angles. The smaller sensor cameras also won't be as "high ISO/low light capable" as the full frame, although the recent models can run circles around any film you might use in your SLRs.

It's also possible to adapt vintage Nikon F-mount lenses for use on Canon APS-C DSLRs or mirrorless cameras (with a slightly different 1.6X "lens factor"), or to use them on APS-C Sony and Fuji (both 1.5X, same as Nikon APS-C) or on micro Four/Thirds format Olympus and Panasonic mirrorless (2X "lens factor"... 50mm will "act like" a 100mm lens would on your SLRs).

Another consideration with modern lenses will be a camera's ability to autofocus in low light conditions. Some of the latest models are really good at this.... I know Canon 6D Mark II (full frame) and 80D (APS-C), for example, can still focus in as low as -3EV (essentially "moonlight"). This won't matter with vintage manual focus lenses, of course. HOWEVER, you instead need to be able to see to focus manually, and many (most?) DSLRs optical viewfinders and focus screens simply don't have any of the manual focus assist features like were found in older SLRs. For manual focusing the mirrorless cameras might be helpful, because many of them have electronic viewfinders that can both amplify the brightness of the view in low light and commonly provide manual assist features such as "focus peaking".

So, depending upon what you want to do... if you want to continue using some favorite lenses, whether or not you mind if those lenses are effected by the "lens factor" of a smaller format sensor, and just how low light you need/want to shoot in, you have a number of possibilities to choose among.

EDIT: Yes, Nikon Df would be a neat choice, too. It's full frame and "retro" styled to look and operate somewhat like 1980s FM2n or FE2 models, for example. Though I don't use one, I love the look and feel of the Df. However, you gotta REALLY want that retro look, because Df are VERY expensive for a 16MP full frame camera! You can get newer, higher resolution, better low light performing cameras for the same or considerably lower cost! A chrome Df body only costs $2100... a black version is $2750. In contrast, a D750 (50% higher resolution) costs about $1700... Or a D810 with more than 2X the resolution is selling for $2800. (Both those are only offered in black versions.)

To make your $ go a little farther with any of these, you might consider refurbished or quality used from a trustworthy store. D810: $1700. D750: $1400. I also found a used Canon 5D Mark III (22MP) for $1000 and 6DII for $1400, original 6D (20MP) for as little as $800. Df (16MP) simply aren't as common used or refurb'd, I did find a black one used for $2000 and some silver ones for as low as $1500.

When it comes to the cost of DSLRs and all digital... The initial expense is considerably more than comparable film cameras (digital cameras are basically powerful little specialized computers). But you will no longer have the ongoing cost of film and processing. Some people say "digital is free".... which isn't really true because you still need to buy memory cards, hard drives for storage, computers and software for post-processing.... and will wear out cameras and repair or replace them eventually. However, it's considerably less expensive than film!

I am retired, so the D5 is definitely out of my price range now. I am the "official event photographer" for the high school, and I know they would not pony up the money for that body. I am drooling over a D3s because I have a friend with one of those and I have been envying his images every time he shoots a concert or a play. His low light images have almost 0.0 noise and the sharpness and color rendition is just like being there. I keep hoping that a UHH member will want to sell his D3s. I regret not buying one of those when I was working and had more disposable income, so I keep my fingers crossed that one with a low shutter count will pop up.