I think a lot of comments - particularly those that offer advise on improving technique - totally missed the point. If a given lens is employed at a particular focal length for a particular scene using the D850, the photographer will have a larger image area than would have been available had he instead employed the same lens at the same focal length against the same scene using the D500. The added image area can be used by the photographer to either capture more of the scene or, as suggested by the OP, to allow for more flexibility in assuring that a dynamic subject does not fall out of the field of view. If the D850 is constrained to be cropped to the DX image size, it does have a very slight disadvantage with respect to resolution as compared to the D500. So, if the shot is taken on the D850 and cropped to a DX size, the resolution would be just a bit less than would have been available using the D500 without any cropping. The OP asserts that the benefits of the D850's expanded field of view greatly outweigh that. I agree.

Some of below is a bit dated, but I found a lot of good info on Nikon flash from this blog series.

http://nikonclspracticalguide.blogspot.com/2008/01/nikon-flash-two-separate-metering.html

maybe below?

https://www.amazon.com/Camera-Bracket-Cameras-Stereoscopic-Photography/dp/B004HK0RXY

...Like many here, I've recently been investing in more capable equipment and making an effort to learn more to help get better results. Regarding focus, I recently went through Steve Perry's ebook on Nikon's autofocus system that I found very useful on the subject. His book covers a full spectrum of Nikons - including D3xxx and D5xxx in the less than $500 range. While I still haven't developed high proficiency in dynamic focusing, I still found much of the information in his book to be very useful. One thing to note is, as recommended in the ebook, I very often will focus static scenes via live view. Live view lets me move the focus point where ever I like and, while slower, can be more accurate than focusing through the view finder. This makes it easier to find a point of focus roughly 2/3rds of the way into the scene. Steve also talks about using a laser-based range finder for that, but I haven't yet explored that option.

It surprises me that many comments indicate that center point focus is almost exclusively used by many. A few comments on that:
- if the intent is to get sharp focus over an entire scene, then isn't best approach to (1) select an aperture that provides a large depth of field; and (2) set the focus point near the hyper focal distance (rule of thumb at roughly 2/3rds into the scene)?
- if there a particular subject in the scene that is intended to be sharply focused with background not sharply focused (shallow depth of field), then shouldn't the focus point be precisely set of that subject (for example, on the eyes of a person in a head shot)?
- if the subject is not in the center of the frame, but compositionally positioned elsewhere (e.g., rule of thirds), wouldn't it be preferable to directly focus the way the shot is composed instead of focusing with center point on subject, locking in the focus, and then recomposing (e.g., focus will not take into account any subject movement after recomposing)?
- When using matrix metering, the exposure is determined based on a combination of the point of focus and total scene. Therefore, if exposure is locked with subject at center of frame and then recomposed, wouldn't exposure be incorrect? Conversely, if exposure is not locked, and recomposed scene maintains focal point at center frame (not on subject), wouldn't exposure still be incorrect? (It seems as though spot metering and locking in both exposure and focus would be required when recomposing after focusing with subject in center frame).

It seems to me that there are two separate topics that are being discussed here. I interpreted the OP question to be relative to camera autofocus characteristics - how many focus points are available and how many of those focus points are "cross focus points." Some of the answers seem to, instead, be talking about the selection of focus points in the pictures that they take. Cameras with better autofocus systems provide a greater number of overall focus points as well as a greater percentage of them that are cross focus points. However, in virtually all still photos, standard advice is to utilize a single focus point when focusing on the scene. For action shots, there are a variety of multiple focus point selection strategies that provide better results in cases where it is difficult to maintain a single focus point on a dynamic subject.

For Nikon DSLRs, the last three generations of autofocus capability are probably just fine for most photography. For use with wildlife photography or other dynamic subjects (e.g., sports), a camera that offers advanced autofocusing capability (such as the 153 focus point system on D500/D850) may be a better choice.

OK, repeated my test with better results. Turns out that that switching out between the longer Tamron lens and shorter Tokina lens shifted the point of focus. For both, I focused in live view at the exact center of the image. On the Tokina, this was the top of a bird bath and on the Tamron, this was leaves on a tree several feet behind the bird bath. I should have selected a scene without a large range distance amongst objects near the center of the image. In the original setup, the Tamron is not as sharp at the birdbath but much sharper on the leaves on the tree.

I just bought it last week at Hunt's Photo. Its only my second full frame lens to go with my D810 and Tokina 24-70 F2.8. I wanted a quality 70-200, F2.8, but had ruled our the newest Nikon based on price. I though about a used older Nikon, the prior version of the Tamron, or this one. Tried both Tamrons out and ended up getting the newest one. Liking it so far - mostly used for grand children closeups. I didn't get the Tamron Tap In Console, and haven't applied any AF fine tuning for the lens. I did do one test from common tripod mounted setup switching between the 24-70 Tokina and the 70-200 Tamron with both set at F2.8, 70mm focal length, spot metered, and focused at a common point about 100 feet away. Using aperture priority, ISO 800, the shutter speed came in at 1/800 with the Tokina and 1/1000 with the Tamron. I'm going to repeat the setup in manual mode with identical exposure settings and focused at closer range. Anyway, with the one test that I did, the 24-70 Tokina is clearly sharper. It is likely that the 70-200 Tamron would benefit from tap In calibration or AF fine tuning.

Posting full downloadable image with EXIF data could show whether focus was correct or not. Otherwise, any response would be just an opinion or guess regarding whether or not you missed the focus vs image blur from low shutter speed. Since, as you say, "the focal point was not staying on her," my opinion is that focus was an issue.

If you post the image for download, the Nikon EXIF data can be looked at - that can typically provide the details of the focus for the shot and whether the intended subject was properly selected as the focus point

interesting. I had never thought of using DX lenses on D810 without auto DX crop. It might be useful - particularly when using longer focal lengths. Here's two photos using 55-300mm lens set to 300mm focal length - these are both SOOC - first with auto DX crop off and second with auto DX crop on.

(Download)


(Download)

Before cloning, you might start with a content aware fill. First, I would create a layer that totally excludes any of the two people that you want to keep in the photo. To create that layer, you can do a fairly quick content aware selection that encompasses all of the image area that contains the two people you want to leave alone. Then, select inverse, then CTRL+J to create a new layer on top of the background that excludes these two people. Then, hide the original (background/full image) layer. On the new layer, use lasso tool to completely select the woman that you want to delete. Select fill, content-aware. She is gone and replaced with content within that layer. You can then restore the background layer visibility and refine the area that content-aware replaced via the clone tool where necessary.

As per below post, the situation being observed may not be at all related to the use of auto ISO, but, rather, the result of a reduced shutter speed that is being avoided when the operator manually sets ISO. For example, assuming aperture priority mode and manual ISO set to 400, the exposure might have the SS set to 1/100 sec. If auto-ISO were set up with a minimum SS of 1/50 sec and an ISO range starting at 200, the exposure for the same shot would result in a SS set to 1/50 sec at ISO 200. This slower SS could be the cause of the problem. In fact, auto ISO is often used to prevent the selection of a shutter speed that too slow for a hand held shot. This is done by setting the auto ISO minimum shutter speed to a value that is fast enough to avoid negative effects from camera shake. I don't often use auto ISO, but have used it to assure selection of an adequately fast SS in Aperture Priority mode.

I interpreted the recommendation regarding separation of subject from background differently than stated below. I don't believe that it was in reference to a potential for the camera's auto focus process to mistakenly focus on the background instead of the subject. Rather, I believe that it was in reference to achieving a more pleasing image by having a subject sharply in focus and a background out of focus. When properly used, the camera'a auto-focus process is more of a semi-auto process in that the photographer should be selecting the focus point in the viewfinder (typically locating the focus directly over the subject's eyes). Whenever I find that a shot that I've taken was focused on the background and not on the subject, it is almost always because I did not correctly position the focus point on the subject when the shot was taken.

as was noted, a full frame 70-200 lens is most likely a higher quality lens than a 70-300 lens, so reduced IQ would typically be expected from the 70-300. Other differences might include slower time to reach focus, reduced bokeh effect at a given focal length, and reduced image stabilization. A 70-300 on a full frame would, of course, have a subject framing capability that is a superset of the framing capability of a 70-200 on a full frame. Note, however, that, a crop sensor would need to cover roughly from 45mm to 140mm to provide that framing capability. Having used a crop sensor setup for a few years now, I recently invested in a full frame with a 24-70 lens. My next lens investment will be a 70-200.

For various portraiture framing (e.g., close up face shot, head shot, head and shoulders, upper body, full body), the distance to the subject can be varied along with the focal length. Tony Northrup has indicated a preference for the longer 200mm focal length for some of the closeup shots. For closeups such as head shots, he indicates a preference to keeping a relatively longer distance to the subject and using the 200mm focal length vs getting closer to the subject and using a shorter focal length. You could experiment with your setup to see the differences. You might also take a look at the calculator below.

https://www.pointsinfocus.com/tools/depth-of-field-and-equivalent-lens-calculator/