Explanation of testresults

Sample for a 4"F/8 Modell

Testreportresults at 532 nm, green wavelength:

wavefront p.t.v. 0.175 = 1/5.7

wavefront RMS 0.017 = 1/58.8

Strehlratio 0.989 = 98.9 %

The german company Carl Zeiss Jena did never print wavefront
p.t.v. results in their testreports due to a good reason. The Interferometer
print the worsest result, which you get from the worsest local error. So if a
Zeiss Lens with a very high strehl would show a middle quality wavefront p.t.v.,
customers maybe thinking Zeiss lenses are not that good, because they are mostly
not able to read a testreport. Therefore we try to explain now how to read such
a testreport:

On the right side of our testreports you read in the topline :
Wave Front's deformation, which is the topographie and also called phasemap,
which is nothing else than a profile of the complete optical surfacearea.

Normally customers guess and read that a overal relation between
wavefront p.t.v. and wavefront RMS should be around 1:4. Checking the relation
on this lens shows us 58,8 RMS : 5,7 PV = 1:10.3. But why is there a difference?
A relation of 58.8 RMS should show (58.8:4 = 14.7 = 1/14.7 wavefront p.t.v.),
again why we have such a difference on this testreports from the general
relation of 1:4?

Let's check the phasemap more carefully now: On the right side of the map we
see a column with letters for diffrent colors. This letter starting on
bottomside at 0, in the middle with 0.0877 and at top 0.175. The 0.175 is the
printed wavefront. This line on the right side show 4 diffrent colors, starting
in the bottom 25% blue color, next 50% green color, next 25% yellow color and
upper 25% redcolor. Now lets see what 's the amount of wavefront we have for
each color:
0.175 : 4 colors = 0.0437 wavefront p.t.v. for each color. That
results as follows:

all surfaces you can see on the map with blue color have a wavefront
p.t.v. of   0.0437 = 1/22.9 wavefront P-V

all surfaces you see on the map with blue and green color have an
wavefront of   0.0437 x2 = 0.0877= 1/11.4 wavefront P-V

all surface we see on the map with blue, green and yellow color have a
wavefront of 0.0427x3 =0.131 = 1/7.63 wavefront P-V

all surface total with all 4 colors have a wavefront of 0.175 , which is
1/5.7 wavefront P-V

Now lets take an look to the phasemap where we see all this 4 colors:

The surface have an amount of about 70% blue surface, which means 70% of
the lens is 1/22.9 wavefront p.t.v. or better

The surface have an amount of 97% blue and green, which means 97% of the
lens surface is 1/11.4 wavefront and better

The surface have an amount of about 99% blue, green and yellow, which
means 99% of the surface is 1/7.6 PV and better

The relation 1:4 between PV and RMS is valid for overal correction and not
for local errors.The red and yellow colors are smallest surface peaks, which
having any time from different reasons, sometimes from lensweight on
lensspacers, sometimes from little glasinhomogenities, sometimes from little
zones, etc.

On this lens now the overal correction is better than 1/11 wavefront PV,
mixed with the 1/22.9PV from the blue color, this results in a fair mixture of
around the 1/14 wavefront which we need to fulfill the known relation of 1:4 .
So please, don' t be surprised if you have a lens with such a testreport
printresult of 1/5.7 wavefront p.t.v., but your startest show better than 1/10
wavefront. The 1%,.2%, 3% small local errors are not detectable for your eyes
and have a not noticable influence in imagequality.

last updated 18/07/02,
© '99 APM Markus Ludes, V 09.99