What is the best focal length for using a DSLR to digitize film negatives?

Question

I have 24-85mm zoom lens with Nikon D750. I am planning to use this setting for digitizing my B&W negative film.

What's the preferred focal length to use for this purpose?

People talk about macro lens, is it really necessary if I already have the above zoom lens?

Answer 1

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:

• What does "magnification" mean?
• How do zoom, magnification, and focal length relate?

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?

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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:

1. The bellows does not have any electrical contacts, so there is no electrical communication with the lens. This means:
2. You won't be able to autofocus (which is fine, because once you set focus distance, you won't ever change it).
3. 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.

4. 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.

5. 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).

Further reading:

• How can I calculate what the effect of an extension tube will be? (Bellows and extension tubes do the same thing: move the lens further away from the camera body to allow for closer focusing / higher magnification.)
• Differences between the PB-4 and PB-6 bellows for my Nikon camera?
• Do I need a macro lens to do macro shots with a bellows unit?
• Review of Micro-Nikkor 55mm 1:3.5 (photographylife.com)
• Macro lenses (Nikon and Olympus) for slide copying (mu-43.com)
• Nikon macro / micro equipment (nicovandijk.com)
• Lots of good ideas here: Let's see your DSLR film scanning setup! (negativelabpro.com)

Answer 2

What's the preferred focal length to use (my 24-85mm lens) for this purpose?

It's not so much about focal length when using a general purpose zoom lens as it is about what type of lens design you should think about using.

People talk about macro lens, is it really necessary if I already have the above zoom lens?

There are a few basic advantages to using a prime macro lens rather than a general purpose zoom lens for digitizing small photographic negatives:

• Macro lenses are optimized for performing best at close focus distances and most are capable of 1:1 reproduction. Most generic zoom lenses are tuned to perform best at much longer than unity (1:1 reproduction) focus distances, and most of them can't even focus close enough for 1:1 reproduction. When imaging something the same size as your camera's sensor, you want to be able to do 1:1 reproduction so that you can use the full resolution of your camera's sensor instead of using just a small fraction of the center of your camera's sensor.
• Macro lenses tend to have significantly better flat field correction than generic zoom lenses. What this means is that when the center of the lens is focused optimally on the center of a flat target (such as a flat negative), then the edges will be more in focus than with a lens not as well corrected for field curvature, which will place the sharpest focus distance at the edges and in the corners slightly in front of the flat subject.
• Macro lenses tend to do less geometric distortion than zoom lenses do, particularly when they are compared to the extremes of the zoom lenses' focal length ranges. Yet to get maximum magnification with most zoom lenses, the longest focal length must be used. Geometric distortion "warps" the subject and makes straight lines appear curved, particularly at the edges and corners of the frame.
• Prime lenses tend to perform better in terms of overall sharpness than similarly priced, or even more expensive, zoom lenses. Prime lenses are optimized for a single focal length. Zoom lenses must balance optical performance at various focal lengths. If we make a design change that gives a 24-85mm lens better performance at 24mm, that same change often causes worse performance at 85mm with gradually degrading performance at the focal lengths in between.
• Roger Cicala, founder and lens guru at lensrentals.com, has noted that most wide-to-normal or wide-to-mild telephoto lenses, such as your 24-85mm zoom, tend to perform worse at the longer focal lengths than they do at the shorter end. He calls it "Roger's Law of Wide Zoom Relativity." Yet the maximum magnification of most zoom lenses, which is important for imaging small things such as 35mm negatives, is only available at the longest focal length, where the lens is not as sharp!

As can be seen above, any focal length using a general purpose zoom lens will be a compromise for doing reproduction work of small flat objects, such as 35mm negatives. To get maximum magnification one must use the longest focal length where such zoom lenses tend to not be as sharp. If you use a shorter focal length, you give up magnification. Even if you use extension tubes or bellows to increase magnification with a general purpose zoom lens, you still must deal with field curvature and geometric distortion. Any increase in magnification using extension tubes or bellows also increases the effects of field curvature and geometric distortion, as well as other aberrations such as chromatic aberration, spherical aberration, etc.

Answer 3

A macro lens is not necessary to get a scan but you get better results with a macro lens because you can get a better magnification level. Whether your lens (and the DSLR solution vs dedicated film scanner) is good enough for your needs, only you can answer that. Essentially you want to be close to the negative to fill the frame but you want to avoid having to correct for wide-angle lens perspective for example. A prime lens is easier to work with because you don't need to worry about accidental focal shift not to be mention the quality will be better. A lot of this will depend on what you want to use the scans for and how many negatives you want to scan. It's cumbersome to use a DSLR especially when you need to repeat the non-trivial precise setup every time you want to scan a negative.

If I had to pick a particular focal length with that lens I might try with 85mm mainly because I don't want to have to worry about focal shift or wide-angle distortion. However that lens would be close to a last resort for me.

Answer 4

I have a Soligor Bellows and slide/film copy attachment with T-mount. Camera was Canon EOS Rebel T6 I found my f4.5 75mm Perfex enlarging lens worked best. I could not set up a El-Nikkor f4.0 50mm to get better than an cropped image of about 80%. I think an enlarging lens is a natural choice but am still experimenting. My light source was a very bright 1 LED flashlight about 10 inches from the slide. The white balance seemed pretty good which I didn't expect. My son with a Nikon PB4 bellows and slide copy uses a flash but I had hundreds of slides and the battery was a nuisance.