Film Scanning: Lens to Film Distance Matrix Reloaded

That is a very common question on film scanning forums. It typically means that the photographer has placed the film holder, macro lens, and camera at incompatible distances, so no matter what adjustments they make, the image never comes into focus—not even close. It's often unclear how to adjust the components, especially if they are held together by bellows and/or extension rings. Typically, the solution involves hand-holding the camera and moving it closer to or further away from the film target, trying to catch a glimpse of a focused image. Once you find the correct focusing distance, measure it with a ruler to determine how much your setup is off.

This article is essentially a reference guide listing the typical distances between lenses and film originals for different film formats, macro lenses, and sensor sizes. This is important because it helps people decide on the height of the copy-stand they are building or the total length of the rings connecting the lens to the film holder (like in the VALOI Easy 35 system). Here, I'm offering the numbers based on actual measurements in real life.

I have my copy stand (LPL Copy Stand Column CSC-10) handy, along with three lenses: 50mm, 70mm, and 90mm. All of them are well-regarded fixed focal length true macro lenses—two from Sigma and one from Tamron. I also have test targets for 35mm film, 120 roll film, and 4x5 inch film. I put the camera with the given lens on the stand, placed the film targets on top of the Kaiser light table, moved the camera up and down to fill the viewfinder, focused the lens, and took the measurements with a tailor's ruler. I repeated that process for both a full-frame camera (Canon EOS R) and an APS-C camera (Canon T8i), using all lenses and targets.

First, a word about what exactly I measured: one measurement I took was the distance from the lens filter thread to the film lying flat on the Kaiser light panel. This is what you see on the chart as the "Lens to Film Distance." See the picture for illustration.

The second measurement is the distance from the camera lens flange, give or take a couple of millimeters, to the surface of the tablet, which is essentially the distance to the test target I use in lieu of real film. You will see this data in the column labeled "Lens Flange to Film." Of course, you may need to distinguish between DSLR and mirrorless cameras as they have different distances between the lens flange and the sensor, but the difference is just 24mm, so that should not affect your copy stand design measurably.

The importance of both numbers lies in the fact that many of these lenses have built-in correction mechanisms. This means the lens is internally adjusted for better performance by moving elements inside to correct aberrations and provide the best flat field of view at different magnifications, up to a 1:1 scale, which is crucial for us. As a result, the length of the lens does not change linearly with changing film formats. Understanding these internal adjustments is important because people need to know at what height they will need to place the camera to capture a certain size of film frame.

By knowing the distance between the lens flange and the film surface, one can estimate the absolute height needed for the camera to be positioned. Additionally, by extrapolating a couple of centimeters between the lens flange and the screw mount on the bottom of the camera, you can get a pretty good estimate of where the copy stand screw should be. This, in turn, gives you an idea of how high the entire column should be and how well it should be stabilized.

Another important consideration from the data in the chart is whether various on-lens adapters like Nikon ES2, Valoi Easy35, or the upcoming VALOI Easy120 will fit your lens and film format. For instance, if you have a 50mm Sigma lens, a full-frame camera, and you want to scan 35mm film, you have only 3.5 centimeters between the lens barrel and the film original. Not many adapters will be able to accommodate that short distance. On the other hand, if the plan is to digitize a 6x9 frame on roll film with a 90mm Tamron lens, you will discover that the total length of the rings you will need is 22 centimeters, which may give you pause as the setup becomes a bit unwieldy.

This is pretty much it. Look at the numbers. In the future, I may provide data for a couple more lenses, like APO Rodagon 4/75, and a few 80mm and 100mm Componon-S lenses, but I think the trend is already clear. However, given that focal lengths of 50mm, 70mm, and 90mm are among the most popular, you should get a rough idea of what sort of copy stand you will need for your preferred film size.

A note about conventional, variable focal length (zoom) lenses: no zoom lens is known to perform well enough for camera scanning, even if they are marked “Macro.” The macro designation on them is mostly a marketing ploy. Yes, you can shoot flowers at close range with shallow depth of field, and the results will be "artistic," but they will never give you an acceptable image in film scanning scenarios, with or without extra extension rings. This is because the edges of the captured frame will never turn out to be as sharp and straight as the center.

Just to reiterate: for camera scanning, we always use fixed focal length lenses. These lenses may come in camera-specific mounts with full electronic coupling, or we can use certain enlarger lenses (like Componon) or truly excellent lenses from industrial vision systems (APO Rodagon).

The lenses in camera-specific mounts have been engineered to work properly at close range and bear the “Macro” designation. The best indication that a lens is a true macro is the presence of markings on the lens barrel indicating different reproduction ratios up to 1:1. These markings indicate that the manufacturer expects the lens to be used at these ratios and guarantees a certain level of quality. These lenses also have built-in helicoids that extend the optical elements far enough to achieve 1:1 magnification and do not need extension rings. For example, the otherwise excellent Zeiss Makro-Planar 2/100 has markings up to 1:2. True to their word, at 1:1 (which requires an extension ring), the lens performs poorly.

Anyway, I hope the chart will be handy and help you with your film scanning exercise.