Lenses ~ adapting m39 Paxette

As a placeholder to a longer conversation...

Leica Thread Mount lenses (m39 LTM) ~

• m39 - 39mm diameter by 1mm thread
• FFD of 28.8mm precisely

Paxette lenses ~ 

• m39 - 39mm diameter by 1mm thread - same as LTM
• FFD of 44mm  - which means Paxette will NOT work natively on LTM adapters, event though the threads are the same

 

 Sony A7RII, Staeble Choro 38mm f/3.5
Tiny little lens, plenty sharp stopped down

So the question arises of how to adapt m39 Paxette to digital mirrorless?  Here are three solutions.

Modify m42 adapter ~

As previously noted, the ffd of Paxette is 44mm.  m42 has a ffd of 45.46mm precisely.  Using this knowledge, here is one solution for adapting Paxette lenses.

• m42 adapter
• Remove the front m42 ring 
• Remove 1.5mm off the rear surface of the ring (machine or sandpaper grind)
• Reinstall the ring into the adapter 
• One each m39 to m42 ring for as many Paxette lenses as are on hand
• Mount m39 to m42 stepup ring on the Paxette lens
• Mount the lens on the m42 adapter 
• Mount m42 adapter on camera
• Take photographs 

The downside of this is one is limited to focusing distances as set by the lens. 

m39 to m39 extension tube on LTM adapter ~ 

Another approach requires a specialty extension tube.  Here's that solution.

• m39 Leica Thread Mount (LTM) adapter
• 14mm m39 to m39 extension tube 
• One each m39 to m42 ring for as many Paxette lenses as are on hand
• Mount m39 to m42 stepup ring on the Paxette lens
• Mount the lens on the m39 extension tube
• Mount extension tube on LTM adapter 
• Mount LTM adapter on camera
• Take photographs 

The downsides of this is one is limited to focusing distances as set by the lens, and the precise extension of 14mm is difficult to find.  I know of two possible suppliers but I'm not sure the part is always in stock. 

Adapt a m42 close focusing helicoid ~

A third approach is proving to be rather flexible. 

• m42 close focus 17mm-33mm helicoid adapter
• One each m39 to m42 ring for as many Paxette lenses as are on hand
• Mount m39 to m42 stepup ring on Paxette lens
• Mount the lens on the m42 helicoid 
• Mount helicoid on camera
• Take photographs 

Simple.  Direct.  Inexpensive. Flexible.

Using a helicoid allows for flexible focusing.  There's no need to measure the adapters for infinity.  Just turn the adapter threw to find the focus point.  And the lenses own focusing ring is still available for use as well.  Lastly, because everything remains m42, this approach is good for Pentacon/Pentax mount lenses and even provides a bit of close focusing capability.

  

 

Sony A7RII, Staeble Telon 85mm f/5.6
Smallest 85mm I've ever seen, plenty sharp 

Why all the Monkey Motion?  

There are more than a few tasty German optics with many aperture blades (think: beautiful out of focus rendition at all apertures) to be found in the Paxette family lens tree.

• Carl Zeiss - 50mm tessar 
• Enna - reportedly excellent, though I've yet to try them
• ISCO - nice, simple optics from a Jos Schneider division
• Roeschlein - I'd not heard of this company until recently
• Schacht - decent contrast and resolution selection of lenses
• Staeble - another decently sharp/contasty selection of lenses
• Steinheil - my current favorite for in-camera soft focus/pictorialist-like work

 

 

Sony A7RII, Steinheil Cassarit 45mm f/2.8
Tiny little lens, pictorialist effects at all apertures
with underlying "sharpness" that'll cut the
paper it's printed on

------------ References ---------------

Lists of Paxette lenses - incomplete

Fitting a m42 adapter for Paxette use 

Lenses ~ another whack at software intervention

On lens "corrections" -

While researching a lens I was interested in I stumbled across a comment that struck me.  It went something like (yes, I'm paraphrasing in the following)...

 "... you can use software to sharpen up this old lens, but you won't have a true understanding of how bad this lens really is..."

I wondered if the writer understood something fundamental to digital photography and current image processing. Software "corrects" for all manner of lens design and implementation "faults."

To see what I mean turn off software intervention.  Specifically, turn off -

• Capture Sharpen - which ostensibly counters AA filter effects
• Lens Correction Profile (LCP)  - which corrects for
• Chromatic Aberration
• Field Distortion 

For in-camera jpgs this means locating and changing the settings there on the camera.  For RAW this means locating these settings in the image processing software, where switches and controls could be very well hidden.

Now have a close look at an image and compare it against a software "corrected" image of the identical scene.

It can take a lot of image processing  just to reach a decent starting point.  What's good for the goose might be good for the gander, right?

Question: Why not apply software "corrections" to old manual focus lenses?

 

 

Sony NEX05T + Sigma 24mm f/3.5 DG DN
Illustrating the results of
all the software interventions applied
by default on file import into a RAW
image processing software 

On Lens Diffraction limits ~ 

Who hasn't read lens reviews that tell us things get mushy when shooting at apertures below the limits of what a sensor can resolve?  The phrase "diffraction limits" comes up shockingly often.

For full frame cameras 12 to 24mpixel that's f/16 and for 40 to 60mpixel that's f/11.  The caution is to avoid those apertures if you want the sharpest rendition possible.  The implication being that images shot at apertures below the resolution limits of a sensor are <insert favorite unsavory expletive>.

In light of software intervention capabilities, I wondered if this was strictly true.

Taking the question seriously, I used a beautiful old Nikon Nikkor 20mm f/3.5 Ai lens, focused two feet, adapted to a 42mpixel Sony A7RII (shooting RAW) and shot three photos starting at f/11, then f/16 and finally at f/22.

Looking at the images with Capture Sharpened turned off I could see a slight difference in the sense of sharpness between f/11 and f/22.  Between f/11 and f/16 it was a little more difficult to tell a difference at full rez or 200 percent rez.

Then I turned Capture Sharpen on and... <drum roll, please>... I see zero sharpness difference between them.  As in, it don't matter (bad English intended).  Software did what it was designed to do: Make things sharp.  OK.  OK.  There was, however, a clear difference in depth of field.  But that's also the point of shooting at small apertures, right?  

To check if this was strictly true I then took a fine little Pentax-M 28mm f/2.8 and reran the f/11, f/16, f/22 comparison.  In this case the sharpness difference between f/11 and f/22 was more obvious, even when using Capture Sharpen.  Because of the way the Nikkor performed, there's likely something in the Pentax-M design that adds a bit more softness at really small apertures.  However...

... for grins, I took the Capture Sharpened f/22 Pentax-M image and applied a gentle UnSharp Mask (USM) and compared the result to a Capture Sharpened bitingly/critically/fabulously sharp f/11 image.  The result is... <another roll on the drum, please> ... zero, zip, nutt'n, nada difference between them.  Software intervention of the kind applied to digital lenses now applied to the wee-Pentax-M is able to make a f/22 image look as good as a Capture Sharpened f/11 shot.

Question: How many people avoid shooting at small apertures because they've been told something awful happens down there? 

Recap question: With the kinds of beneficial image improving software tools available to us, why not use them, regardless of the lens? 

Lenses ~ Steinheil

I wanted to write a little about what I'm experiencing using lenses from a former German manufacturer.  C. A. Steinheil Söhne Optical and Astronomical Works, as they were first known, made early and significant contributions to lenses for photography.

 

Sony A7RII
Steinheil 45mm f/2.8 Cassarit at f/11 

A little history ~ 

In 1866 Carl August von Steinheil patented the Rectilinear four element two group symmetrical design lens.  He beat Dallmeyer by a week or two to the patent.  The importance of this early design can't be over-stated.  Many subsequent designs descend from this idea.

There was also an early design for what would later by patented by Carl Zeiss as the tessar four element three group design.  Steinheil patented theirs in 1881, which was two decades before Zeiss.  Zeiss inverted the Steinheil optical layout and successfully claimed uniqueness of concept before the patent office.

Steinheil continued to develop lenses for photography through the 1960's.  They had Dagor/Protar designs. They offered their Antiplanet inverted tessar.  They developed Cooke triplets.  Many of these lenses are available for not much money on the used market these days.

Soft focus lens search ~ 

I've been looking for lenses that gently introduce optical imperfects into images made on miniature formats.  

Wollensak offered a Velostigmat Series II that allowed users to move the first/front element away from the second and third element and group.  Various focal lengths implemented this feature to be used on various large film formats  The lenses introduce soft focus effects that were sought after by Pictorialists and portraitists at the time.  I am looking for something similar for use on miniature formats.

I stumbled across a Japanese first element focuser, bought one, and found it does the trick, just like the Velostigmat Series II.  Then, a friend alerted me to the fact that certain German lenses from the 1950's and 1960's were also first element focusers.

In these I found the soft focus lenses for miniature formats that I was looking for. 

On vacation ~ 

This past winter we headed to Italy to escape the cold, gray, set skies of Paris, only to have those clouds and wet follow us.  No matter.  We needed to get away.  

I hauled a Sony A7RII with three lenses.  One of the lenses practically lived on the A7RII was a Steinheil 45mm f/2.8 Cassarit.  This is a unit focuser in a m39 Paxette mount (44mm FFD, not the more comment 28.8mm Leica Thread Mount spec).

This unit focusing 45mm Steinheil went along because I'd not correctly/fully cleaned a 50mm f/2.8 Auto-Cassaron Edixa my friend originally alerted me to.  I didn't think the 45mm would have much majick.  I was wrong.  These Steinheils they have a range of soft focus-ness that can be quite useful.

The soft focus images made around Napoli and Rome were made with the 45mm f/2.8 Cassarit unit-focuser. 

Designed differently ~

Steinheil unit and first element focusers seem to share a common trait.  They render very crisply with evident underlying sharpness.  They also exhibit an overlaying reduction in overall contrast and highlight bloom.

Characteristics of two soft focus candidates ~ 

• Ricoh 55mm f/2.2 
• four element four group
• First element focusing 
• ISCO Iscotar 50mm f/2.8 
• Cooke triplet 
• First element focusing
• Both lenses
• Soft wide open
• Good contrast from wide open
• Center sharpening up as aperture closes
• Edges struggling to sharpen up even as aperture closes
• Exposures normal ~ comparable to modern AF optics 
• Highlight bloom diminishes with aperture closure 

Characteristics of Steinheil soft focus candidates ~

• Sharp across from wide open
• Veiling softness at all apertures ~ diminishing slightly as aperture closes
• Tendency to feel over-exposed on Sony mirrorless ~ tonal distribution crowded to the high end of the curve
• Highlight bloom largely unaffected by aperture closure 

Note: I also have a 135mm f/4.5 Culminar 4 element 3 group tessar formula Steinheil lens.  It's not as sharp from wide open as the shorter focal length lenses listed above.  Stopping down does little to improve resolution.  However, there remains the underlying veiling and highlight glow of the two shorter focal length optics.  This is different than how a 135mm f/4.5 Staeble behaves.  The Staeble has good contrast from wide open, which improves with the closing of the aperture.  I'm beginning to think that soft image rendition is a Steinheil trait.  If anyone knows a Steinheil lens designer who wouldn't mind commenting, I'm all ears.

This underscores something I've come to appreciate.  That is, a photographic lens of the same optical layout designed by two different teams can and often do render differently.  Sometimes dramatically differently.

To me this means any majick found in lenses is not simply the result of the lens type (tessar, plasmat, triplet, Ernostar, Sonnar, etc).  Rather, differences in rendering are the result of the calculations and decisions made in details, such as lens curvatures, glass types, and element placement.

 

----------- Resources -------------

Steinheil lenses with design cross-sections 

Steinheil company history 

Lenses ~ thoughts on Viltrox 28mm f/4.5 FE

On a lark I purchased a ridiculously cheap Viltrox 28mm f/4.5 FE.

 

What I found can be summed up in the following two sections. 

Pros ~

• Very inexpensive (pleasantly low impact on the wine budget)
• Very small
• Very light
• Surprisingly sharp 
• Autofocus... sort of...
• Cute little built-in lens cover that is actuated by a small knob on the front of the lens that helps keep the pocket lint out of the optic 

Cons ~

• Fixed f/4.5 aperture
• Prone to flare in strongly off axis lit scenes 
• Slow/inconsistent startup  
• Dodgy AF 
• Works only on certain Sony cameras depending on firmware version (or so I'm told)
• Periodic AF startup failures on AF cameras 
• Periodic AF failures if camera left on too long
• Regular AF failures in lower (note: not yet low) light
• Firmware updates are an absolute disaster
• Some versions are available through a Viltrox app for cellphones
• Other/newer (?) updates are only available through Microsoft/Apple desktop computers.

Note 1: AF failures require a re-power start.  Failing that, I need to unmount/remount the lens.  Failing that, I need to drop the battery out and reinstall.  There must be something going on with the way the Viltrox is programmed and is trying to interact with the camera.

Note 2: Because I run Linux and the cellphone app appears limited to earlier firmware versions there is no way to update the firmware in my lens to see if AF performance has been improved in more recent versions. 

From this you can easily guess that this is a troublesome lenscap of a lens.  On the one hand it fits in my pocket when mounted on a small APS-C Sony E (in my case - A5000).  On the other, startup time and AF have proven to be a frustration.

None of this prevented me from taking it on holiday this past winter.  Only once did I put a Sigma 19mm EX DN on the A5000.  Everything else, litterally, was shot using this Lens of Frustration.  I suffered through it's faults to enjoy the portability aspects of the setup. 

Is sainthood granted for living under these (well, OK, self imposed) conditions?

There are a couple Flickr albums filled with images made in this way.  

See - Napoli and Rome

The sharp images in those albums are from the Viltrox 28mm.  The soft images are from an A7RII/Steinheil 45mm Cassarit setup I also took along for the adventure.

I wish the Viltrox 28mm f/4.5 FE were a more reliable optic.  It'd never leave one of my cameras if that were the case.  Alas... well... I still enjoy using it... and get to practice breathing slowly when the little lens starts acting goofy... which seems like every 3rd or 4th startup...

 

Lenses ~ short list of Soft Focus lenses from 1866 to 1945

Similar to my wondering about lens histories for sharp lenses, I wondered when soft focus came into being.

 

 

Sony A7RII + Steinheil 45mm f/2.8 Cassarit
Showing in-camera lens induced
soft focus effects are indeed
possible when shooting 
miniature formats 

It turns out, Dallmeyer did the job back in the mid-1800's.  This is a few decades before the start of the "pictorialist" movement.  Several things I've read suggested these Dallmeyers didn't sell all that well (Portrait nor Bergheim - are they in fact the same lens?  someone please correct me if I'm wrong) at first.  When the "pictorialist"s showed up the Bergheim was resurrected.

For me, that's not the most surprising thing.  Rather, it's the fact that an American company, Wollensak, offered a very broad range of soft focus lenses with introductions of new optical formulas spanning more than 20 years.  Then there's Kodak.  They put to market two Portrait lenses in the 1940's.  Which seems rather late to me.

The reason this surprised me is that for many years writers claimed that soft focus and "pictorialism" died during the First World War.  It turns out that they were wrong.  "Pictorialist" photography continued to be practiced well into the 1970's.  Well, it was in America, at least.

With the adoption of smaller formats (120 and 35mm) soft focus lenses continued to be introduced.  They were a "thing" on the Japanese "scene" with some of the product leaking out into other markets.  I've written about this at length in prior posts.  

For this entry I would simply like to put a few place-holders in history as way posts along the road of soft focus lens history. 

Keeping in mind:

• There were many many opticians who offered single element lenses that were adaptable to soft focus photography 
• Due to uncorrected optical aberrations there was a difference in points of focus between what was seen on the ground glass and which portion of the color spectrum the UV/blue/slight-green sensitive materials recorded 
• The Eidoscope being the first lens to allow accurate focus correspondence between the ground glass and light sensitive materials - sort of (see next comment)
• Soft Focus lenses achieve the effect in part by under-correcting for spherical aberration, which means there can be a broad range of possible focus points (depending on film sensitivity).  Several sources suggest when deploying for portraiture to focus on the nose and the to let the aberrations do what they do behind the point of focus. 

Brief list of dates and manufacturer for large format cameras: 

• 1866 ~ Dallmeyer Portrait
• 1868 ~ Dallmeyer Bergheim
• 1891 ~Hans Watzek Meniscus described
• 1890s ~ Dallmeyer Bergheim reissued
• 1903 ~ BOM Hermagis Ediscope corrected for ground glass focus

... and then something I very seriously underestimated: Wollensak

• 1903 ~ Wollensak Achromatic 
• 1906 ~ Wollensak Royal Portrait 
• 1906 ~ Wollensak Portrait Series A - less expensive than the Royal
• 1909 ~ Wollensak Vitax 
• 1911 ~ Wollensak Velostigmat Series II
• 1911 ~ Wollensak Verito - design borrowed from Bodine Pictorial lens
• 1912 ~ Wollensak Vesta - no diffusion adjustment
• 1926 ~ Wollensak Varium - Cooke Triplet giving gentle SF effects

... then...

• 1945 (approx) ~ Kodak Portrait 12inch and 14inch f/4.5

------------- Reference Materials ---------------

Rapid Rectilinear 

Optical design forms 

Soft focus ~ why 

Wollensak compendium 

Karl Struss Pictorialism 

Wiki on the Cooke triplet 

DPReview forum thread on lenses that preceded the tessar 

Tessar thoughts 

Tessar formula recalculations 

 

Lenses ~ short list of 1800's optical designs

Over the winter I wondered what the history of lens development for photography might look like.  While there are many many variations on the theme, I found a way through the Madness that made sense to me.

 

Keeping mind that optics were well known before photography, their adaptation to the photographic process is interesting.  It turns out, a certain Dr. Petzval played an important role.  It's so important, that in the 1902 Camera Club of Paris magazine they published the following description of the installation of a monument in the likeness of Dr. Petzval at the Imperial University in Vienna.

Translated from French -

"...  The series of festivities given by the Vienna Photographic Society concluded with the formal presentation of the Petzval monument to the Imperial University. This monument, modeled by Brenek and executed in marble, bears the following inscription:

DR. JOSEPH PETZVAL

PROFESSOR DER MATHEMATIK 1837-1877

geb. 1807; gest. 1891

gewidmet von der PHOTOGR. GESELLSCHAFT IN VIEN

In other words: To Joseph Petzval, Professor of Mathematics, born in 1807; died in 1891; dedicated by the Vienna Photographic Society

The monument depicts, in high relief, a remarkably expressive portrait of the great scholar. It is framed by a foliage-adorned border, and on the entablature, a laurel branch rests on a lens board. This detail recalls the portrait lens invented by Petzval, as noted in the document signed by Mr. Schipper, Rector of the University of Vienna, in which he declares his acceptance, on behalf of the Academic Senate, of the gift offered by the Photographic Society..."

Keeping in mind that:

• Dr. Petzval and Voigtländer had a business agreement until a falling out separated them
• Carl August von Steinheil was awarded a patent for his Rectalinear lens a week or two _before_ Dallmeyer ~ the designs/calculations appear to be strikingly similar
• Carl Zeiss was awarded a patent for their Tessar in spite of the fact that both Dr. Petzval and Steinheil had similar, though inverted, lenses decades before
• The Englishman Dennis Taylor worked from concepts (and wrote and spoke often about his approach), where the Germans preferred to work from calculations
• Germany in an Axis technology transfer to Japan before the outbreak of the Second World War shared their calculation approach to optical design, which now the adopted/accepted solution for making lenses today 

Here's a brief timeline that I've found useful for understand how photographic lenses came to be.

• 1840 - Voigtländer and Petzval - Portrait 4 elements 2 groups ~ *sharp resolution drop-off from center to edges
• 1857 - Petzval Orthoskop ~ 4 elements 3 groups ~ inverse of what later became tessar
• 1866 - Carl August von Steinheil preceeded Dallmeyer by a week or two - Rectalinear  4 elements 2 groups ~ *corrected the sharp drop-off of the original Petzval/Voigtländer Portrait 
• 1881 - Steinheil Antiplanet (Triplar, Culminar with examples made into the 1970's) ~ 4 elements 3 groups ~  inverse of what later became tessar
• 1890 - Rudolph Zeiss Protar ~ Anastigmat 4 elements 2 groups ~ similar to the earlier Rectilinear, though with different calculations and glasses
• 1892 - Emil von Hoegh - Goerz ~ Dagor 6 elements 2 groups 
• 1893 - Dennis Taylor ~ Cooke Triplet 3 elements 3 groups ~ outstanding corrections across the field
• 1893 - Steinheil Orthostigmat ~ Dagor-type 6 elements 2 groups 
• 1895 - Voigtländer Collinear ~ Dagor-type 6 elements 2 groups 
• 1900 - Hans Harting ~ Voigtlander Heliar 5 elements 3 groups Cooke derivative with cemented doublets on both ends of a symmetrical triplet layout
• 1903 - Zeiss Tessar ~ 4 elements 3 groups

Of course if a person digs just a bit deeper they'll find hundreds and hundreds of opticians who made photographic lenses and contributed to the development of optics for photography.  So the field of knowledge can get very muddy very quickly.  I stripped everything to just the simplest of skeletons.  Relevant details are left to the reader to explore.

To me, the important years would be 1840, 1866, and 1893.  Everything seems to descend from design advancements patented in those years.

------------- Reference Materials ---------------

Rapid Rectilinear 

Optical design forms 

Wiki on the Cooke triplet 

DPReview forum thread on lenses that preceded the tessar 

Tessar thoughts 

Tessar formula recalculations 

Chasing "softness" in small formats ~ troisieme part ~ Showing My Homework

I recently wrote a blog entry where I talked about lenses that seem to fill the gap between Full Blown Soft Focus for 35mm format and "normal" sharp/clinical lenses.  What I shared were mainly Lens Porn, er, sorry, Portraits illustrations because I find these cheap lenses beautiful to take photos of.  I did not share my homework, even though I wrote at length about differences between the optics.  This blog entry corrects this omission.

Keep in mind that the trick to these first element focusing lenses is that the greatest optical imperfection effect is generally found in subjects closer/closest to the camera.  Of course it depends on lens design, but this is the tendency I've seen thus far.

Often these first element focusing lenses for 35mm format that are easily adaptable to digital derive from or are implementations of the early Cooke Triplet.  The design is two positive elements on each end with one negative element in the middle.  This is pretty simple, easy to manufacture, and can be very low cost.  Lenses on the used market can be nearly Give Away cheap.  I recently picked up a digital adaptable first element focusing Cooke triplet design lens for less than 9Euro.

Homework ~

 

Steinheil Auto-Cassaron
50mm f/2.8 at f/4
processed using a film sim
that I like just to see how
the low contrast of the lens
might play out against a
contrast-inducing LUT

 

Comments ~ (borrowed in large part from an earlier blog post)

In reverse order, from bottom to top... 

Ricoh 55mm f/2.2 Riconar - Optical imperfections galore - the kinds of imperfections vary depending on subject distance.  While I don't show this here, it's easily seems comparing close to distance focused subject at f/2.2 and f/4.  It delivers rather decent contrast, actually.  

Interesting highlight "glow" effects.  Controllable by aperture and subject distance, which is potentially useful.  One would have to map out distance/aperture to know which settings to use.  

f/11 can make a pretty sharp image of distant subjects.  Regarding the effects I'm going after, between f/2.2 and f/5.6 the optical imperfections at all distances can play well toward generating a decent "Pictorialist" style enlarged to "normal" viewing size/distance.

ISOC Iscotar 50mm f/2.8 - Optical imperfects somewhere between the Ricoh and Steinheil.  Good contrast.  

Interesting highlight "glow" effects.  Controllable, as with the Ricoh, by aperture and subject distance.  f/8 and f/11 can make a decently sharp image at greater subject distances if desired.  

Between f/2.8 and f/5.6 the optical imperfections can play well toward generating a decent "Pictorialist" style enlarged to "normal" viewing size/distance.  I think of the ISCO as a slightly more rational German Riconar.

Steinheil Auto-Cassaron 50mm f/2.8 - Subtle optical imperfections, spherical aberration at all apertures and all subject distances. Low contrast.    Using the "haze" removal control during processing cleans up a scene, but why use it if I'm looking for "soft focus?"  Have I mentioned this is a low contrast lens?  There must be an echo in here.  Either that or it's strongly evident from looking at the results.

To me this lens is like using a Heliar large format lens from Voigtlander.  Back in the day I owned two of these, a 15cm and 21cm f/4.5, both in Compound shutter.  These lenses had similar underlying detail to what I see with the 50mm.  Missing the rendering of those old lenses I'm happy to discover the Steinheil.  The more I stare at Steinheil images the more I wonder if this isn't a basic trait to how Steinheil designed their optics?  As with the Heliars I find this a really interesting way to make an image.  There seems to me to be a lot of potential for processing unique small format digital images.  I have at least one more Steinheil optic coming to try to confirm/deny this line of thinking.

As the lens is stopped down the underlying detail begins to extend from the center toward the edges of the field.  The effect is common to how triplets behave and I saw this most particularly in a Meyer Domiplan 50mm I once had.  It was sharper from wide open than any Zeiss Tessar I ever saw (and I had more than a few of these over the years).  The ISCO behaves this way too.  That is, wide open the center of the field can be surprisingly sharp and the mid to edge of the field showing subtle/not-so-subtle optical defects of various kinds.  These clean up as the aperture is stopped down and the sharpness spreads out.

With my Auto-Cassaron it's as if the lens designers kept/allowed the spherical aberration to gain consistency in other areas of optical design.  Resolution, field distortion, chromatic aberration and coma are better controlled than in the Ricoh and ISCO.  While more subtle than many large format soft focus lenses from Back in the Day, the Steinheil for small format might make for a decent "Pictorialist" style lens where image viewing sizes can vary depending on the electronic display system. 

 

Images made with a Steinheil
Auto-Cassaron 50mm f/2.8 at f/2.8 or f/4