Grok replied thusly:
Your friend’s comment is a sharp observation, and they’re absolutely right to point out an imprecision in how I explained things. Let me clarify and correct my earlier description based on their feedback.
### The Misstep
In my explanation, I said the 10 lp/mm lines measure "contrast" (coarse structures) while the 40 lp/mm lines measure "sharpness" (fine detail). That’s an oversimplification. Your friend is correct: **all MTF lines measure contrast**, just at different spatial frequencies (how closely packed the details are). I shouldn’t have implied that only the lower frequency (10 lp/mm) relates to contrast—it’s not that coarse structures "don’t demonstrate contrast" versus fine ones. Instead, it’s about how well the lens preserves contrast across different levels of detail.
### The Correct Picture
- **MTF is all about contrast**: The vertical axis (0 to 1, or 0% to 100%) shows how much contrast is maintained between black and white lines at a given frequency. A higher value (e.g., 0.95 or 95%) means the lens keeps more contrast—whether it’s big, bold patterns (low frequency, like 10 lp/mm) or tiny, intricate details (high frequency, like 40 lp/mm).
- **Frequency differences**:
- 10 lp/mm (lower frequency) tests contrast in larger, coarser structures—like distinguishing a black fence from a white wall. A high MTF here (e.g., 95%) means those big patterns stay punchy and clear.
- 40 lp/mm (higher frequency) tests contrast in finer details—like the texture of that fence’s wood grain. A high MTF here (e.g., 95%) means those tiny details still have strong contrast, not blurring into gray mush.
- **Your friend’s example**: They’re spot-on. If one lens has 95% MTF at 40 lp/mm and another has 70% at the same frequency, the first lens has *higher contrast* in those fine structures. The 95% lens keeps the blacks blacker and whites whiter in small details, while the 70% lens lets them fade more toward gray, reducing clarity.
### Why My Wording Was Off
I tried to simplify by tying 10 lp/mm to "contrast" (think big, bold tonal separation) and 40 lp/mm to "sharpness" (think crisp edges on tiny stuff). But sharpness and contrast aren’t separate—sharpness *is* a result of high contrast at higher frequencies. My shortcut muddied the waters, and your friend caught it. Kudos to them!
### Revised Explanation
For a layman photographer:
- Think of an MTF chart as showing how well a lens keeps things clear and distinct (contrast) at different scales.
- The 10 lp/mm line shows how punchy big patterns look (e.g., a bright sky vs. dark trees).
- The 40 lp/mm line shows how crisp tiny details stay (e.g., individual leaves).
- Higher lines = better contrast = clearer images, whether it’s big stuff or small stuff. A lens with 95% at 40 lp/mm beats one with 70% because it keeps fine details more vivid.
Excellent lenses have higher, flatter lines at all frequencies, meaning better contrast everywhere. The average lens drops more, losing contrast, especially in fine details. The error was in my wording, not the overall trend.
Does this clear it up? I’d love to hear if your friend has more thoughts—they’ve got a good eye for this! And thanks for passing along their feedback; it keeps me sharp too.
