All About Lenses

The lens is critical because it's primarily responsible for image quality across the frame. A good lens can capture sharp, clear images and minimize distortions, color fringing,  ensuring the fine details in your film are preserved consistently from center to edge. 

For film digitization, you want a lens with flat field focus to ensure edge-to-edge sharpness. A lens with a high resolving power will capture the fine details of your film. Also, consider a lens with low chromatic aberration to avoid color fringes on high contrast edges. 

Certainly there is. Richar Karash spent huge efforts to test the majority of lenses available today. That includes old vintage enlarger lenses, process lenses and lenses from all modern brands. The must-read Richard's article "How good a macro lens do you really need for dslr/mirrorless camera scanning?" is available here. The list is also available here. 

If you are looking for the quick answer, the Sigma 2.8/70 Macro Art lens is one of the best true macro lenses and it is produced in mounts for Canon, Sony, Nikon, etc. So you get fully mechanically and electronically coupled lens which extends to 1:1 scale and you are ready to digitize the moment you mount the lens on your camera. By all means this is not a cheap lens (US$469) though and it's mentioned here just as an example.

Here is the top 8 lenses out of 56 from Richard's list. As you can see this is a mix of camera lenses and specialty lenses (Tominon, Rodagon, CoolScan):

50 mm f/2.8 Sigma EX DG macro

50 mm f/4.5 Tominon

55 mm f/2.8 Micro-Nikkor AI or AIS

70 mm f/2.8 Sigma Macro ART

75 mm f/4 APO Rodagon-D 1x Copy Lens

75 mm f/4.5 APO Rodagon-D 2x Copy Lens (normal orientation)

90 mm f/2.8 Sony Macro

100 mm f/2.x CoolScan 8000/9000 lens 

Read the article to really get sense what to look at when evaluating the lens.

You should witness the debates over which lens is the best for camera scanning! Sometimes the debate is even more intense than the Nikon vs. Canon or Android vs. iPhone discussions!
Now you can be the judge! Below are actual images captured with a full-frame Canon EOS R camera compared to Vlads Test Target on 35mm film. Google Drive folder is open for anyone with this link: https://drive.google.com/drive/folders/1b9pWfaZls2q8LU8PfKq6HLuatLLuX5N1?usp=sharing

Check out the major players in the game along with their approximate second-hand prices. (Note: the Sigma 2.8/70 Macro Art is the only one still available new.)

APO Rodagon D 2x 4/75         $250

Tominon 4.5/50                       $50

Sigma 2.8/50 EX DG macro    $150

Sigma 2.8/70 Macro Art         $470

Componon S 4/80                    $80

Componon S 5.6/100                $80

Carl Zeiss Micro-Planar 2/100 $900

Missing from comparison are very popular lenses 55 mm f/2.8 Micro-Nikkor AI or AIS,90 mm f/2.8 Sony Macro, Canon 2.8/100 mm Macro , Canon 60 mm macro - the author simply didn't have access to those.
For each lens the shot with each f-stop was taken , the results without any processing and at full resolution exported in PNG files and placed on Google drive so anybody with the link can download files and review them in any application - Lightroom being one of the easiest to compare images side by side.

The Excel spreadsheet in the same folder has files names and corresponding f-stops. Review usually involve finding the smallest resolvable element in USAF 1951 chart (of which ten are placed on the single frame).  It's very important to review image quality in the center and in one of the corners. The winner image is the one which has highest resolution in both center and the corners and shows little fringing and blurriness). 

If you want to contribute: Vlad will gladly supply test target for you to use with your equipment with the only condition that the in full resolution results of your scans be made public.

While you can use a kit lens for digitization, a specialized macro lens will often yield better results. Macro lenses have a flat field of focus and are optimized for close-up work, making them ideal for capturing the fine details of film negatives and slides. Kit lenses are typically zoom lenses and not really designed to work at required reproduction ratios. 

Focal length affects the magnification and working distance in film digitization. A longer focal length will draw on the sensor a more uniform image of the film original, but at the expense of working distance. You'll need to experiment to find the balance that works for your setup. The rule of thumb  is that effective focal length of the lens should be at least two times the diagonal size of the camera sensor. 70-80+ mm lenses are good for full-frame digital cameras, 50-60 mm lenses are good for APS-C cameras.  

Enlarger lenses are designed for reproducing flat film negatives onto flat paper, so they are very good at providing flat field sharpness (when used close to the prescribed reproduction ratio range something like 1:4 to 1:20). However, they most likely do not have focusing mechanisms, have screw mount M39 or M42,  aperture has to be set manually and they will need an adapter to fit on your camera. "Normal" camera lenses are more convenient to use and can provide excellent results, especially if they are fixed focal length true macro lenses and can focus to 1:1 scale without extension rings. 

While all macro lenses are suitable for close-up work, not all are equally good for film digitization. Some may exhibit curvature of field, strong light fall-off or other aberrations that can affect the quality of the digitized image. It's best to research lens reviews or test a lens yourself before committing to buy. 

You can use various test targets (like Vlads Test Target various offerings) or charts with fine details and various contrast levels. Photograph the target with your digitization setup, then examine the image at 100% view on a computer screen to assess sharpness, contrast, uniformity and any lens aberrations/fringing. 

It's crucial to ensure your lens and camera body are compatible at least on paper. The lens' distance to the sensor at required reproduction ratio should exceed the camera flange distance by at least 1-2 cm so there is a space for lens adapter. Ideally you would want to fully control the lens aperture and focusing from your camera. While adapters are available for many lens and camera combinations, they may not support all features. Thoroughly research lens compatibility before purchasing. Enlarger lenses always require some sort of adapter/extension rings/bellows. Keywords are focal length, flange distance, type of mount. If lens' mount is M39 or M42 and focal length 50 mm or above there is very high chance that adapter can be found, but double check nevertheless.