Assuming you're scanning 35mm or medium format negatives, and that you want to use the full resolution of your DSLR to digitize your negatives/slides, you can't do it with your setup (D750 + 24–85mm Nikon lens) alone.
Why? You have to think in terms of magnification (also called reproduction ratio). See also:
Your Nikon d750 has a full frame sensor, meaning it's the same size as a 35mm negative frame. For maximum resolution of the digitized negatives, you want them to fill the DSLR's sensor area. So you're looking for a 1:1 reproduction ratio.
Your lens has a maximum reproduction ratio of 0.22x, which means that when your lens is set up at the minimum focus distance (0.38 m / 1.25 ft) at the longest focal length, a 35mm negative/slide will only fill slightly less than a quarter of the width and height of your camera's field of view. Your D750 has 24 megapixels (6016 by 4016 pixels), so after cropping, your digitized images would be (6016 × 0.22) × (4016 × 0.22) = 1324 × 884 pixels = 1.17 megapixels.
For comparison, a flatbed scanner at 1200 DPI would produce images at 1700 × 1134 pixels, or about 1.93 megapixels.
See also: What are the major differences when digitizing slides/negatives with a DSLR vs. a scanner?
I have digitized a few thousand old slides, film, contact prints, etc., my father inherited from his parents. I used:
- My Nikon D800E DSLR
Nikon PB-4 bellows with PS-4 slide copying adapter, about $80 on eBay. Here is my PB-4 bellows with PS-4 slide copy adapter attached to the right end:
Nikon Micro-Nikkor Auto 55mm 1:3.5 lens, about $90 used. Here is the setup with the Micro-Nikkor 55mm lens attached to the bellows. Notice that the PS-4 slide copy adapter has its own little bellows that meets lens and attaches to its filter ring threads, to keep stray light out of the light path:
a daylight color-temp incandescent lamp to backlight the slides when scanning (or daylight from a window)
- some scrap lumber and 1/4–20 nuts and bolts to make a solid table mount for the bellows base
Some points of note:
- The bellows does not have any electrical contacts, so there is no electrical communication with the lens. This means:
- You won't be able to autofocus (which is fine, because once you set focus distance, you won't ever change it).
- You won't be able to control the lens aperture from the camera. This is fine if using any Nikon D lens (i.e., with an aperture control ring) or older, such as the Micro-Nikkor 55mm 1:3.5. This means you can't practically use a Nikon G lens such as your 24–85mm ƒ/3.5–4.5G(which does not have an aperture control ring) or the newer electronic aperture "E" lenses. But you wouldn't want to use your zoom lens on the bellows anyways.
Important: the Micro-Nikkor 55mm 1:3.5 (and other Nikon lenses of the same vintage) are non-Ai, so make sure it has been converted to "Ai". Otherwise, you could physically damage your D750.
See also, Can I use a Nikon 50mm f/2 pre-AI on my Nikon D5100? (different specific lens, but same same vintage, same issue). Please see the link in mattdm's accepted answer to that question.
- Whatever lens you use when doing this,
- Set your camera to full manual exposure, and manual focus (if not using a bellows)
- Focus using your widest aperture
- Once you've set focus, tape the focus ring so it won't change
- Stop down the aperture a bit to the lens's sharpest aperture (ƒ/5.6 for the Micro-Nikkor 55mm 1:3.5, ƒ/5.6–8 for your 24–85 ƒ/3.5–4.5G)
If you have never done any macro photography, or used bellows or film/slide copying setups before, it takes a bit of fiddling to understand how focusing, magnification, and focus distance interact. But for digitizing several hundred slides and negatives at high resolution with your DSLR, this is the recommended setup (bellows, slide holder, cheap old 50–65mm macro lens).