BEYOND THE GLASS: TAMING THE LIGHT WITH FILM HOLDERS

While not strictly necessary, a film holder is highly recommended for camera scanning. Trying to scan without a holder can result in warped, unfocused images. The chances of scratching or damaging the film also shoot thru the roof. It's also much more difficult to consistently position and align the film without a holder. In some cases, you could use a piece of cover glass to hold the film flat, but this introduces the risk of newton rings. Importantly, film holders also help to shield the film from non-image forming light, preventing it from reaching the camera sensor and potentially reducing image quality. 

Yes, there are. There are dedicated film holders for slides, negatives, and various film formats. These can be bought off-the-shelf from various manufacturers or built DIY-style. Notable commercial options include holders from brands like Negative Supply, Pixl-latr, VALOI and Skier, and some camera scanning setups like the Nikon ES-2 come with their own holders. See Film Holders for Camera Scanning  

Film holders can be made of a variety of materials. Plastic is common for mass-produced ones because it's inexpensive and light, yet sturdy. Some film holders are made of metal for enhanced durability. DIY film holders can be made from anything from cardboard to 3D-printed resin to Lego pieces. 

A glass film holder uses two pieces of glass to hold the film flat. This can ensure absolute flatness, but introduces the risk of newton rings. Also glass serves almost like a magnet for dust particles. You may use special anti-newton glass but that may cause certain fuzziness in scans. 

 A glassless holder relies on tension to keep the film flat. Glassless holders are easier to work with and don't risk newton rings, but might not hold film as absolutely flat as glass. 

Newton's rings are an optical phenomenon that can occur when a piece of film comes very close to glass.  They appear as odd, rainbow-like bands on scans. Newton's rings are a pattern of interference caused by the reflection of light between two surfaces - a spherical surface and an adjacent flat surface. They can be avoided by using glassless film holders or special Anti-Newton Ring (ANR) glass, which has a texture to prevent contact points large enough to create Newton rings.

Absolutely. Making a DIY film holder is quite feasible if you're handy. You can use materials like mat board, cardboard, or 3D-printed parts. The challenge is to make a holder that is precise, durable, and easy to reliably affix to the scanning rig or light source in the desired orientation. Unfortunately, majority of 3D printed film holders do not have any means to secure them properly, so advancing film easily displaces those holders forcing operator to refocus camera with each shot. 

The cost varies depending on the brand, material, and specific features. Plastic holders from mainstream brands and 3D printing shops  can range from $20 to $60. More specialized or premium holders, such as those from Negative Supply, can cost several hundred dollars. DIY options can be very cheap, especially if you already have suitable materials on hand. But one has to keep in mind that film holder by itself - not being integrated part of bigger, heavier , well-constructed, steady setup - is pretty useless as it moves freely over the surface of the light tablet and have to be kept in place by tape, or hand, or some sort of paperweight. See Film Holders for Camera Scanning  

The film holders should be cleaned regularly to avoid dust, which can end up on the scans. You can use a rocket blower or an anti-static brush to remove dust. If your holder uses glass and it becomes dirty or smudged, clean it with a lens cleaning solution and a microfiber cloth. Also, ensure the surfaces adjacent to the film are sufficiently black and matte to absorb any stray light.  

Some film holders can handle different formats, while others are designed for a specific format. Universal film holders often use adjustable or replaceable gates to accommodate different sizes. Most likely they are re-purposed holders from enlargers. Dedicated holders for a specific format, such as 35mm or medium format, are often simpler and quicker to use, and they are sturdier. VALOI 360 system is the good example of how multiple film format holders  can reuse the same base and light source.