Lens Distortion Vs MTF Radial / Tangent curves

[Tim Povlick]

Greetings,

I have been trying to find this answer in optics books and in the archives but not luck.

Does the separation of the radial curve from the tangential curve on the MTF graph indicate the amount of distortion a lens has? Is it a direct indication?

For example: This MTF curve:
https://www.schneideroptics.com/pdfs/photo/datasheets/ApoSymmarL/ApoSymmarL_56_120_2.pdf

shows the R&T lines track each other fairly well. The distortion is down in the dirt:
https://www.schneideroptics.com/pdfs/photo/datasheets/ApoSymmarL/ApoSymmarL_56_120_1.pdf

That lens is smokin' !

Whereas this lens shows a bit more separation:
https://www.schneideroptics.com/pdfs/photo/datasheets/super-symmar_xl/super-symmar_xl_56_150_2.pdf

and the distortion is higher (although still no a concern):

https://www.schneideroptics.com/pdfs/photo/datasheets/super-symmar_xl/super-symmar_xl_56_150_1.pdf

Best Regards,

Tim

[Dan Fromm]

Not distortion, astigmatism.

[Tim Povlick]

Hi Dan,

That makes a lot of sense! Thanks for you expertise.

Regards,

Tim

[rdenney]

Just to provide an example: There are fisheye lenses that show very good sagittal and tangential MTF, but obviously have quite a lot of (intentional) distortion. Or, we could call rectilinear projection the distortion, and fisheye projection then undistorted image, depending on whether we preferred round accuracy or straight-line accuracy.

Rick "astigmatic and distorted" Denney

[ic-racer]

Circles of confusion and Airy disks become oval at the edge of the image circle due to the aperture becoming a crescent, rather than a circle near the edges of the image circle. Thus, in one dimension they will be smaller than in the other and may produce better resolution. Perfect rectilinear corrected lenses show this effect and it has nothing to do with image distortion.

As a simple test, I compared the far axis aperture of two retrofocus Zeiss lenses. A rectilinear 18mm and a fisheye 16mm. The distortion of the aperture was similar off axis in both. The fisheye did not produce a more round aperture off axis.

[Tim Povlick]

Hi Rick,

Thanks for the tip about the Fisheye lens. I checked "Modern Lens Design" book and can see from the graphs what you are saying.

Appreciate your help, thanks!

Best Regards,
Tim

[Tim Povlick]

Hello ic-racer.

I was trying to figure out the difference between distortion and astigmatism. Your explanation is very clear. Many thanks!

All the best,

Tim

[Struan Gray]

Nail in the coffin:

Imagine photographing a square array of tiny lights, like the nets of fairy lights you can buy for little girls' bedrooms. Aberrations and diffraction tell you how much each little dot will be smeared out. Distortion tells you how much the lines of dots will deviate from being a perfect square net.

The remarkable and surprising thing is, the two very different effects are related, and can be traded off against each other when designing the lens. In terms of their effects on the image though, they are very different.

[Emmanuel BIGLER]

Another example of a lens where all aberrations are highly corrected, but distorsion, are so-called f-theta lenses for scanning devices.
In those lenses (Rodenstock has a line of such lenses bearing the 'ronar' name but they are very different from the good ol' apo ronar), a very specific form of distorsion is mandatory. Hence, those lenses obey different trade-off rules in their design, and are probably useless to the photographer as plain taking lenses.
However, the photographer uses them without seeing them, in certain scanners...

[rdenney]

As a simple test, I compared the far axis aperture of two retrofocus Zeiss lenses. A rectilinear 18mm and a fisheye 16mm. The distortion of the aperture was similar off axis in both. The fisheye did not produce a more round aperture off axis.

On the other hand, look at the projected aperture shape from a double-biogon wide-field lens like the Super Angulon. Those are actually quite good at maintaining the roundness of the aperture even to the edges of the coverage.

The answers to a question like this soon overwhelm the simple initial question and begin to play off one another for the amusement of the answerers more than the enlightenment of the original poster.

Rick "self-incriminated" Denney

[ic-racer]

On the other hand, look at the projected aperture shape from a double-biogon wide-field lens like the Super Angulon. Those are actually quite good at maintaining the roundness of the aperture even to the edges of the coverage.


Yes. And perhaps an advantage to a Biogon over a retrofocus desgn.

Here is a 28 biogon MTF. Indeed the radial and tangental resolution is similar.
http://img81.imageshack.us/img81/2081/zeisscontaxg28mmbiogonmzk8.png (http://img81.imageshack.us/i/zeisscontaxg28mmbiogonmzk8.png/)

And here is the 28 Distagon (retrofocus) which has a more crescent aperture off axis. The tangental and radial resolution is quite different, as expected.

http://img.photobucket.com/albums/v670/ic-racer/28distagon.jpg

[Dan Fromm]

But ic-racer, MTF charts don't show resolution, they show contrast delivered as a fraction of contrast received. At 1., top left corner of the chart, contrast delivered = contrast received.

Each pair of lines (solid and dashed) in the MTF charts you posted corresponds to a resolution. I haven't looked for the charts you posted so don't know what resolution each pair of curves corresponds to. Whatever they are, the bottom pair of lines (least contrast delivered) is for the highest resolution.

Here http://www.dioptrique.info/OBJECTIFS16/00753/00753A.HTM are calculated curves for the 1952 f/4.5 Biogon. The lower left chart (Ast. Tan and Ast. Sag at the bottom) shows the positions of the two surfaces (tangential and sagittal) of best focus relative to the point of best focus at the center of the field.

Oh, Eric follows the convention that focal length = 100. You'll see this a lot in lens patents. And I erred in an earlier comment, the air gaps' thicknesses give the elements' spacing.

[ic-racer]

Here is some additional info on the subject from Zeiss. See page 19.
http://www.zeiss.de/C12567A8003B8B6F/EmbedTitelIntern/CLN_30_MTF_en/$File/CLN_MTF_Kurven_EN.pdf

[Tim Povlick]

Hi Emmanuel,

Thanks for bringing up the f-Theta (telecentric). I did manage to find it on the Rodenstock site. I didn't realize they were used in scanners.

When I see your responses to optical questions I always read them closely as they are very informative.

Hi Ic-racer.

That 1'st MTF curve is really something how the S and T are so tight.

Thanks to all others who contributed also! It's very interesting.

Best Regards,

Tim

[Dan Fromm]

Here is some additional info on the subject from Zeiss. See page 19.
http://www.zeiss.de/C12567A8003B8B6F/EmbedTitelIntern/CLN_30_MTF_en/$File/CLN_MTF_Kurven_EN.pdfI've been to p. 19. So?

[ic-racer]

The diagram on p. 19 shows graphically how the ellipsoid presentation of the point spread affects the MTF in two perpendicular orientations.

[Dan Fromm]

ic-racer, fine, wonderful. But I already knew that. No need to reply, but what I questioned was your reading of the MTF curves (see post 11 in this thread) as showing resolution.