MTF curves

ray_parkhurst · Post by **ray_parkhurst** » Wed May 17, 2017 8:23 pm

The MTF curves I've seen recently for the inspec and other lenses here:

http://www.photomacrography.net/forum/v ... hp?t=29363

are confusing me more than MTF curves usually do.

Is there a standard for stating magnification? What do these mean?

-0.5
0.5
-3.5

Why is it stated in that post that -0.5 is the same as 0.5 and is 1/2x magnification? What does the "-" mean?This is extremely confusing and I finally decided to ask.

I do understand the basics of MTF so don't try to explain frequency or modulation. I just want to know about how the quantities are reported in the graphs, and how to interpret them.

This may be explained in an FAQ but I've read about a dozen explanations of MTF but not a single one talks about negative magnification.

I have further issues with curves I've seen and will probably ask in this thread.

rjlittlefield · Post by **rjlittlefield** » Wed May 17, 2017 9:28 pm

Negative magnification just reflects the fact that the projected image is inverted.

See https://en.wikipedia.org/wiki/Magnification#Photography for some discussion.

Note that the use of negative magnification differs between communities and can be a source of great confusion when reading the other group's papers. For example, in https://en.wikipedia.org/wiki/Numerical_aperture#Working (effective) f-number , the formulas simply give the wrong answers if you plug in positive magnifications instead of the negative ones that are expected.

--Rik

ray_parkhurst · Post by **ray_parkhurst** » Thu May 18, 2017 6:23 am

rjlittlefield wrote:Negative magnification just reflects the fact that the projected image is inverted.

See https://en.wikipedia.org/wiki/Magnification#Photography for some discussion.

Note that the use of negative magnification differs between communities and can be a source of great confusion when reading the other group's papers.
...

--Rik

So does this mean the inspec lens quoted in my link above does not invert the image? Or is Linos/Rodenstock just being inconsistent/sloppy?

rjlittlefield · Post by **rjlittlefield** » Thu May 18, 2017 9:39 am

I'd go with inconsistent/sloppy. It's weird to see such inconsistency in two graphs that look like they came from the same source & setup, but I don't know how else to explain the discrepancy.

--Rik

RobertOToole · Post by **RobertOToole** » Thu May 18, 2017 5:40 pm

rjlittlefield wrote:I'd go with inconsistent/sloppy. It's weird to see such inconsistency in two graphs that look like they came from the same source & setup, but I don't know how else to explain the discrepancy.

--Rik

Schneider Kreuznach uses the same system of negative values for ß' (magnification). Now I am curious.