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 photos 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.

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 provide a larger image of the film, 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). However, they most likely do not have focusing mechanisms, 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 fixed focal length true macro lenses.

While all macro lenses are suitable for close-up work, not all are equally good for film digitization. Some may exhibit curvature of field 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 deciding.

You can use various test targets 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 screen to assess sharpness, contrast, and any lens aberrations. Vlads Test Target has been designed specifically to test lenses' suitability for digitizing. 

It's crucial to ensure your lens and camera body are compatible at least mechanically. 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.