What is MTF?
MTF is a comprehensive measure of a lens's ability to transfer contrast and resolve fine details from the object being photographed to the image sensor. Think of it as a lens's report card, grading its performance.
* Modulation: Refers to the contrast of the image. A high modulation value indicates that the lens is good at maintaining contrast between light and dark areas. Low modulation means the contrast is reduced, leading to a muddy or washed-out image.
* Transfer Function: Describes how well the lens transfers contrast at different spatial frequencies (i.e., how closely the image matches the source).
* Spatial Frequency: Measured in line pairs per millimeter (lp/mm), it represents the density of lines the lens can resolve. Higher spatial frequencies mean finer details.
Understanding the Chart:
Most MTF charts look like graphs with several lines. The key elements are:
* X-axis (Horizontal): Represents the image height, the distance from the center of the image towards the edges. 0 represents the center of the image, and the rightmost point usually represents the corner or edge of the image.
* Y-axis (Vertical): Represents the MTF value, usually ranging from 0 to 1 (or 0% to 100%). 1 (or 100%) means perfect contrast and resolution.
* Lines: These are the curves plotted on the graph. Each line represents a specific measurement:
* Spatial Frequency Lines:
* High Spatial Frequency (e.g., 30 lp/mm, 40 lp/mm): Represents the lens's ability to resolve fine details (sharpness). Higher values are better.
* Low Spatial Frequency (e.g., 10 lp/mm): Represents the lens's contrast rendition. Higher values are better.
* Orientation Lines:
* Sagittal (S) Lines (often solid lines): Measure performance along a radial line from the center of the image.
* Meridional (M) Lines (often dotted lines): Measure performance along a tangential line (perpendicular to the radial line). Sagittal and Meridional lines represent the lens's response to lines oriented in different directions. Large separation indicates astigmatism.
* Aperture: Some MTF charts will have different sets of lines, representing the performance at different apertures (e.g., f/2.8 and f/8).
Interpreting the Chart:
Here's how to interpret the information on the chart:
1. Overall Height:
* Higher lines generally indicate better performance. A lens with consistently high lines across the image area is considered excellent.
* Pay attention to the Y-axis scale. Seemingly small differences can be significant.
2. Shape of the Lines:
* Flat Lines: Indicate consistent performance across the image frame (center to edge).
* Drooping Lines: Indicate decreasing performance as you move away from the center of the image. The steeper the drop, the more significant the performance degradation towards the edges and corners.
3. Sagittal (S) vs. Meridional (M) Lines:
* Close Proximity: Indicates good correction of astigmatism. Ideally, you want the Sagittal and Meridional lines to be close together.
* Large Separation: Indicates astigmatism. Images may appear sharper in one orientation than another, or sharpness may be inconsistent across the frame.
4. High vs. Low Spatial Frequency Lines:
* Both High: Indicates a sharp lens with good contrast. This is the ideal scenario.
* High Contrast (Low Frequency) but Lower Sharpness (High Frequency): Indicates a lens with good contrast but potentially limited resolution for fine details. This is common with older lenses.
* Low Contrast (Low Frequency): Indicates a lens that produces images with a washed-out or muted look.
5. Aperture Comparison:
* f/2.8 (or widest aperture) vs. f/8 (or smaller aperture): Look for improvements in performance as you stop down the lens. Many lenses perform better at smaller apertures. This shows how much the lens improves when you are not using the maximum aperture.
Example Interpretation:
Imagine a lens MTF chart showing the following:
* 30 lp/mm lines (S and M) starting at 0.9 at the center and dropping to 0.7 at the edge of the image. This suggests good sharpness, even towards the edges, but with a slight decrease in sharpness as you move towards the corners.
* 10 lp/mm lines (S and M) staying around 0.95 from center to edge. This suggests excellent contrast performance across the entire image frame.
* S and M lines are close together. Suggests well corrected astigmatism.
* Lines for f/8 are slightly higher and flatter than f/2.8. Suggests the lens performs better when stopped down.
What MTF Charts Don't Tell You:
While MTF charts are useful, they don't capture *everything* about a lens's performance. They *don't* directly measure:
* Bokeh (Out-of-Focus Rendering): The aesthetic quality of the blurred areas of an image.
* Vignetting (Darkening of Corners): Although some manufacturers include vignetting data separately.
* Distortion (Curvature of Lines): Though, again, some manufacturers provide separate distortion data.
* Color Rendition (Color Accuracy):
* Flare Resistance: How well the lens handles bright light sources in the frame.
* Autofocus Speed and Accuracy:
* Build Quality and Handling:
* Subjective "Look" or "Character" Some lenses have a distinctive rendering style that isn't captured in an MTF chart.
Where to Find MTF Charts:
* Lens Manufacturers' Websites: Most reputable lens manufacturers (e.g., Canon, Nikon, Sony, Zeiss, Sigma, Tamron, Fujifilm, Olympus) publish MTF charts for their lenses.
* Review Sites: Some in-depth lens review sites may also provide their own MTF measurements.
Key Takeaways:
* MTF charts provide a quantitative measure of a lens's sharpness and contrast performance.
* Higher lines generally indicate better performance.
* Flatter lines indicate more consistent performance across the image frame.
* Look for close proximity between Sagittal and Meridional lines.
* Consider both high and low spatial frequency lines.
* Compare performance at different apertures.
* Use MTF charts as one tool among many when evaluating a lens. Don't rely on them exclusively. Also consider real-world sample images, reviews, and your own shooting needs.
By understanding how to read MTF charts, you can make more informed decisions when choosing camera lenses and get a better understanding of their strengths and weaknesses. Good luck!