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Archive 2012 · when sagittal and tangential Spatial frequency curves don...
  
 
Rodluvan
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p.1 #1 · when sagittal and tangential Spatial frequency curves don't coincide


I hope this is the least 'wrong' category to post this in.

How does wildly non coinciding sagittal and tangential Spatial frequency curves manifest itself visually in a photo (I know they only coincide fully in a theoretical perfect lens)?



Jan 17, 2012 at 09:26 AM
S Dilworth
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p.1 #2 · when sagittal and tangential Spatial frequency curves don't coincide


That depends on why the MTF curves differ for the sagittal and tangential orientations. (I guess you're talking about MTF curves.)

For example, it might be caused by lateral chromatic aberration, in which case you would see off-axis fringes of false colour on tangentially orientated high-contrast structures.

It might be caused by astigmatism, in which case the sagittal and tangential focal surfaces wouldn't coincide. Astigmatism is visible as poor off-axis sharpness of sagittal and/or tangential structures, depending on where you choose to place the focus. It's often much worse at the extremes of the field of view. The best tactic may be to focus on a compromise surface bisecting the sagittal and tangential surfaces, though it really depends on what you're photographing.

Astigmatism can also affect bokeh, since structures in one orientation may be farther out of focus than the other orientation.

Stopping down keeps astigmatism in check, but doesn't reduce lateral chromatic aberration. So if you have a series of MTF charts at different f-numbers, you can sometimes guess the reason for the difference in sagittal and tangential curves. Do you have a specific lens in mind?



Jan 17, 2012 at 10:27 AM
Rodluvan
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p.1 #3 · when sagittal and tangential Spatial frequency curves don't coincide


S Dilworth wrote:
That depends on why the MTF curves differ for the sagittal and tangential orientations. (I guess you're talking about MTF curves.)

For example, it might be caused by lateral chromatic aberration, in which case you would see off-axis fringes of false colour on tangentially orientated high-contrast structures.

It might be caused by astigmatism, in which case the sagittal and tangential focal surfaces wouldn't coincide. Astigmatism is visible as poor off-axis sharpness of sagittal and/or tangential structures, depending on where you choose to place the focus. It's often much worse at the extremes of the field of view. The best tactic may
...Show more

Thanks.

I am struggling with the concept in general, and I was going through the MTF charts of ZF lenses and noticed that some of them, like the 3.5/18 has an horrendous looking chart in that respect and I wondered how it would manifest itself:

http://applications.zeiss.com/C12578620052CA69/0/219DD210CBF9AB29C1257864004CC097/$file/distagon_35_18.pdf

The MP100 otoh, is fantastic apparently, but I now it exhibits CA if used carelessly.
http://applications.zeiss.com/C12578620052CA69/0/B06F47F367D29FB3C1257864004D344F/$file/makroplanar_2_100.pdf



Jan 17, 2012 at 01:44 PM
S Dilworth
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p.1 #4 · when sagittal and tangential Spatial frequency curves don't coincide


I wouldn't call the curves for the 18 mm f/3.5 Distagon horrendous (a 100-degree angle of view is hard!), but I see what you mean. In this case, since the tangential curves don't greatly improve when going from full aperture to f/8, you can guess the lens probably has significant lateral chromatic aberration. You can see this in real-world photos, too. Other aberrations are no doubt present in addition to lateral chromatic aberration.

The 100 mm f/2 Makro-Planar has very low lateral chromatic aberration. Longitudinal chromatic aberration is also pretty low, allowing nice MTF curves at f/2 for white light. That doesn't mean the lens doesn't produce false colour in out-of-focus regions, which might be what you're thinking about. Modern lenses with a good correction of the monochromatic aberrations often produce saturated false colours just outside the focal plane. This can be caused by wavelength-dependent spherical aberration (spherochromatism), which is ironically made worse by the strongly curved lens elements used to correct other aberrations.

Bottom line: if you'd like to know how the lenses perform, look at photos produced by them! The MTF curves don't reveal everything.



Jan 17, 2012 at 03:35 PM
skibum5
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p.1 #5 · when sagittal and tangential Spatial frequency curves don't coincide


S Dilworth wrote:
Longitudinal chromatic aberration is also pretty low, allowing nice MTF curves at f/2 for white light. That doesn't mean the lens doesn't produce false colour in out-of-focus regions, which might be what you're thinking about.


But that is what longitudinal chromatic aberration is, is it not?



Jan 17, 2012 at 07:17 PM
 

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Rodluvan
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p.1 #6 · when sagittal and tangential Spatial frequency curves don't coincide


Much obliged!

S Dilworth wrote:
I wouldn't call the curves for the 18 mm f/3.5 Distagon horrendous (a 100-degree angle of view is hard!), but I see what you mean. In this case, since the tangential curves don't greatly improve when going from full aperture to f/8, you can guess the lens probably has significant lateral chromatic aberration. You can see this in real-world photos, too. Other aberrations are no doubt present in addition to lateral chromatic aberration.

The 100 mm f/2 Makro-Planar has very low lateral chromatic aberration. Longitudinal chromatic aberration is also pretty low, allowing nice MTF curves at f/2 for white light.
...Show more



Jan 17, 2012 at 08:53 PM
S Dilworth
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p.1 #7 · when sagittal and tangential Spatial frequency curves don't coincide


skibum5 wrote:
But that is what longitudinal chromatic aberration is, is it not?


Well, no, because it's defined at the focal plane. A lens with longitudinal chromatic aberration would produce false colour just outside the focal plane, but so might a lens with very little longitudinal chromatic aberration, for other reasons (such as spherochromatism). This is why modern lenses, despite having less longitudinal chromatic aberration than older lenses, often have more saturated false colour just outside the focal plane.

Even some apochromatic lenses (with three colours brought into focus and very minimal tertiary colour) have lots of false colour just outside the focal plane.



Jan 18, 2012 at 11:23 AM
JohnJ
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p.1 #8 · when sagittal and tangential Spatial frequency curves don't coincide


S Dilworth wrote:
Well, no, because it's defined at the focal plane. A lens with longitudinal chromatic aberration would produce false colour just outside the focal plane, but so might a lens with very little longitudinal chromatic aberration, for other reasons (such as spherochromatism). This is why modern lenses, despite having less longitudinal chromatic aberration than older lenses, often have more saturated false colour just outside the focal plane.

Even some apochromatic lenses (with three colours brought into focus and very minimal tertiary colour) have lots of false colour just outside the focal plane.


Sorry to keep off topic but isn't this the common misconception about LoCA, that it is the purple/green colour cast in OOF areas whilst my (admitedly poor) understanding is that it's actually spherochromatism, as you've described? It appears to be a very common error in terminology.

JJ



Jan 18, 2012 at 12:13 PM
skibum5
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p.1 #9 · when sagittal and tangential Spatial frequency curves don't coincide


JohnJ wrote:
Sorry to keep off topic but isn't this the common misconception about LoCA, that it is the purple/green colour cast in OOF areas whilst my (admitedly poor) understanding is that it's actually spherochromatism, as you've described? It appears to be a very common error in terminology.

JJ


But it is the purple cast in front and green cast behind that is LoCA is it not? They adjust for B and bring it up to match R's plane of focus and then you have G plane of focus and R/B plane of focus just a bit in front. As you stop down the issue becomes less and less problematic. Lenses such as the 300 2.8 IS that also try to bring G to R/B don't really show this affect at all even under some pretty nasty circumstances.

I just looked and even in technical documents from Canon's own optics lab this is how they define Longitudinal CA.


EDIT: yes I just looked up some info on spherochromatism and as I thought it's not like LoCA at all and the whole purple in front and green behind across the entire frame is LoCA. Spherochromatism is said to be generally less present than LoCa and it tend to be worse in certain parts of the frame, which makes sense, than others and you might have say B in front of R in some parts and R in front of B in others.



Jan 18, 2012 at 07:45 PM
skibum5
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p.1 #10 · when sagittal and tangential Spatial frequency curves don't coincide


S Dilworth wrote:
Well, no, because it's defined at the focal plane. A lens with longitudinal chromatic aberration would produce false colour just outside the focal plane, but so might a lens with very little longitudinal chromatic aberration, for other reasons (such as spherochromatism). This is why modern lenses, despite having less longitudinal chromatic aberration than older lenses, often have more saturated false colour just outside the focal plane.

Even some apochromatic lenses (with three colours brought into focus and very minimal tertiary colour) have lots of false colour just outside the focal plane.


isn't spherochromatism not at all uniform across the image though? And I believe you can see LoCa spread well into OOF areas at times. Take a 300 f/4 non-IS L and shoot into some pine woods at f/4, you see a green clouding around needlesbranches farther back and purplish in front (uniformly across the frame, same degree in center as mid point as at edges and not swapping of which color is smeared front vs back)



Jan 18, 2012 at 08:01 PM





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