Film Flatness Test: 3 Film Holders in the First Quantitative Comparison

This article documents an attempt to quantitatively compare film flatness in three DSLR scanning film holders. The method is based on depth-from-focus analysis using a macro rail. The measurement accuracy is limited; the results show trends and relative differences, not absolute values.

Why film flatness matters

Film flatness is the most common complaint in the DSLR scanning community. The film bows in the holder, and the corners lose sharpness. The physics behind it is well known: at typical scan settings, the depth of field is only a fraction of a millimeter. A bow of 0.3 mm is enough to cause visible loss of sharpness in the corners.

Lots of opinions, no measurements

Forums and Reddit are full of statements about film flatness: “The corners are soft”, “Holder X is better than Y”. What you won't find: measurement data. As far as our research shows, nobody has quantitatively determined and published the deviation of a film in a holder. All comparisons are based on subjective assessment of scan sharpness.

This article is an attempt to change that.

The test strip

The test strip used, Kodak Gold 200, shot with an Olympus OM-2n and Zuiko 50mm f/1.8 at f/5.6. A relatively dense negative with evenly distributed image information.

The same film strip and the same frame were used for all measurements. The choice of subject matters: the depth-from-focus method needs texture in the image (film grain, edges) to determine the focus point. A uniformly bright or dark subject (sky, wall, snow) provides no usable signal. Branches and leaves work well: dense, high in contrast, evenly distributed across the entire frame.

Method A: The reflection check (qualitative)

The film surface reflects light. If you hold the mounted film under a ceiling lamp, unevenness becomes visible as distorted reflection lines. This method is simple and shows fine details, but it can't be translated into numbers.

The same film strip in four holders, each photographed under the ceiling lamp:

Reference: The same film held by hand only, without a holder

What you can see with the naked eye: None of the holders keeps the film perfectly flat, and the reflection lines are distorted in all three. But all three do a visibly better job than no holder at all. Which one performs best can at most be guessed from the reflection images. The curvature cannot be quantified; whether the deviation is 50 or 500 micrometers cannot be read from the reflection. Hence the second method.

How the three holders guide the film

The three tested holders use different design principles to hold the film in position. This affects where and how much the film can curve.

Negative Supply: The base has a panorama format gate, and the film is only guided along the two long edges (top and bottom). There is no guidance along the length. There are masks that slide in from below (for half frame, 35mm or panorama), but even with a mask the film remains free in the lengthwise direction. This means: in the middle of the gate, the film has the most room to sag.

Valoi: There is a separate holder cast in one piece for each film format. The film is guided both across its width and along its length. The design of the lengthwise guidance cannot be seen from the outside, as it is integrated into the housing.

Ausgeknipst: A combination of both approaches. Like Negative Supply, the base has a panorama format gate and guides the film only at the edges. Interchangeable tops are placed on it, which also guide the film along its length from above and press it down. Without a top, the holder behaves like the Negative Supply approach (edge guidance only). With a top, lengthwise guidance is added.

What this means for the film edges: All three systems guide the film at exactly the area you would need to expose in order to scan the perforation and edge markings as well. A holder that shows the edge can no longer grip the film there, and the control measurement further down shows where that leads: without the cover plate, the same strip deviates from the plane 2,3 times more, at 381 µm RMS. With this design, edge scans and flatness are mutually exclusive. If you want both, there is no way around two anti-Newton glasses, and you pay for it in speed, because each frame is loaded individually.

This difference in guidance is relevant for the measurement: since the gate width differs for each holder, the evaluation was limited to the inner 80 % of the film frame. The edge areas contain little image information and are cropped away during scanning anyway; they are not included in the comparison.

Method B: Depth-from-Focus measurement (quantitative)

The camera moves through the film plane on a macro rail in defined steps. An image is taken at each position. Each region of the image reaches its maximum sharpness in a different frame, depending on how far it is from the lens. From the position of the sharpness maximum, the height of the film surface can be calculated at every point.

Setup

Measurement setup: camera on a macro rail, film holder on the light source. The Post-its under the light pad serve as shims; they compensate for minimal height differences at the corners so that the film plane is parallel to the sensor.

Close-up: macro rail with dial gauge, 0.1 mm step size

The details:

• Camera: Sony ZV-E10
• Lens: Carl Zeiss Jena Tessar 50mm f/2.8 on macro bellows
• Aperture: f/2.8 (wide open, for maximum sensitivity to focus shift)
• Macro rail: 0.1 mm step size, 21 shots per run
• Alignment: Mirror method

Mirror alignment: The lens reflection has to sit exactly in the center so that the optical axis is perpendicular to the film plane

Analysis

A Python script handles the analysis. It divides each image into a grid and determines, for each cell, in which frame the sharpness is highest. From this, the height of the film surface can be calculated at every point. Two corrections follow: First, the global tilt is subtracted out (the sensor is never perfectly parallel to the film). Second, the field curvature of the lens is removed, so that only the pure film topography remains.

Three runs per holder, shuffled, averaged.

Technical details on data processing

For the analysis, the script divides each image into a grid of 20 x 30 cells. For each cell, the Laplacian variance is calculated across all 21 frames, a measure of how much high-frequency contrast (film grain, edges) the cell contains. The cell is sharpest when the focal plane lies exactly on the film surface.

The resulting sharpness curve has one peak per cell. Its position is determined by 3-point parabolic interpolation in order to achieve a finer Z resolution than the step size (0.1 mm). A best-fit plane is then fitted to the entire Z-map and subtracted (tilt correction). When comparing multiple holders, the mean of all Z-maps is subtracted as system bias (common-mode rejection); this removes the field curvature of the lens, which is identical across all measurements.

The analysis is performed on the inner 80 % of the film frame. The edge areas are discarded, as they contain little to no image information and are cropped off during scanning anyway.

The scripts are written in Python (numpy, opencv, matplotlib). If you want to review the code or recalculate the raw data, feel free to get in touch; if there is justified criticism of the method, we will adjust the analysis and publish the corrected results.

Worst-case reference: film guided only at the edges, without cover plate, maximum sag

Results

Three runs per holder, averaged. Measured on the inner 80 % of the film frame (edge areas excluded, as the holders guide the film over different widths, see above). Lens field curvature removed via common-mode rejection.

PV = Peak-to-Valley: the largest deviation between the highest and the lowest point.
RMS = Root Mean Square: the average deviation, considerably more robust than PV, because a single outlier does not distort the result.

Heatmaps

The heatmaps show the topography of the film plane. Red means the film bulges towards the lens. Blue: it sinks away. White is the ideal plane. The scale shows micrometres.

3-holder comparison: red = film closer to the lens, blue = further away

Control measurement without cover plate (left): significantly more deviation than the holders with a cover plate

And the individual heatmaps, each averaged over three runs:

Ausgeknipst, averaged over 3 runs

Valoi, averaged over 3 runs

Negative Supply, averaged over 3 runs

What the data says

Validation: does the method work?

The sprocket control (film without a cover plate) shows 2.3x higher RMS values than the best holder. This is the most important data point of the entire measurement: it confirms that the method resolves real differences in film flatness and that the results are not lost in measurement noise.

Ausgeknipst (RMS 163 um)

The lowest RMS value in the test. The heatmap shows a relatively even distribution without dominant hotspots. The interchangeable top presses the film down both at the sides and along its length, which is reflected in even guidance. The run-to-run variation (how much the results differ when the same film is loaded repeatedly) was 176 um, comparable to Negative Supply.

Valoi (RMS 175 um)

Just behind Ausgeknipst. The heatmap shows slight wave patterns that could stem from the S-curve guidance of the Valoi channel design; the film is guided through a curved channel as it is inserted. At 210 um, the run-to-run variation was somewhat higher than with the other two holders. Whether this is due to the construction or to the way the film is threaded cannot be deduced from the data.

Negative Supply (RMS 202 um)

The highest RMS value of the three holders. The heatmap shows more contrast than with the other two, and areas with stronger curvature are more clearly visible. Negative Supply guides the film only along the long sides, not lengthwise. The lack of lengthwise guidance could explain why the deviation in the middle of the gate is somewhat higher. At the same time, the run-to-run variation of 175 um was the lowest in the test; the film sits consistently in the same position every time it is loaded.

The comparison

The factor between the best and the worst holder is 1.2x (163 vs. 202 um RMS). In absolute terms: a difference of 39 micrometers. That is less than the thickness of a human hair.

For context: at f/8, the aperture most people scan at, the depth of field at the negative is about 500 micrometers. All three holders keep the film within this tolerance. The difference of 39 um will not be visible in a finished scan at this aperture.

At wider apertures (f/4 or f/2.8, as used in high-end scanning setups), the depth of field shrinks to less than 200 um. In this range, the measured differences could become relevant, but even then the effect is hard to separate from other sources of error (sensor alignment, lens field curvature, film curl of the specific film strip).

What do real scans from the 120 holder look like?

The measurement above was done with 35mm film on the macro rail, wide open and with a single test strip. The four images below show what scans from our holder look like in a normal workflow. Italian photographer Osea Costantini tested our film holder for 120 roll film in 6×6 format with his own black and white negatives, in his own scanning setup. We gave him the holders for testing.

Disclaimer: We sell some of the products that appear here. Even so, the article is meant first and foremost as help and not as a sales pitch.

We looked at the corners first, because when film curves in a holder without glass, they are the first to go soft. In these scans, sharpness holds right to the edge of the frame. You can see this clearly in the shot of the painter in the sunflower field: at the bottom right, the seam on his jacket pocket is as sharp as the canvas in the center of the image. At the bottom left, the veins in the sunflower leaves are clearly visible.

This is not a measurement. The negatives are different from our test strip, the format is larger, and the files in this article are reduced to web size. The images only show what the result looks like when someone digitizes their own film with the holder.

Scans: Osea Costantini, 120 film in 6×6 format, digitized with our film holder and published with his permission. You can find more of his work on his YouTube channel.

Conclusion

All three holders keep the film measurably flatter than an unsupported film strip. The differences between them are small; the factor between the best and the worst result is 1.2x.

The products differ in many other aspects (material, workflow, compatibility, price). Film flatness is one factor among many. On this one factor, the three tested holders are close together.

A note on the setup

Measurements were taken at f/2.8 (wide open). That is not common for scanning, as the imaging performance of the lens drops wide open, especially in the corners. The reason for the open aperture: the depth of field has to be small enough for measurable differences in the film plane to show up at all. At f/5.6 or f/8, the DoF would be too large to resolve film curl. A 100mm macro at a 1:1 reproduction ratio, slightly stopped down, would have been the better measuring instrument, but was not available.

To the community: help us measure better

This experiment was a first attempt with limited equipment. The method has weaknesses, which are documented above. If anyone in the community knows a more precise, affordable method (laser interferometry, moire topography or something else), we would be glad to hear about it. The tests will be repeated and the raw data published.

The goal is not a marketing comparison. The goal is to improve our own design based on measurement data.