What do Thalmann’s lens tests tell us about lens sharpness and MTF curves, and how do they connect to the quiet power of large-format film?
Factors behind Thalmann’s Future Classic lens selection
In my previous post, I examined the factors that influence Thalmann’s Future Classic lens selections. One major factor I identified was weight—that is, portability played a significant role.
An open question was whether MTF specifications or resolution performance influenced Thalmann’s selections. To clarify this, I reached out directly to Tim Parkin, who wrote an insightful article in On Landscape discussing Thalmann’s Future Classics list. He confirmed that Thalmann’s choices were primarily driven by a balance between weight, image circle, and resolution, with weight being particularly important for landscape photographers, as Thalmann himself was. Tim also pointed me to the well-known Hevanet test, a collaborative effort between Kerry Thalmann, Christopher Perez, and Mike McDonald (circa 1999). Hereafter, this test will be referred to as the Thalmann Test for simplicity. This test provides additional insight into the performance characteristics that likely informed their evaluations.
Lens Test Data and Measurements
Thalmann and collaborators measured the resolution of several lenses, evaluated together with film at specific tested apertures. Each entry in the corresponding column lists the aperture (f-stop) at which the test was conducted, followed by three resolution values. These values represent the measured resolving power at the center, midpoint, and corner of the image field, respectively. All results are expressed in line pairs per millimeter (lp/mm), a standard unit of spatial resolution, where higher values indicate better performance. For example, an entry of f/16: 60 54 32 means that at f/16, the lens achieved 60 lp/mm at the center, 54 lp/mm at the midpoint, and 32 lp/mm at the corner.
According to additional details shared on Christopher Perez’s blog, all lenses in the test were evaluated at a 1:20 magnification ratio, representing a typical working distance for large format photography. The test setup used TMax 100 film, developed in D-76 according to a calibrated system of exposure time and temperature. A variety of 4×5 cameras were employed, including the Canham DCLH, Linhof Technika III, Linhof Master Technika, Linhof Master Kardan, and the Tachihara wood field camera. The Edmund Scientific Lens Resolution Chart was used for measurement. Exposures were carefully controlled, targeting Zone VII–VIII for detailed highlights and Zone III for detailed shadows, ensuring consistent tonal evaluation across all negatives.
Interpreting the Results
Based on this description, it becomes clear that Perez and Thalmann did not measure full Modulation Transfer Functions (MTFs), but rather reported the combined resolution of the lens–film system. The distribution (histogram) of these measured resolutions, taken at the center of the lens at f/22 (as an example), is shown in Figure 1.
The measured resolutions range from 35 to 75 line pairs per millimeter (lp/mm), with the most frequent value (mode) occurring at 60 lp/mm—a range where Schneider and Rodenstock lenses are particularly well represented. Fuji measurements tend to cluster in the mid-resolution range, while Nikon entries are spread more evenly across the spectrum. Very high resolution values above 70 lp/mm are rare, with only a single Rodenstock entry reaching that level. The chart highlights Schneider’s strong and consistent performance across the higher-resolution bins, suggesting robust resolving capabilities under the testing conditions applied by Perez and Thalmann.
Statistical Analysis of the Lens Test
To assess whether Thalmann’s selection of “Future Classics” relied mainly on lens resolution, I grouped the measured data for lens–film systems into two categories: Future Classics (in red) and the original Perez–Thalmann Test dataset (in blue). If resolution had been the decisive factor, the red group would show a clear shift toward higher resolution values. Figure 2 shows otherwise. Many of the Future Classics fall within the same resolution bins as the common entries from the Perez–Thalmann Test, particularly at 50 and 60 lp/mm. This means the Future Classics delivered resolution performance comparable to other respected lenses. Only a few exceeded 65 lp/mm, which suggests that ultra-high resolution was not a dominant selection criterion.
The evidence instead points to broader factors shaping Thalmann’s choices. Build quality, rendering style, or reputation may have carried as much weight as sharpness. It is reasonable to conclude that measured resolution was not the primary factor in designating a lens as a Future Classic.
Figure 3 adds another dimension by comparing listed price with measured resolution. The graph shows no strong correlation between lens price and optical performance. Some high-priced lenses achieve excellent resolution, but others fall short, while several mid-range lenses perform surprisingly well. The downward trendline confirms that a higher price does not necessarily mean better sharpness. Notable cases include the Xenar and G-Claron, which deliver high resolution at relatively low cost, and the Super-Angulon, which commands a premium despite modest resolution. Overall, these results suggest that lens sharpness is not a reliable predictor of market price in this category. The price data came from the MKD45 database.
The Unresolved Puzzle
Once the main selection criteria became clear, two key questions still remained:
- Can the compound resolution measurements from the Perez–Thalmann tests be estimated from the MTF curves published by the manufacturers?
- If yes, which MTF(%) best represents the lens resolution reported in this test?
Only Rodenstock and Schneider-Kreuznach publish MTF data that are publicly available. Nikon and Fujinon do not, which limits broader comparisons.
Before answering these questions, I will explain three things: what MTF (Modulation Transfer Function) means, how to combine the MTFs of a lens and film, and how to estimate average lens sharpness. These steps will make comparisons across different models possible.
MTF in a nutshell
What Is the Modulation Transfer Function?
The Modulation Transfer Function (MTF) describes how well an optical system—such as a lens, film, or scanner—preserves contrast at different spatial frequencies, usually measured in line pairs per millimeter (lp/mm). It quantifies the system’s ability to reproduce fine details, with values ranging from 0 (no contrast) to 1 (perfect contrast). In practice, the MTF curve shows how sharply and accurately different levels of detail appear in the final image. A more complete explanation can be found in several books and links, so I will not repeat it here.
Every optical system—including the human eye—has its own MTF. When several components form an imaging chain, their individual MTFs combine multiplicatively. For example, in a system made up of a lens, film, and scanner, the overall MTF equals the product of the MTFs of each component:
\[ \text{MTF}_{Total}= \text{MTF}_{Film} \times \text{MTF}_{Lens} \times \text{MTF}_{Scanner} \]
How Is Spatial Frequency Measured?
Norman Koren created a MATLAB script to calculate MTF. Since I prefer open-source tools, I rewrote his code in Python. The Python version (mtf.py) is available on GitHub for anyone interested in exploring or analyzing MTF data.
As an example, I estimated the MTF for TMax 100 film paired with a 150mm f/5.6 Schneider Apo-Symmar lens, focused at infinity and stopped down to f/22. The script generated the MTF curves shown in Figure 4. I kept Koren’s example and notation so the comparison remains easy to follow. At this point, a short explanation of the underlying theory helps prepare for the discussion.
Limitations of MTF
The MTF of film is usually easier to measure than that of lenses. Lens MTF depends on many more variables, including:
- Distance from the image center (field position)
- Aperture (f-stop)
- Wavelength or light spectrum
- Focal length
- Focusing distance
- Orientation, typically measured in two directions:
– Sagittal: along the radial direction from the center
– Tangential: perpendicular to sagittal, following a circular path around the image center
Because of this complexity, manufacturers seldom publish simple MTF graphs that show contrast versus spatial frequency for each lens. Instead, they provide a series of plots for every lens model. These plots represent performance at different magnification ratios and aperture settings.
Each plot is a 2D graph. The x-axis shows image height (distance from the optical center), and the y-axis shows contrast (MTF). Within each plot, separate curves illustrate several fixed spatial frequencies—typically 5, 10, and 20 line pairs per millimeter (lp/mm) for large-format lenses. These curves correspond to coarse contrast, mid-level detail, and fine resolution. For each frequency, manufacturers usually draw two curves: one sagittal and one tangential. This allows users to evaluate astigmatism and off-axis performance.
Figure 5 shows an example of such a chart, published by Schneider-Kreuznach for this lens. I retrieved this graph from the Internet Archive.
Film and Lens MTF Combined
MTF of Films
In a perfect lens, the MTF curve shows high contrast at low spatial frequencies and then decreases smoothly to zero at the diffraction limit. The curve remains identical in both radial (sagittal) and tangential directions, so no astigmatism or field-dependent aberrations appear. It also stays flat across the image field, maintaining equal performance from center to edge. In such a case, all MTF curves overlap cleanly, showing perfect symmetry, no falloff, and a graceful decline governed only by diffraction—not by lens imperfections.
With MTF plots from manufacturers, we can model the overall system performance by applying straightforward mathematics to both film and lens components.
To introduce the concept, let’s begin with the MTF of the film itself (see Figure 6). A typical film MTF curve can be approximated with a Lorentzian function defined by two parameters: the characteristic frequency \( f_{50} \) and the falloff rate \( \gamma \). The equation is:
\[ \text{MTF}_{\text{film}}(f) = \frac{1}{ 1 + \left( \frac{f}{f_{50}} \right)^{\gamma}} \]
I retrieved the MTF of Kodak Ektachrome E100VS from the Web Archive.
Understanding \( f_{50} \) and \( f_{10} \)
Here, \( f_{50} \) represents the spatial frequency at which the MTF drops to 0.5 (50%), while \( \gamma \) is an empirical exponent that controls how sharply contrast falls with increasing detail. Values usually range between 1.5 and 2.5. For T-Max 100 film, \( \gamma = 2.24 \), a value chosen to fit measured data and to keep the function stable when inverted. Green light is most relevant in resolution analysis because the human eye responds far more strongly to green than to red or blue. In the example shown in Figure 6, the \( f_{50} \) value for green is about 41 line pairs per millimeter. The parameter \( f_{50} \) provides one of the most meaningful indicators of real-world image sharpness. Unlike theoretical limits defined at 10% or 100% contrast, \( f_{50} \) represents a practical midpoint that aligns closely with how the human eye perceives detail. It offers a stable metric for comparing lenses and films and is especially useful when modeling overall system performance or estimating usable detail in final prints. In short, it captures the balance between resolution and contrast that defines visual sharpness. By contrast, \( f_{10} \) often serves as a practical indicator of a system’s theoretical resolution limit—the finest detail a lens or film can still reproduce with minimal but usable contrast. This metric will be important for answering the earlier question because it marks the boundary between visible detail and theoretical resolution.Film Development Effects
Photographic films usually show isotropic MTF, meaning performance is the same in all directions. Development, however, can shift the curve significantly. Developer type, dilution, agitation, and temperature all affect contrast and grain. Table 1 lists typical \( f_{50} \) values for several well-known films. As shown in Table 1, T-Max 100 stands out with a remarkable \( f_{50} \) value of 125 lp/mm, making it an exceptionally high-resolution film. This outstanding performance was one of the main reasons Perez and Thalmann selected it for their lens tests mentioned earlier.
MTF of the Lens
The same Lorentzian equation used for film MTF also describes lens performance. To adapt it, we replace \( f_{50} \) with \( f_{\text{lens}} \), following the notation introduced by N. Koren. Here, \( f_{\text{lens}} \) marks the spatial frequency where the lens MTF drops to 0.5, or 50% contrast.
We can estimate \( f_{\text{lens}} \) directly from manufacturer MTF data. The process is simple: start with the Lorentzian equation adapted for lenses and solve for \( f_{\text{lens}} \). The result is:
\[ f_{\text{lens}} = \frac{f}{ \left( \frac{1}{ \text{MTF}_{f}} -1 \right) ^ {\frac{1}{\gamma}} } \]
The values of \( \text{MTF}_{f} \) and \( f \) come from the published MTF charts, such as the one in Figure 5. As a rule of thumb, choose the curve closest to the diffraction limit, usually the one plotted at the smallest aperture. For the Schneider-Kreuznach 150 mm lens discussed earlier, this is the f/22 panel at infinity focus. For the Perez–Thalmann tests, use the panel that matches the test’s magnification factor. To simplify, I take the MTF values at the image center.
Which spatial frequency should we use? For large-format lenses, 20 line pairs per millimeter often serves as a representative measure of sharpness. Therefore, I set \( f = 20 \) lp/mm. In the example from Figure 5, this gives \( \text{MTF}_{20} \approx 66\% \). Substituting into the equation yields \( f_{\text{lens}} = 28 \) lp/mm for this lens.
Once we estimate \( f_{\text{lens}} \) for a given lens, we can calculate the modulation transfer function of the complete system—lens plus film—using the total equation introduced earlier.
Summary of MTF Findings
The probability density distribution of the estimated \( \text{MTF}(f_{\text{test}}) \) values for the 17 lenses with available MTF data appears in Figure 7. The mean MTF is approximately 0.13 ± 0.03. This result, combined with the known limitations and statistical uncertainties of the Perez–Thalmann measurements, shows that the reported resolution values align closely with the \( \text{MTF}_{10} \) level (10% contrast). That level marks the system’s theoretical resolution limit — the finest detail still captured with minimal but perceptible contrast.
This outcome is consistent with the stated goals of Thalmann and collaborators. It indicates that lenses performing near the 10% MTF level operate close to their practical resolution limits. The tests therefore provide a useful reference when comparing MTF data to real photographic performance. They also demonstrate that film is not the limiting factor for image sharpness. As Figure 4 shows, film contributes relatively little when defining overall resolution.
Practical Implications of MTF for Photographers
For the Schneider-Kreuznach 150 mm lens in the earlier example, the characteristic frequency is \( f_{\text{lens}} = f_{50} = 28 \) lp/mm, focused at infinity. A high-quality 35 mm lens designed near the diffraction limit typically reaches \( f_{\text{lens}} = f_{50} = 37 \) lp/mm (see Koren). At first glance, the large-format lens appears less sharp. But to judge fairly, we must estimate the total detail each system delivers and compare their ratios using the following formula:
\[ \text{Relative Total Detail} = \frac{L_{\text{LF}} \times f_{50,\text{LF}}}{L_{\text{35mm}} \times f_{50,\text{35mm}}} \]
Here, \( L_{\text{format}} \) denotes the linear size (or scaling factor) of the format or sensor, and \( f_{50,\text{format}} = f_{\text{lens},\text{format}} \) represents the frequency at which lens MTF falls to 50% contrast for that format. Comparing 4×5 film with 35 mm film (cropped to the same aspect ratio), the linear factors are 4 and 1, since 4×5 covers roughly four times the image height of 35 mm. Substituting values gives:
\[ \text{Relative Total Detail} = \frac{4 \times 28}{1 \times 37} \approx 3.03 \]
Why Format Size Matters
This means a 4×5 image can resolve about three times the linear detail of a 35 mm system, assuming both use good technique: excellent lenses and film, optimal aperture, precise focus, solid support, and favorable conditions.
Even though a large-format lens may show a lower \( f_{\text{lens}} \) than a top 35 mm optic, it still records far more image information. The advantage comes not from higher lp/mm performance but from the much larger film area. This enables greater enlargement while retaining fine detail, as confirmed in comparisons by R. Clark. In real terms, resolution depends not only on sharpness per millimeter but also on the total area that records that sharpness.
Conclusions
This analysis set out to test whether the resolution values from the Perez–Thalmann study can be interpreted through MTF theory and what that reveals about sharpness in large-format photography.
By estimating the MTF at the frequencies reported in the tests, we found that most lenses cluster around \( \text{MTF} \approx 0.13 \). This result shows that the reported “resolution” corresponds to the point where system contrast falls to about 10–15% — a threshold often used in optics as the limit of practical resolving power. In other words, the lenses were evaluated near their limits, and the results reflect real-world usability rather than idealized lab conditions.
We also saw that large-format lenses, despite having lower \( f_{\text{lens}} \) values than some excellent 35 mm lenses, still deliver more total detail. The larger film area multiplies the usable resolution, so a 4×5 image can contain up to three times more linear detail than a 35 mm image.
Finally, these results confirm that resolution was not the primary factor in Thalmann’s choice of “Future Classic” lenses. As Figure 2 shows, many selected lenses share the same resolution range as lenses not included. This indicates that Thalmann valued a broader set of qualities — portability, image circle, and rendering — over maximum sharpness. His selections reflect a practical balance of usability and performance, well suited to large-format landscape work.
