Yes, yes, appreciate the comments and Frank's images. (look very nice at work
but will pixel peep on calibrated monitor this weekend ;-) ) My fault for not
linking images. Roger's comments though very helpful were from 2013.
As far as the too thin adapter issue we are NOT talking
subtle, pixel peeping 200% in the corners type of effect. This is a SUBSTANTIAL
image mucking effect. People pay big bucks for top performing lenses and if top
performance is lost, why bother?
Film works and as Moose will testify my OM bodies are not sitting on the shelf,
but Mag LV with focus peaking in a nice EVF is the cat's meow
for adapted MF lenses. I suspect if AG received an A7II in the mail, we would
hear him sing its praises for use with OMZ lenses.
People have been terribly dissapointed with their Zeiss 21/2.8 (normally
stellar on Canyon/Nykon--ask Bob) on A7 series--a properly shimmed adapter
fixed it. The same has been true of the latest 16-35/4 IS Canyon and probably
the Canyon 24 T-S. WA's with floating elements seem to be affected the most.
The OMZ 24/2.8 or the 21/3.5 would not be expected to show much of an issue
though I would think the Z. 21 f2 (has floating element) that I have, might.
I am not sure, but having the adapter shimmed to a proper hard stop at infinity
seems highly advisible. This seemed much less an issue with Canyon as digital
back, but not clear to me why.
The sensor stack issue is also complex and, for the most part, am less
concerned as it pertains mostly to RF WA lenses. Still the ZeissDistagon T* 28
F2 behaves much better on the thin stack modified A7r which make me wonder
about other SLR lenses. This seems to be a function of lens design and some
possible correlation with how close the exit pupil is to the sensor.
Is there a downside of the Kolorvision modified sensor stack?
http://www.lensrentals.com/blog/2015/01/a-thinner-sensor-stack
Native lenses as above don't suffer much but Zeiss designed lenses for the
stack being there and reports filtering in that the Loxia line perform better
in the corners with the unmodified version. How 'bout an electro-optical
material in the stack with tunable refractive index simulating different sensor
stack thickenesses? Probably as much chance of Sony working on that as the
messiah arriving next Tues at 2:30 or Oly designing a proper FF Digital OM body.
Extreme fretting in the defense of image quality is no vice, Mike
Dr. Focus responds:
Thou dost fret too much. You forgot to read the most important part of
Roger's
message:
----------------------------------------------------------------------
What
Does It Mean in the Real World?
Like a lot of laboratory testing, probably
not a lot. Adapters couldn't
all stink or people wouldn't use them. Like a lot
of tests, you can
detect a very real difference in the lab that doesn't make
much
difference at all in the real
world.
----------------------------------------------------------------------
ps:
Even 50 lines/mm resolution (let alone 80) is very difficult to
achieve unless
you're on a rock solid tripod.
Dr. Focus
On 3/16/2015 8:05 PM, Mike
Gordon via olympus wrote:
> I am very comfortable carrying my OM bodies and
Canyon digital back but prefer to lug less heavy stuff these days.
> Thus my
toe dip into MFT and possibly sony in the near future. I really like the new
high performing lenses these days
> but miss some "rendering character" of my
lenses built for film. Many newer ones are very sharp inded but tend to have
>
a sameness about them and fair to middling bokeh---this is a vast
generallization with some glaring exceptions. Perhaps the aspheric elements play
a role. Anyway I enjoy the character and adapting of my old friends.
>
> I do not have a good handle on the complexities of determing how to achieve
optimal performance. The adapter quality/precise dimensions seems to one
variable. (Expense did not guarantee a good match for a lens in Roger's tests at
lensrental)
>
http://www.lensrentals.com/blog/2013/09/there-is-no-free-lunch-episode-763-lens-adapters
>
This is not a trivial issue for wide angles and seems to be especially importnat
for those with floating elements (may interact with sensor stack thickness)
Minor offsets in thickness effect the peripheral rays in a nonlinear way and not
corrected by depth of focus considerations. The corners just become lousy. Some
lenses seem to developefield curavture. While the inherent amount of curvature
in the optic will certainly vary from lens to lens, moving a given lens forward
or rearward will have a non-linear (curvature is non-linear) relationship change
between the distance to the center of the film plane vs. the distance to the
corner. Shimming the adapter to get infiinty spot on corrects this. I don't
understand this fully. Dr. Focus was skeptical of any serious issue.
> Another
issue is the stack thickness already discussed and posted previously.
>
>
http://www.lensrentals.com/blog/2015/01/a-thinner-sensor-stack
>
> There is now a commercial correction for this!! Some WA's as long as 28mm are
affected.
>
> http://www.kolarivision.com/thinfilterconversion.html
>
>
From Dr. Nasse at Zeiss--mostly an issue on tangential plane:
>
> From
Nasse:
>
> "Lenses with a very large beam tilt react in a much more sensitive
manner to a change of refractive index in the image space caused by filter
plates in front of the sensor (such as low pass and IR-blocking filters). If the
filter plate is not considered in the design of the lens, the edge definition
will suffer. The effect of the additional path through the glass grows
exponentially with the beam inclination. A Distagon which never achieves more
than 20° beam tilt in the corner of the image reacts more tolerantly than a
symmetrical wide- angle lens, which might reach a 45° tilt. This is why filters
in digital Leicas are very thin – to remain compatible with older optics. If the
filter is significantly thicker, the contrast transfer for the image edge
becomes worse for tangential structures. In the graph of the curves, this looks
like the old retrofocus lenses but is caused by astigmatism rather than lateral
chromatic aberration. The focus is shifted to greater distances
for tangential structures by the additional path through the glass. If the best
edge definition is to be achieved, then all that can be done is to stop down
further."
>
> I wish I had a good handle on these issues.
>
> Adaptation bewilderment, Mike
>
>
--
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