I've noticed that some lenses start off with the sagittal and tangential lines very tightly overlapping (i.e. well corrected), and then as you stop down to f/2.8 / 4 / 5.6 / 8, etc. the lines begin to separate and continue getting farther apart as you stop down more.
Then, there are lenses that seem to function in the opposite direction.
Meaning, I've noticed that some other lenses start off with the sagittal and tangential lines with wider gaps (i.e. not well corrected), and then as you stop down to f/2.8 / 4 / 5.6 / 8, etc. the lines begin to come closer together and becoming "tight" as you stop down more.
Tight > Loose
Loose > Tight
I'm curious to understand what is the driver of lens design that determines the "direction" of whether a lens is best corrected WO vs. best corrected stopped down. I recognize that most of optics is quid pro quo, so I understand that there may be things in play where you "can't have it both ways". Just trying to understand what drives that.
CA may impact differential MTF and not decrease as much at smaller apertures compared to other aberrations that decrease at smaller apertures to increase MTF overall.
RustyBug wrote:
I've noticed that some lenses start off with the sagittal and tangential lines very tightly overlapping (i.e. well corrected), and then as you stop down to f/2.8 / 4 / 5.6 / 8, etc. the lines begin to separate and continue getting farther apart as you stop down more.
Then, there are lenses that seem to function in the opposite direction.
Meaning, I've noticed that some other lenses start off with the sagittal and tangential lines with wider gaps (i.e. not well corrected), and then as you stop down to f/2.8 / 4 / 5.6 / 8, etc. the lines begin to come closer together and becoming "tight" as you stop down more.
Tight > Loose
Loose > Tight
I'm curious to understand what is the driver of lens design that determines the "direction" of whether a lens is best corrected WO vs. best corrected stopped down. I recognize that most of optics is quid pro quo, so I understand that there may be things in play where you "can't have it both ways". Just trying to understand what drives that.
I can't answer your question fully, but I think I have seen a partial answer but can't remember the source. As EB-1 noted CA (but specificaly lateral CA) doesn't diminish as you stop a lens down (axial CA does diminish as you stop a lens down). If I am rembering correctly, one of the major lens designers, perhaps Nasse from Zeiss or Karbe from Leica, or the guy who designed the Coastal Opitic 60 f/4 Macro, noted that lateral CA because it doesn't diminish as you stop down will result in a spread of the sagital and tangential lines as you stop down. Since reading that I have noticed that lenses that have more lateral CA do tend to have that pattern where the gap between the lines widens as you stop down. Lateral CA may not be the only cause of that pattern, but I believe it is one cause of it.
Steve Spencer wrote:
I can't answer your question fully, but I think I have seen a partial answer but can't remember the source. As EB-1 noted CA (but specificaly lateral CA) doesn't diminish as you stop a lens down (axial CA does diminish as you stop a lens down). If I am rembering correctly, one of the major lens designers, perhaps Nasse from Zeiss or Karbe from Leica, or the guy who designed the Coastal Opitic 60 f/4 Macro, noted that lateral CA because it doesn't diminish as you stop down will result in a spread of the sagital and tangential lines as you stop down. Since reading that I have noticed that lenses that have more lateral CA do tend to have that pattern where the gap between the lines widens as you stop down. Lateral CA may not be the only cause of that pattern, but I believe it is one cause of it....Show more →
So, if the Lateral CA doesn't diminish ... what does that infer about how you should interpret the MTF's when those two lines "spread out" like that? By that, I mean do you pay more attention to the Sagittal or to the Tangential performance (beyond the presence of Lateral CA), as the primary reference ... or, do you kinda "split the difference" between them, etc.
Mar 09, 2026 at 01:59 PM
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RustyBug wrote:
So, if the Lateral CA doesn't diminish ... what does that infer about how you should interpret the MTF's when those two lines "spread out" like that? By that, I mean do you pay more attention to the Sagittal or to the Tangential performance (beyond the presence of Lateral CA), as the primary reference ... or, do you kinda "split the difference" between them, etc.
I don't really know the answer to the question you are asking, and importantly for this issue I don't know how cleaning up the lateral CA in post (which if often easy to do with a simple click) affects the performance. My guess, and it is just a guess as I don't really have the background to answer the question is that the outer frame where the lines diverge the most will be affected even if you clean up the lateral CA in post, but I could be wrong and I am not sure if the impact would be relatively minimal or pretty severe.
