Hi C.H. and all,
> From: "C.H.Ling" <ch_photo@xxxxxxxxxxxxx>
>
>Unfortunately, for this great Tessar I see not that nice bokeh - the leaves
>and branches.
Being an aesthetic matter, bokeh may be highly subjective... however, it
usually isn't an issue for me. Remember that Tessars are relatively slow
designs, which gives less freedom for bokeh control.
Neverthless, I find this picture (taken with the Tessar 45/2.8) nice, at
least to my taste. YMMV: <
http://cjss.sytes.net/web/www.supercable.es/~santis/etc/tessar.jpeg>
I believe the strong points on the Tessar are: extreme flare resistance,
good-but-not-excessive image sharpness (maybe not wide open, but doesn't
look bad either) and the very good colour correction -- see next paragraph.
>Also, strong CA near the corner (not so corner for a 1.6x
>sensor) of the crop from the tram in city :-)
I don't think it's CA... the Tessar wide-open shows a certain amount of
primary, monochromatic aberrations (like spherical, coma, astigmatism) --
the "fringe" looks blue because there's blue in the nearest regions!
OTOH, the test pics on that web from the Sonnar 135/2.8 <
http://galactinus.net/vilva/retro/eos350d_sonnar135.html> show what I think
it's _longitudinal_ CA -- otherwise it should be an superb lens, or so
performs mine!
Tessars, especially those with Lanthanum glass, are said to be extremely
well colour-corrected, with residual CA levels almost as low as modern APO
designs! In a tests I did a while ago (sorry, no pics available) the Tessar
45 was able to portray sharply a very distant, fancy coloured clothes-line
which otherwise had about the same luminance than the background -- it was
nearly invisible thru other lenses!
>Talking about good pancake, I would recommend the Zuiko 40/2.
Sure, it's a GREAT little lens! One of my favourites in the OM-System, I
always take it in trips -- my usual light kit is either 21/2 + 40/2 + MC
85/2 (as seen in my trip to Senegal) or 24/2.8 + 40/2 + silvernose 85/2
(which I took to Uruguay)
IMHO, it is a fine performer. And the 40mm focal length is IDEAL for me... I
also like the compact size; handling may suffer because of this, but with a
UV filter the aperture is much easier to handle.
>It has the largest aperture of all pancake (?).
I'm afraid not... there was a K*nica Hexanon 40/1.8 -- reportedly one of the
sharpest lenses ever made, even wide open.
>Nice bokeh and good CA control at F4
>and below.
I used the 40mm for my entry in TOPE 20, shot on film: <
http://www.tope.nl/tope_show_entry.php?event=20&pic=30>
Another bokeh sample from the Zuiko 40, this time with digital (I think I
posted this already): <http://cjss.sytes.net/atachaos/audrey40.jpeg>
Why I wrote so firm about the color fringe issue is I have read lots of test
> reports and tested many lenses myself, most if not all color fringe probem
> can be reduced/eliminated by stopping down the lens.
This is an interesting issue. CA is a rather complex matter... According to
'Optical Instrumentation - Theory and Design' by Begunov, Zakaznov,
Kiryushin and Kuzichev (Mir Publishers, Moscow 1988), CA can be of three
different types, usually mixed:
1) Axial (or longitudinal): the focus point simply shifts depending on
wavelength. In reasonably corrected systems (any photographic lens) the
focus point goes back and forth, coinciding at two wavelengths, usually
around red and blue -- APO systems may coincide at THREE wavelenghts.
Axial CA affects all of the image (centre and corners) equally. It shows as
a blue-violet halo around highlights, most noticeable on simple, long focal
devices (i.e. telescopes). Being a focus shift, its visible effect
diminishes when stopping down, due to higher depth-of-field (or, more
precisely here, depth-of-focus) although the amount of shift itself remains
the same.
2) Transverse, AKA chromatic difference of magnification, AKA "lateral
colour": Its effect is ZERO at the center, growing towards the edges. It
shows as fringing of one colour at the "outer" side of lights, and another
colour at the "inner" side -- these colours are blue-purple and orange-red
for uncorrected systems, and typically "apple green" and "cherry red" for
corrected ones.
Please note that this aberration already arises at the paraxial region; that
is, it won't be reduced by stopping down! However, its amount also depends
on the degree of correction of axial CA.
3) Spherochromatism, AKA chromatic variation of spherical aberration: like
monochromatic spherical aberration, light rays piercing the lens at
different distances from the centre will be focused at slightly different
distances -- now varying with a different pattern depending on wavelength.
Stopping down the lens will reduce the range of distances where the rays may
pass thru the lens, thus greatly reducing the effect of this aberration.
Should affect all the image area.
You can see an example of spherochromatism by looking at the different
colour channels of this picture taken with a Tamron 35-70/3.5 -- the blue
channel, although with similar resolution, is much less sharper than the
other colours: <http://cjss.sytes.net/atachaos/Esther/Marita_RGB.jpeg> (look
at the shirt's flaps)
However, after all this long explanation... I want to point an important
issue: the effect of aberrations (chromatic or not) on the response of the
image sensing device, be it film or digital. It may affect _severely_ the
way we see the CA and/or other lens aberrations -- or induce us to think of
other issues as aberrations!
For instance, when comparing the (really excellent) Tamron SP 70-210/3.5
with the Sonnar 135/2.8, besides the notable difference in micro-contrast
and resolution, I noticed that the film's grain was FINER in all the Zeiss
shots -- the very same roll of film in the very same camera, taking the very
same subject. I simply could not believe my eyes... LATER, I learned from
some Leica users that extemely highly corrected lenses (like theirs!) do
render a finer grain with any film -- the scattered rays on other lenses
tend to agglomerate silver halide molecules in bigger chunks, read 'grain'.
I think this is what was happening to me...
OTOH, digital sensors may have different issues. When a pixel is lit by very
intense light, it shows a blooming effect which extends to neighbouring
pixels -- film may show a similar effect, but only in extreme cases. I may
be wrong, but I don't think the purple fringe around the brightest point is
caused by CA -- see this sample taken with the (sharp!) Zuiko 35-70/3.5-4.5,
which includes a crop of actual pixels: <
http://cjss.sytes.net/atachaos/bleeding.jpeg>
As already pointed out, lateral colour shouldn't improve by stopping down...
I propose the following experiment: with a lens which supposedly improves
lateral CA when stopping down, take a picture wide open, another slightly
stopped down (with a longer exposure time, of course) and a third also wide
open, but with a ND filter equivalent to the stops changed in the second
pic, with an equal exposure time.
And not counting other disturbing effects: lens elements' decentering, stray
rays on neighbour cells...
Here is an interesting site that provide ISO12233 chart test results that I
> can visual the results myself instead of just looking at the interpreted
> data:
>
> http://www.the-digital-picture.com/Reviews/Canon-Lens-Reviews.aspx
>
Very interesting link! Thanks a lot!
Cheers,
--
Carlos J. Santisteban Salinas
IES Turaniana (Roquetas de Mar, Almeria)
<http://cjss.sytes.net/>
--
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