Leica MTF Graphs

[Mustafa Umut Sarac]

I looked too many MTF graphs of Leica , Zeiss and japanese lenses and one thing I have just noticed.

When you look to Summicron 50 , sagittal and tangential curves compensate each other in a very smooth way. When sagittal dives down , tangential up or reverse. Same is nearly true for Zeiss but they have more aggressive curves.

Modern Canon lenses have no relation with that concept and two cuves are as a horizontal line at the top or aggressively at the bottom.

Could be that Leica speciality and dance between two curves , responsible for 3d look ?

Thank you ,

Umut

[Wolf Rainer Schmalfu]

Originally Posted by Mustafa Umut Sarac

I looked too many MTF graphs of Leica , Zeiss and japanese lenses and one thing I have just noticed.

When you look to Summicron 50 , sagittal and tangential curves compensate each other in a very smooth way. When sagittal dives down , tangential up or reverse. Same is nearly true for Zeiss but they have more aggressive curves.

Modern Canon lenses have no relation with that concept and two cuves are as a horizontal line at the top or aggressively at the bottom.

Could be that Leica speciality and dance between two curves , responsible for 3d look ?

Thank you ,

Umut

Most of the lens manufacturer publishing the theoretical MTF graphs, and not the real practical MTF values. Of course, they look much better! Only Zeiss is publishing the real practical MTF measures!

[ssmc]

Diverging sagittal and tangential MTF curves are responsible for astigmatism and/or field curvature.

Getting the curves to match as closely as possible is a laudable design goal for the best edge/corner performance in a plane - but since many subjects are not planar, a lens that tests great on a flat chart may in fact have odd properties either side of the plane of best focus, and vice-versa.

I seriously doubt that Leica or Zeiss intentionally design lenses with astigmatism or severe field curvature but it may be an unavoidable consequence of other factors such as low distortion, and very importantly for a rangefinder, compact size. If a lens is too big it will block too much of the finder which renders it less useful for many purposes. SLR lenses can be as large as they need to be, with the only real design restriction (other than cost) being the minimum flange focal distance required to clear the mirror.

Regards,
Scott

[Mustafa Umut Sarac]

Lets analysis the Scott generously gave us.

What are astigmatism and field curvature , how it deformates the image and what can it cause something positively at the image.

The biggest difference between Leica Supertele images and Nikon Supertele images is , the Nikon have many out of focus areas at the image. If a bird photographed , Leica presents a full in focus photograph but Nikon have many ugly out of focus areas on bird.

Can above two properties causes it ?

Thanks,
Umut

[semilog]

The biggest difference between Leica Supertele images and Nikon Supertele images is , the Nikon have many out of focus areas at the image. If a bird photographed , Leica presents a full in focus photograph but Nikon have many ugly out of focus areas on bird.

I know people who take serious photos of birds (Nikon, Canon, Leica), and I've never heard a claim remotely like that one. If you're going to make claims like that you really need to provide well-controlled example images.

[ssmc]

I too have never heard of such claims regarding "superteles". But anyway...

Astigmatism causes tangential structures to be focused in a different plane (or curved surface) to radial structures. It should never be present in the center of an image unless the lens had been assembled incorrectly or one of the elements is ground wrong.

Wikipedia has some decent articles on the 7 first-order aberrations, with reasonable illustrations, though the ones for astigmatism are not great, so here is a real-world example of astigmatism taken from the upper right corner of an image. Note how the wires are rendered differently depending which direction they run. Also note the very bad effect this has on the foliage at lower right and the sharpness of the pole (this example was caused by front-focusing)



Here is a bigger (100%) version - for some reason the link won't work:

http://www.pbase.com/smcleod965/imag...3/original.jpg

Here are some links to the wiki articles:

http://en.wikipedia.org/wiki/Lens_Aberrations

http://en.wikipedia.org/wiki/Astigmatism

http://en.wikipedia.org/wiki/Petzval_field_curvature

http://en.wikipedia.org/wiki/Distortion_(optics)

Hope this helps
Scott