Back in 1997, Carl Zeiss published this explanation of what THEY saw as the
limits of lens resolution. I think it supports what Robert is saying.
-------------------------------------
Resolution of camera lenses where are the limits - and why?
Some users of Zeiss lenses are so enthusiastic about these optics that they
believe Carl Zeiss can do won-ders. Well, this is not exactly true. The fact
is: Carl Zeiss has to adhere
to nature's laws just as anybody else. So far, Carl Zeiss's physicists have
found no way around it.
One of these laws defines the resolution limit of any optical system, any
camera lens, even a perfect one with absolutely no lens errors. It is the
law of diffraction. This law states that a sharp point in an object will not
correspond to a sharp point in the image thus having a diameter of exactly
zero, but rather to a small diffraction disc (physicists call it "Airy
disc"). This disc has a certain dia-meter, which varies with the aperture of
the imaging system. The smaller the aperture (e. g. f/22) the larger the
disc. And the larger the disc, the lower the resolution!
So, even a perfect lens with no lens errors is limited in its resolution!
(Such a perfect lens does not exist. Even the most sophisticated lenses on
this planet, the Carl Zeiss S-Planar lenses for the semiconductor industry
are only approximations to the perfect lens, although very close ones.) Thus
the term "diffraction-limited" has become the synonym for lenses that are so
good their only per-formance limit is the law of diffraction. According to
this law no camera lens used in photography, still or motion, can produce
resolutions higher than given in the following table (Approximate, rounded
off values for white light spectrum of even energy distribution. In unevenly
distributed spectra of gas discharge light sources somewhat higher
resolutions may be achieved.)
f-no......................resolution
.............................(line pairs per millimeter)
.45.............................35
.32.............................50
.22.............................70
.16...........................100
.11...........................140
...8...........................200
5.6...........................280
...4...........................400
2.8...........................560
In practical photography other limitations of resolution occur, too. The
existing depth-of-field concepts, for example, lead to a limit at 30 line
pairs per millimeter, simply because they assume that the image of a sharp
point may be considered sharp as long as the unsharp disc it actually is
(called the "circle of confusion") grows no larger than 1/1000th of the
focal length of the respective standard lens. In 35 mm photography only
1/1500th is allowed. 1/1500 th of the 50 mm standard focal length equals
1/30th mm. So 30 fit into one mm, which means, we are talking 30 line pairs
per millimeter (CLN 1 has more details in "Depth of Field - An Insider's
Look Behind The Scenes", an article that meanwhile has been reprinted in
many publications around the world). This is about the same limit set by
diffraction for f/45 which was an aperture setting quite popular with 5 x 7
inch large format photography, producing images that can be viewed without
any subsequent magnification.
Today's high quality color films do reach resolutions in the region of 140
line pairs per millimeter with Kodak Ektar 25 leading the field at 200! The
full resolution potential of these films cannot be utilized with existing
depth-of-field concepts nor f-settings of f/11 and beyond. On the other hand
all real lenses on the market today are limited not only by diffraction, but
by lens errors also. Some of them quite heavily.
Carl Zeiss has always strived to develop very elegant sets of lens error
corrections and to deliver a high degree of lens performance, second to
none. So, with normal Carl Zeiss lenses set at f/8 and f/5.6 resolutions at
the very limits of the best color films have been reached.
Objects of 4 millimeter in size (approximately 1/6 of an inch) have been
imaged from almost 400 meters distance (more than 1.000 feet) with a 100 mm
Carl Zeiss Makro-Planar lens at f/5.6 and a Contax RTS III 35 mm SLR camera
featuring the unique Contax vacuum pressure plate. Similar results were
obtained with Contax AX 35 mm autofocus SLR, Contax RX 35 mm low noise SLR
and Makro-Planar 60 mm, Planar 85 mm f/1.2, Planar 100 mm f/2, Aposonnar 200
mm f/2, Vario-Sonnar 28-85 mm and 35-70 mm zoom lenses, even with moderate
priced Planar 50 mm and Distagon 28 mm.
Resolutions on the same level have been achieved with Carl Zeiss lenses in
Hasselblad medium format cameras, proving that at Carl Zeiss, medium format
lenses, contrary to popular belief, offer no lower resolution than the very
best 35 mm lenses.
Other camera systems which achieve performances in the same premium class
with their Carl Zeiss lenses include the medium format SLR cameras of Rollei
fototechnic with the Rolleiflex 6008 integral as their latest top of the
line product and the discontinued Rolleiflex 3003, the only 35 mm SLR with
inter-changeable film backs since the demise of the Zeiss Ikon Contarex and
Contaflex.
The most obvious limitation of resolution in everyday photography, however,
is unwanted motion with handheld cameras. Even a seasoned photographer with
a very "calm" camera will hardly find resolutions higher than 40 line pairs
per milli-meter in his photographs unless he uses an adequate tripod. CLN
will detail on this topic in a later issue.
Camera Lens News No. 2, fall 1997
"Robert Monaghan" <rmonagha@xxxxxxx> wrote in message
news:9d713e$lpe$1@xxxxxxxxxxxxxxxx...
> Hmmm? I thought that _was_ his point; namely, that most normal lenses are
> diffraction limited around f/8 anyway, so regardless of which lens you
> use, whether it is a $20 fast f/1.4 50mm takumar (price I recall paying
> for mine) or $2,000 Leica super-lens, the results in terms of image
> resolution ("sharpness") will be limited by diffraction in a good lens
> design. Most of the normal 50mm-ish lenses designed in the last 40+ years
> are diffraction limited over this part of their range for 35mm SLRs.
>
> What this means is that folks with older or lesser brand cameras need not
> feel their results are compromised in terms of resolution ("sharpness")
> against much more pricey cameras. They can simply shoot at f/8 on a tripod
> etc. and get similar diffraction limited lens resolution performance as a
> Leica. That's what diffraction limited means, and if you don't understand
> this, you simply need to review any good book on optics (Cox,
> Kingslake..). Leica lenses can't break the laws of optical physics either.
>
> What Keppler didn't point out was that for many (normal) lenses, the
> highest resolution values will be at this same f/8 diffraction limited
> setting, due to the tradeoff between factors such as film resolution,
> aberrations, and diffraction. see "sweet spots of lenses" showing tradeoff
> example: http://www.smu.edu/~rmonagha/mf/lenslpm.html - max resolution is
> at f/8; this is why many books advocate using these mid-range f/stops for
> minimum aberrations and maximum resolution performance. This is also the
> unstated reason why Keppler chose this aperture; it is the maximum
> performance spot for the majority of normal 50mm lenses for SLRs/RF.
>
> what this means is that not only would the $20 pentax equal the
> diffraction limited resolution of the $2,000 Leica lens at f/8, but the
> pentax lens at f/8 would probably beat or equal the Leica lens resolution
> at f/2 and f/4.. and at f/16 etc. This applies only to resolution (lpmm).
>
> Again, the Leica lens would probably have superior aberration control,
> maybe nicer bokeh (subjective), and maybe less light falloff or be sharper
> in the corners at various f/stops, but the maximum high resolution values
> for both lenses, at the f/8 resolution point, would be the same, and
> limited by diffraction.
>
> Again, I find this to be good news for the average joe with a decent SLR.
> They often wonder if putting kilobucks into switching to Leica lenses will
> "improve" their photography and result in higher resolution ("sharper")
> images. The short answer is good technique and shooting at f/8 "sweet
> spot" will provide them with maximum performance which is equal to the
> Leica lenses (also diffraction limited at f/8) at their best resolution
> setting too. Again, this is only true for resolution, not distortion etc.
>
> And as the above URL shows, if you are using color films, you probably are
> more limited by the film than your lenses, even if your old used lenses
> only cost $20 ;-) It is much cheaper to switch to a higher resolution film
> to get better results (e.g. ektachrome to velvia).
>
> grins bobm
> --
> * Robert Monaghan POB752182 Dallas Tx 75275-2182 rmonagha@xxxxxxxxxxxx
*
> * Third Party 35mm Lenses: http://www.smu.edu/~rmonagha/third/index.html
*
> * Medium Format Cameras: http://www.smu.edu/~rmonagha/mf/index.html
megasite*
< This message was delivered via the Olympus Mailing List >
< For questions, mailto:owner-olympus@xxxxxxxxxxxxxxx >
< Web Page: http://Zuiko.sls.bc.ca/swright/olympuslist.html >
|