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Re: [OM] MTF Lens tests

Subject: Re: [OM] MTF Lens tests
From: Winsor Crosby <wincros@xxxxxxxxxxxxx>
Date: Mon, 22 Jul 2002 15:37:05 -0700


 I still have trouble wrapping my mind around the meaning of MTF.  How
 do you interpret the data?

Hmm, Higher is better :-)

MTF is determined with several test pattern (grids), consisting of line pairs
(black and white, or respectively tranparent or not).
The lens is projecting such a  pattern on a sensor which is meassuring the
maximum and minimum brightness (contrast).
This difference is compared with the min-max. difference of the
orinal pattern.
The MTF-value compares  the contrast (differences) of the orginal
pattern with the contrast of the projected pattern.

When the projected contrast is the same as the grid the MTF would be 100%.
A fine grid wich is not resolved any more appers as a even gray area:
No difference -> No contrast -> MTF = 0%.

A high MTF indicates that the lines are saturated black and white.
Lower MTF indicates that it is becoming dark and bright gray instead.

The pattern do have (5 lp/mm), 10 lp/mm, 20 lp/mm and 40 lp/mm.
(often refered as  frequency, eg. = 10 lp/mm)
The value of 5 lp/mm (= line pairs per mm on the negative) was
meassured only by Color Foto in the later test (4 lines per diagram!).

5 and 10 lp/mm describe the contrast of image structures.
20 and 40 lp/mm rather indicate the resolution of the lens.
As reference. For dof-calculations 0.033 mm max. c.o.c. equals about 15 lp/mm

On the left side of the diagram (MTF-axis) you see the values of the
center, at 21mm image heigt the edge at the end of diagonal of the frame,
where you can see corner falloff.

Hmm, the values of the table are simply the average for a frequency
over the diagonal. No centerweighting like photodo does. I´ve used the
calculator an simply summed them together to one value.

I hope this is understandable and helps a bit.

Frieder Faig


Aha! What a great explanation. I have filed it away, but I doubt I
will have to refer to it. Thank you.It helps much more than the any
explanation I have seen before, if they bother to explain it.

Hmm. It kind of pleases me that my 21/3.5 did so well against the 21/2.

Can you tell me what the dotted lines represent in the line graphs?

--
Winsor Crosby
Long Beach, California

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