Gear

= Lenses = It's already established that DSLR scanning involves taking digital pictures of 35 mm or type 120 film in a setup where the reproduction ratio is at 1:1 or 1:1.6 or somewhere close. The ratio of the subject size on the film plane (or sensor plane) to the actual subject size is known as the reproduction ratio. The 90% of success depends on the camera lens used in the process. No amount of digital post-processing can compensate for an inadequate lens choice.

If you are completely new to subject matter, this article on Macro photography will be very handy.

The one peculiar thing about shooting at a reproduction ratio close to 1:1, is that not that many lenses produce sharp corner-to-corner and a geometrically undistorted image of the flat original, such as a negative or a transparency. Even great, legendary sharp regular lenses will most likely fail at the very close range.

Regular lenses
"Regular" lenses are those which are designed for shooting at the distances typically many times over the lens focal length.

Macro lenses
Most likely the class of lenses one should be looking at with great attention for the purpose of DSLR scanning is true 'macro' lenses. This is a special class of lenses specifically designed for close-up work, with a long barrel for close focusing and optimized for high reproduction ratios. They may not necessarily be built with flat original in mind, but should still perform reasonably well."It's important to note that some makes of general purpose zoom lenses mark some of them as ' macro'. This is at least misleading as those lenses typically allow to shot at relatively close range, but it is never expected that the original will be flat. Bee on the flower will come up just fine as the frame corners will be blurred and that is just fine too for the purpose of that sort of shots. But for the stated purpose of reproducing flat original those lenses are basically useless." Let's examine one which is already known to be very close to the top of the class: Sigma 2.8/50 macro xd. First of all, this is a true macro lens: it barrel can move optics that far ahead that lens does not need extension tubes. The barrel already marked with magnification scale  from 1:1 to 1:10 making it obvious that 1:1 scale is in its working range out of the box. The lens comes in popular mounts Canon EF and Nikon and... Having dedicated mount means that lens supports such features as autofocus and electronic aperture control which makes shooting with this lens as simple as shooting with any other dedicated lens.

The camera will work in automatic mode leaving one with two responsibilities: advancing the film and watching out for histogram. The lens will work fine with full frame and APS-C form factors without issues. For APS-C that lens beats most of the competition as it has to work in comfortable 1:1.6 scale. The lens no longer manufactured for Canon mount but still can be relatively easy found on secondary market for around $120 (as of Oct, 2019)

There is also a number of other lenses, but often they are not available in the native mount and while adapters can be easily found almost for any lens-camera pair the automatic features become unavailable. The later is not a showstopper but still adds certain extra burden to the process.

There is only one considerable drawback in Sigma lens - it's relatively short focal distance leaving little legroom in front of the lens to the film holder, light-protecting  hood, etc. Say using Sigma 50 mm with the popular and well made Nikon ES1 film holder becomes problematic at 1:1 scale because of the length of ES1 barrel - and we are talking just about extra 2-5 mm!

Anyway if one pays attention and does not fall for marketing gimmicks there is a number of true macro lenses available on the market.

Wikipedia has a list of macro lenses at https://en.m.wikipedia.org/wiki/Macro_photography in the section titled "Macro photography lenses".

Enlarger lenses
The next class of lenses which are relatively cheap and widely available on the second -hand market (eBay.com) is enlarger lenses. Those lenses have been typically designed to work with the flat originals and reproduce them properly in the range of scales from 1:2 to 1:20. There is a myriad of those lenses which differ in their quality and price. One word of caution: vintage lenses may be very good for  b/w work and produce horrible results in color. One really need to consult with the already expansive collections of data re lens properties before committing to one or snatching  one on the eBay auction.

Enlarger lenses often come in M39 mount, but may come in variety of mounts and one should pay attention and be ready to employ some sort of adapter. They also lack any focusing mechanism and may have a flimsy plastic front - which is perfectly adequate to control aperture but may not be suitable to carry any mechanical function like supporting the film holder. The front thread diameter may be anything like 37 mm or 43 mm and finding step up rings may be an issue.

Special purpose lenses
Number of devices used for film scanning has internal high quality lenses which enthusiasts have been able to extract and repurpose.

Lens comparison
One of the most authoritative sites with lens comparison data is http://coinimaging.com/macro_lens_tests.html?

External resources
= Cameras =

Camera form factor
The physical size of the camera sensor has a critical role in DSLR scanning from engineering point of view. The selection of the lens becomes somewhat easier as the reproduction ratio drops to 1:1.6. A good 50 mm lens will shine in that setup, as corner sharpness can be easier to achieve.

Point-n-shoot
= Light sources =

A note about color temperature
What we perceive as light is a mixture of waves of different wavelengths. The shorter the wavelength, the more reddish and yellowish, the longer, the more blueish the perceived light color will be. The color temperature is measured in Kelvin, so typical examples are:


 * 2700K: Warm white, e.g. traditional light bulbs
 * 3200K: Halogen bulbs
 * 4000K: Neutral white, e.g. Fluorescent lamps
 * 5000K: Morning and evening sun
 * 5500K: Forenoon and afternoon sun; also photographic flash lights
 * 5800K: Noon sun
 * 6500K: Overcast sky

Color negative and slide films are commonly sensitized to “average daylight” with 5500K per definition, and this is why photographic flash lights produce light with 5500K to 5600K.

Regardless of the light source we choose for digitizing negatives, a color temperature of 5500–5600K is a basic requirement.

Note about Color Rendering Index (CRI)
While the color temperature describes the overall “yellowness” or “blueness” of light, the Color Rendering Index (CRI) is a measure for the accuracy of color rendition. Artificial lights may not have a more or less yellowish or blueish character but also may have “gaps” in certain areas of the visible spectrum where no light is emitted, as opposite to natural sun light with its gap-less spectrum. The CRI thus indexes natural sun light with 100% and any other light source usually with smaller values, depending on its spectral gaps.

Gaps in the spectrum may cause dull or alienated colors which is bad for digitizing negatives: It is then nearly impossible to restore the original positive color the original negative color stood for. So our basic requirement for a digitizing light source is a high CRI light source. The better ones advertise themselves with “High CRI” or “CRI >90”, really good ones reach a CRI of 95 or even 97. One should stick with these and ignore those not telling anything about their color characteristics.

LED illumination
Video Lights are powerful light sources which helps keep expose time short. The only true requirement is a high CRI value.

Other
= Stands = Enlarger stands

Copy/repro stands

Optical bench

= Film holders = Film holders from flatbed scanners

Film holders dedicated to camera scanning

Negative holders from enlargers

= Macro bellows =

= Cardboard products for scanning with the smartphone =