Hi--

>Dear Bob Neuman (or is it David Ruether, I'm confused on that issue)

DR - I use Bob's account, and have been writing under his name
and "HTH" for the last 2-years or so...

Thank you for the long and thoughtful response. While I cannot question your
knowledge, technique, or thoroughness, I am still surprised by your results...
(and it is fun meeting a fellow lens checking nut - and one even more
intense and thorough than I am...;-). You may find the last bit of
"SUBJECTIVE Lens Evaluations (Mostly Nikkors)" kinda interesting...;-)
It can be found at these sites:

-- Jan-jaap Aue: (nice text format)
http://www.phys.rug.nl/mk/people/aue/nikon/david.html
-- Quang-Tuan Luong: (nice text format)
http://www.cs.berkeley.edu/~qtluong/photography/35mm/nikon-neuman.html
-- Niklas Nikitin: (nice table format)
http://www.cs.hks.se/~nicke/private/photo/lenstest/david.html
-- Henrik Elowsson (nice table format)
http://www.island.liu.se/~i96henel/nikon/slemn5a.htm
-- Robin Kanta (nice table format)
http://www.worldphoto.com/david.html
-- Leo Verwoerd (plain text and table formats)
http://utopia.knoware.nl/users/leover/5a.htm
-- Bo-Ming Tong: (plain text format)
http://www.cs.arizona.edu/people/bmtong/nikon/c.html#13

The checking methods used were informal, and the evaluations *subjective*,
but I trust it can serve as a useful guide for those into sharpness.
A new version will come out eventually, in which I will try to evaluate
the zooms better (their performance is so much more complex, and the
sample variation is so much greater than with primes, that one-number
evaluation ratings are close to useless - and the numbers in "5a" are
higher in general for the zooms than they will be in "6").
I also like to use good lenses optimally when I can (it is surprising
that most people are unaware that the focused distance can be very
important to performance level, though many seem to be aware that
some stops may produce sharper images than other stops - and some realize
that sample variation and version variation can be relevant, also, to
image quality.
As I have pointed out several times recently, I am moving back toward
the use of primes (I prefer good corner performance), and the slower
versions of those at that (mostly for weight/size reasons), so the
20-35mm has not been very interesting to me (the 20mm f2.8 is a fine
lens...).
To comment on a few other things in your letter...:
- I do notice the decline (with the expected diffraction-limiting)
of performance with moving toward smaller stops - but my observation
(admittedly less thorough and precise than yours - but good enough to
see the results) indicates that all the lenses I've tried are still
useable (if not optimum, in most cases) and similar at f16.
- I have noticed that the resolution of the 300mm f2.8 does not
improve appreciably when stopping down to mid stops, but the contrast
does ("sharpness" for me is a combination of resolution and contrast,
not just one or the other...), at least until one rounds the mid-aperture
peak and begins the (gradual..., at least with most films...) diffraction
slide down hill... (Velvia, being unusually sharp, would somewhat
exaggerate that slide, but should wind up no worse at f16 than other
films, just better at optimum stops).
- I would not trust Nikon's service for adequately checking anything - my
experience with them is that they can discern only gross alignment errors...
- I find Velvia to be 40, relative to the vast majority of other slide film
speed ratings, but Nikon meters are often off the equivalent 1/3rd stop...
- I applaud the thoroughness of your set-up (but, ah, what distances do you
check at...?
- You are right about using optimum stops - it is surprising how many people
ask questions about which lens is best, but are unconcerned about using them
at good apertures/distances - my point was that at f16, most good lenses are virtually identical, at least in the center (though obviously better at mid
[or wider] stops [in the center], and away from less-than-optimum focus
distances).
- >(my name above isn't the way it is written in Norwegian and my ears cringe
>by the very thought of how the above is pronounced. However, given the
>American dominace in cyberspace, we minority people have no choice but
>following suit and try to use CyberEnglish)
Ah, yes, we do forget that we are blabbing in our native tongue, completely
unaware that most folks out there are having to struggle along wi' thet thar
ferrin lang-goo-ij...;-) ("with that there foreign language", if you needed
a translation...;-) Your English is superb, and considerably better than us American's general grasp of our native language...;-)

Thanks, again, for writing (You should see the differences of opinion
in audio! At least in photography, there is SOME rooting in reality...!;-).
David Ruether


>I'm quite sure that we see more or less the same trends when we scrutinise a
>Nikkor lens. For example, your recent report on the 28 f/2.8 AIS Nikkor (the
>"good one") was virtually identical to my own. So, why is it we seemingly
>disagree on the 20-35 or other lenses? I have given that some thoughts, and
>they follow below.
>
>First of all, I would like to say that being the owner of some 75 Nikkor
>lenses and a professional nature photographer, implies I try to get the most
>out out of my tools (= the lenses). After all, I try to make a living out of
>nature photography, and the clients expect the very best quality. My
>educational background is a PhD in statistics and hydrological optics, so
>feel entitled to make inferences from testing .... I therefore test my
>lenses thoroughly, not only in the lab, but also under true field conditions
>to ensure I know the weak and the good points of any of these Nikkors. To
>make sure I can reliably and repeatably get the data I want, I shoot under
>standardised conditions employing a carefully calibrated Nikon body (F4
>before, now F5). This camera is checked out on the importer's professional
>service facility. I always use the best professional tripod available to me,
>i.e., the Sacthler tripod (which, incidentally, is vastly superior to the
>Gitzo/Bogen/Manfrotto or any other), and for any lens above 200 mm, I employ
>a Sacthler *pentapod* instead of a tripod. Mirror lock-up is used on long
>lenses although I must say I never noticed any difference on either the F4
>or F5 (presupposing the use of a tri/pentapod of course, and electric
>shutter release). The film is invariably Fuji Velvia which likewise always
>is exposed at 50 ISO (its true speed) and processed professionally within 2
>hours. The images are examined at 20-80X magnification with a Nikon
>stereoscope using diffuse cold-light illumination. I file the test films and
>all lenses used regularily are re-tested every year to make sure they are in
>good working condition.
>
>So, given this long preamble, how is it we disagree?
>
>It is fairly clear to me that modern computer-designed lenses in fact do
>differ from older designs. As a general rule, on-axis sharpness in
>particular of zoom lenses tend to be slightly higher. The same can be said
>for contrast of many new lenses. However, the central image isn't everything
>and in order for a picture to come out properly in printing, also the
>corners may need to be good. As you indicated in your e-mail and elsewhere
>in usenet discussions, the corners of many lenses do tend to be soft at
>large apertures. I agree with that. The culprit is very often the designer's
>use of the Slussarev effect, i.e. combatting light fall-off and vignetting
>by introducing coma into the exit pupil to enlarge it off-axis. The added
>lens elements to reduce this error "downstream" may be insufficient at large
>FOV's, for instance, at 20-24 mm for the Nikkor 20-35 f/2.8. I suspect
>chromatic errors are downweighted amngst the criteria used for optimising a
>new lens design. However, some of the off-axis softness is also due to
>curvature of field to show up very clearly on flat-field targets while being
>less objectionable for field photography. This
>brings me to my main objection to using the 20-35 or other similar designs
>for landscape photography. Chromatic errors may be insignificant within the
>in-focus zone whilst they produce a "roughening" of the zone that is outside
>the best focus to give a significant loss of detail. This results because
>the primary colours do not coincide in their focal points and therefore,
>within the theoretical DOF there may not be sufficient actual sharpness.
>This can easily be seen under high magnification. It has nothing to do with
>my 20-35 being a bad sample, I have tried 4 or 5 of these with identical
>results, and my observations comply entirely with lab tests run in European
>journals.
>
>Now, is the 20-35 Nikkor a bad lens? - no way, it is in its own a splendid
>performer and my top choice for my F5. For photojournalistic use, where
>great DOF is not needed, it is a joy to use and gives me high quality images
>that print very well for front covers etc. Here I use f/2.8 to f/8 and these
>apertures work very well. However, I will always take my 25-50 f/4 Nikkor
>for landscape use because this lens produces so nicely imagery throughout
>the DOF, in fact, much better than the 20-35.
>
>You commented that you hadn't noticed sharpness declines on the high-speed
>long Nikkors at f/16. Each and every of these Nikkors I own, and they amount
>to many, show exactly the same pattern of sharpness vs f-number. For
>example, the 600 f/4 looses sharpness already 1/2 stop down from wide open.
>It is a terrific performer at f/4, so why would one want to stop it down?
>You gain negligible amounts of DOF with such a lens anyway, no matter what
>aperture is used. At f/16, longitudinal chromatic errors from the secondary
>spectrum are just awful with the 600. The 200 f/2, 300/2, 300/2.8, 400/3.5,
>800/5.6, and 800/8 behave the same way as the 600. These are designs
>optimised for using them wide- or nearly wide-open. I think you are misled
>by the *contrast* increase they shown upon stopping down. This increase goes
>on to f/8 or something. However, enhanced contrast (mostly coming from
>reduction of flare, to which these lenses are extremely susceptible) cannot
>make up for the rapid loss of sharpness and fine detail from elevated LCA
>that arises from stopping them down.
>
>People should pay more attention to the optimal apertures for their lenses
>to get much higher quality from their 35 mm pictures. Too many people shoot
>landscapes with 20mm lenses at f/22 and complain about poor image quality
>afterwards. I am extremely critical with the aperture setting, and try to
>make the best compromise between the demands for DOF and high sharpness. To
>that end, tilt/shift lenses sometimes are the only feasible choices and I
>accordingly use them a lot. They include my converted 28 f/3.5 Tilt/Shift
>Nikkor, Canon 35 f/2.8 TS adapted to Nikon mount, and the Bellows-Nikkors
>105 f/4 and 135 f/4 on a Zoerkendorfer tilt/shift adaptor. By using these TS
>lenses the need for stopping down to gain DOF becomes less acute.
>
>With best wishes,
>
>Bjorn [NFOTO Bjorn Rorslett ]
>(my name above isn't the way it is written in Norwegian and my ears cringe
>by the very thought of how the above is pronounced. However, given the
>American dominace in cyberspace, we minority people have no choice but
>following suit and try to use CyberEnglish)
>
>