Text of Zeiss paper (looooong)

[T3D john bercovitz <bercov@xxxxxxxxxxx>]

Resolving Power and Contrast

Erich Heynacher and Fritz Kober

Reprinted from
ZEISS INFORMATION No. 51

Interest in photography - be it 
professional or as a hobby - is growing 
constantly. It is therefore not at all 
surprising that more questions are 
asked about the performance of lenses 
before a person decides on the purchase 
of a camera or a new lens. Such 
questions cannot be answered in just a 
few words because, in evaluating a 
photographic lens, a great number of 
factors has to be taken into 
consideration. The following article 
may furnish valuable hints to anyone 
interested in the study of image quality 
in photographic lenses.
Lens manufacturers are frequently asked 
what the resolving power of their 
lenses is, in the assumption that the 
resolving power is a criterion for 
quality. However, the German optical 
industry does not disclose any figures 
concerning resolving power for the 
following three reasons: 1.	The 
disclosure of figures on the performance 
of lenses is rather problematical. A 
proper evaluation of the quality of a 
lens is possible only if several 
numerical values are known, which - 
for example - give information on the 
sharpness of the lens over the entire 
image area from the center to the very 
corners of the field, the image quality 
at different lens openings, starting with 
full aperture,  the image field 
illumination (vignetting to- ward the 
corners), the distortion, etc. These 
performance data must be care- fully 
balanced and different emphasis must 
be placed on the various values 
depending on the intended use of the 
lens, e. 9. for general photographic 
purposes, for portraiture, for 
enlargements, or copy work. 
Consequently, only a person fully 
experienced in the testing of 
photographic lenses will be able to 
evaluate such data without the risk of 
misinterpretations. Furthermore, any 
data on the performance of a certain 
lens is useful only if these values can 
be fully applied in the regular 
production run of a given type of lens. 
2.	When determining the 
resolving power, it is possible to 
influence the results quite considerably 
by using photographic emulsions and 
development techniques which deviate 
from the common practice. As a result, 
as long as no standard specifications 
exist, manufacturers may be tempted to 
apply measuring techniques which may 
upgrade the results but do not 
correspond to actual conditions.

3.	Finally- and this is the main 
reason why the German optical 
industry refrains from giving lens 
resolution figures - the re-solving 
power is not as important a criterion 
for image quality as is generally 
believed. This statement will be 
proved in the following paragraphs. 
A number of photos were taken with 
perfectly uniform image quality over 
the entire field, so that it is not 
necessary to balance center sharpness 
against edge sharpness. Neither do 
these pictures show any visible 
vignetting nor distortion. We may, 
therefore, use these photos without any 
reservation for comparative image 
quality tests. Let us first consider 
photos 1 and 2. They are both of poor 
quality. If you had to choose, however, 
which of the two would you prefer? At 
first glance, you would probably select 
photo 2. At least, that is what 
everyone did who saw the pictures up 
to now. Photo 2 appears to have much 
higher contrast than photo 1, the latter 
giving the impression of being fuzzy. 
However, if you take a close look, you 
will notice that photo 1 has a 
considerably higher resolution and 
better definition than photo 2 which, 
upon close examination, is rather 
unsharp. We do not know for which of 
the two pictures you will finally settle. 
Your decision will largely depend on 
your personal preference. In any case, 
however, you will certainly not find 
photo 1 so much better as the 
resolution figure would have it. 
Actually, the lens resolution figure for 
photo 1 is twice as high as that for 
photo 2. Let us now turn to photos 3 
and 4. Their image quality, or at least 
the image quality of photo 4, is much 
better than that of the pictures 
previously studied. And yet, it is 
photo 3 which has the higher 
resolution. This is easily recognizable 
in the ornaments of scepter, crown and 
robe. But unless we take a very good 
look, we do not realize it. The higher 
resolution, therefore, is of no 
consequence for the impression created 
by the picture. Far more striking 
examples could be presented in photos 
of really good image quality. 
Unfortunately, this cannot be done here 
because the screens employed in the 
printing process would destroy the fine 
detail which we need to prove that a 
poorer photo actually can have the 
higher resolution. But with the aid of a 
trick we can create similar conditions 
to those en- countered in very good 
pictures. For this purpose, we need 
only choose a larger viewing distance, 
in other words, we observe photos 3 
and 4 from a distance of, say, 3 or 6 
feet instead of from the normal reading 
distance of approximately 10 in. There 
will then be absolutely no doubt about 
which of the pictures is the better one, 
and we realize of how little avail high 
resolution can be. The fine detail 
reproduced in the photo with the 
higher resolution can no longer be 
clearly seen. In other words, it does 
not matter whether it is resolved or 
not. The above examples have 
demonstrated that it is possible to have 
poor photos show- in high resolution, 
and good pictures with moderate 
resolution. Two photos of identical 
resolution may be entirely different in 
image quality. Just compare photos 1 
and 4 from a distance of about 3 feet. 
Both pictures have the same lens 
resolution figure - but what a difference 
in image quality. It is evident that the 
resolving power - or at least the 
resolving power alone - is not the 
decisive criterion in evaluating the 
quality of photographic lenses, and this 
is what we have tried to show. At this 
point the question may be raised 
whether the poor image quality in 
photos 1 and 3 may not primarily be 
the result of softer printing as compared 
to the printing of photos 2 and 4. The 
answer is no because all four examples 
were taken on identical photographic 
material and treated alike during 
processing. There is, of course, a 
possibility of improving photos 1 and 
3 to a certain extent during processing 
(by using high-contrast photographic 
material with the added control of 
dodging, redevelopment etc.), but the 
improvement will be rather 
insignificant even with a subject of 
very few intermediate tones, as is the 
case in our examples. With true half-
tone pictures, the above technique 
would be practically use- less, because 
any attempt at improving the image 
through the use of high-contrast 
photographic material would 
automatically lead to a decrease of 
tonal gradation. But, you will ask, 
what is it that distinguishes photo 4 
with its modest resolution from photo 
1 which has the same resolution as 4, 
and photo 3 with high resolution? 
What then, if not lens resolution, is a 
valid criterion for image quality? Our 
comparative photographs have shown 
that the image quality is not so much 
deter- mined by the definition of fine 
detail as by the manner in which the 
more easily perceptible, larger 
structural elements in the picture are 
reproduced. The more faithful the 
contrast ratio, the better the image 
quality. It is obvious that the degree of 
accuracy of contrast rendition in an 
image depends on how coarse or fine 
the respective structural elements of the 
image are. The contrast in very coarse 
structural elements will always be 
reproduced to a fairly accurate degree. 
On the other hand, there is no such 
thing as "true contrast rendition" as 
soon as we go beyond the limit of 
resolution. Details, however, the size 
of which lies between these two 
extremes, will not be reproduced 
absolutely true, but still with more or 
less good contrast. In order to get a 
better idea of the image-forming 
properties of optical system, it seems 
appropriate to look at contrast 
rendition as a function of the size of a 
given detail. A very simple method is 
employed for determining contrast. 
Screens are used with equidistant white 
and black lines of the same width. The 
number of lines in 1 mm space serve as 
a measure of the size of a detail. Figure 
5 shows sketches of two such screens. 
Figure 6 indicates the contrast 
rendition as a function of the screen 
size employed for the test. Line 5 
represents the ideal contrast rendition 
in a perfectly "true" optical imageU 
Curves 1 to 4 represent the contrast in 
our four sample photos. The numbers 
of the curves correspond to the 
numbers of the illustrations. It will be 
noted that in photo 1 the contrast of 
even coarse details (a few lines per 
millimeter) is strongly reduced.  This 
is the reason why the picture looks 
"soft" or "fuzzy-. By comparison, the 
contrast of photo 2 falls off more 
slowly from the ideal value. This 
photo therefore seems to be richer in 
contrast. It is true, of course, that 
photo 1 contains much finer details 
than photo 2U though at the sacrifice of 
contrast

In order to render fine details visible, 
little contrast is required. Under 
favorable viewing conditions, about 
5% will suffice. This value is marked 
in the curves by a circle. Each circle 
indicates the limit of resolution, and 
the resolution figure is given by the 
respective number of lines/millimeter. 
Thus, in photo 1 the resolution is 
about twice that of photo 2, while it is 
identical to photo 4, as was mentioned 
before. - What was said about photos 1 
and 2 is applicable also to   It is non-
existent in practice because a certain 
unsharpness is caused by diffraction of 
the light at the diaphragm opening 
even in a perfectly error-free lens 
Moreover, an additional unsharpness is 
produced by the scattering of light in 
the photographic emulsion It is known 
that any unsharpness leads to a loss m 
contrast transfer. pictures 3 and 4 with 
the only difference that the curves of 
contrast transfer show higher values 
than the curves 1 and 2, thus 
representing a higher image quality. 
All this can be read at a glance from 
the contrast transfer curves. But that is 
not all.
If we include in our study the limit of 
resolving power of the human eye, as 
represented by the vertical line 6, this 
line will divide the range into two 
regions, one in which fine detail is 
perceived by the eye (left of the line), 
and the other in which fine detail is 
invisible to the eye and thus not of 
interest (right of the dividing line). 
Line 6 applies when looking at the 
photos from normal viewing distance. 
As the viewing distance increases, the 
number of perceptible details decreases, 
and the dividing line is displaced to 
the left. For observation from a 
distance of approximately 3 ft., for 
example, line 7 applies. Therefore, 
within this range of detail perception, 
contrast transfer curve 4 is closer to the 
straight line 5, representing the ideal 
lens, than contrast transfer curve 3. 
Consequently, photo 4 must have a 
better image quality than photo 3.
Considering all these facts, there is no 
doubt why the resolving power as such 
is not a suitable criterion for image 
quality. It is only a point on the curve 
of contrast transfer, and we have seen 
that in good images this point even 
lies far outside the important image-
forming range. What really matters is 
the contrast rendition within that part 
of the range in which detail can 
actually be perceived by the human eye 
at normal viewing distance.
Fig b: Examples of One screens as 
used for the measurement of contrast 
transfer. The screen size is expressed 
by the number of lines/millimeter 
(frequency).
Fig. 6: Contrast transfer curves for the 
ideal image (5) and sample photos 1 to 
4 (curves 1 to 4) The limit of resolving 
power of the eye for a viewing distance 
of 10 in is shown by the straight line 6 
for a viewing distance of approximately 
3 ft. by line 7. Multiplied by to the 
figures given for detail size could be 
the conjugate numbers of 
lines/millimeter in the negative.  


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