Legend has it . . . that this is one of the highest resolving lenses, ever.  

In a Zeiss newsletter from 2006, Camera Lens News, it was stated that the Zeiss ZM Biogon 25/2.8 had set a new resolution record approaching 400lp/mm.  That may be "theoretical" and not exactly 100% achievable in real world conditions, but, nonetheless, that is significant, to say the least. What does that mean for us, without getting into white papers, lens tests and politics?  Extremely sharp, extremely capable - in a tiny lens that is about as inconspicuous as it is beautiful (in silver anyway ;).  

Indeed, the lens is impressive.  I had dreamt about this lens for some time, despite understanding it's really not a great fit for the Sony sensor.  However, upon trying it on a Leica M, what it was designed for, essentially - the Leica M models, I found uncomfortable truths there, too.  In fact, unless you have a special Leica, the frame lines won't match and thus you can't compose with precision.  But, it's home.  It just fits right. This insanely high resolving legendary lens should show all it's capable of.  But, No.  If it's capable of this fantasy resolution I'm excited to see it.  On my iMac 5K, one of the ultimate viewing options in early 2018, it's not even filling my screen. Nothing to zoom in on.  Frustrating.  Looks great! Though, not what I saw and composed, nor fills my screen entirely, and it's nice to see that my image was in focus since the rangefinder doesn't give precise feedback at a distance.  I realized that the dream scenario wasn't exactly perfect.  I was even fantasizing about buying a native body just to take full advantage of this lens!  It's that good.  But 24MP just wasn't the fireworks this resolution whore longed for.

As it was, I had found I could easily upscale images from the Sony A7R2 when using the Otus 55 in ideal lighting conditions, etc.  So, 42.2MP plus - like 50MP, even 100MP or more - was intriguing, and far more so than 24MP, particularly once I had to start making compromises, and at a great expense.  Yet, because of the rear lens element being super close to the film plane, or, in the case, the digital sensor, the images had edges that were distorted and fuzzy, in terms of coloring and even focus.  A real annoyance that ate into the real estate of otherwise incredible imagery.   

Fortunately, some other crazies (HaruhikoT on Fred Miranda Forums) found that by reverse engineering this similar to correcting vision, one could place a filter onto the lens that, if calibrated properly, would . .  could . . theoretically . . . correct this and give us a flat image on the sensor without compromising other desirable qualities like sharpness.    Mind blown.  

BastionK over at phillipreeve.net got involved with putting this information together and has assisted getting this out.  He compiled a list of M mount lenses known to have rear elements near the sensor and thus issues that could be corrected with this method.  Many contributed to the effort from their own experiences, including myself.   Part of this means that by adjusting the optical profile you lose your calibration for distances, including infinity.  This can be corrected by opening the lens and removing some internal shims - super thin spacers.  Who in the world would be crazy enough to open their lens up?!  I did. After all, there are no electronics.  

I found that my 2nd hand copy had something that seemed some  attention and cleaning anyway, and from watching others it seemed reasonable.  It was.  Not a problem, but you do need a special spanner wrench and enough basic skills to warrant this.  Or, some say that some M adapters actually allow you to reach infinity without having to open your lens.  That's more ideal for sure, but I was skeptical, didn't want to spend another $200 or so, and was incredibly impatient because all else was right in front of me. 

I bought a Hasselblad Proxar bayonet filter, cut it out with a Dremel and put it into a new filter holder.  

Front End Filters addressed at phillipreeve.net

I have, indeed, seen that the sharpness and resolution of details is improved in corners and that the color shift and focus fuzziness pretty much disappeared. Although, in either case, closing down the aperture to 5.6-11 is really where this lens comes into that legendary quality it's known for - which coincidentally lessens such issues.    

If I were shooting film, Tech Pan would be the film of choice.  TMax 100, even TriX 400 would be fine, and nice.  But, it really is in a special category of capability when you have film or a sensor matched to take the most advantage of that sharpness.  For some, this is just a great street photography or landscape lens.  Others, the crazies that tend to marvel at details captured and every little pore on a face or crack in a wall, piece of sand on a beach, this lens delivers when used well.  

Surprise.  As a food photographer, I was not expecting this to be a tool for that kind of work.  It's not a fit with the norm.  With a helical adapter, like the Voitlander Close Focus adapter, you can get surprisingly close to food and with a very different perspective and field of view compared to a classic food tool, the 100 Macro.  The relationship of the central subject on a food plate can be entirely manipulated via angles and relative proximity to other items or background that is completely altered.  This, is of course, also true with other subjects like the red car pictured below.   It can generate a lot of drama, in a very different voice compared to a 100 Macro, or even 50/60mm.  Set wide open you still get that dramatic focus fall off akin to use of other lenses, and yet have a story of elements in the background if not an entire scene.  Longer focal lengths require compacting into a compact space.  It's simply a very different shot and story.   Varying tools allowing varying options is idea to have in one's arsenal for shooting - whether food, portraits, scenes, video/cinema, etc.  And for food - this lens is incredibly sharp, as noted, but also has that micro contrast detailing that makes images pop and with that "3D" element many speak of relative to Zeiss lenses.   This is no different.  Unique, strong, sharp, contrasty, mitigated flare, pretty ideal.  And 42MP to bring it to life.  It also gives the ability to crop, should I need.  Or, make an absurd sized print.  

On that note, the size is something to really appreciate.  Tiny to have in a bag, even a pocket.  Tiny on camera gives is a super nice feel.  Helical adapter allows for super close, essentially macro focusing (though I'm not sure of what factor).  EVF allows me to see what I'm getting and actually nail that focus and ensure that I am. This is a massive gain compared to rangefinder use.  Shooting in the street with a Leica and this closed down to 6.3 or greater, sure, easy to get your scene in focus.  Fully open, nailing that optical plane of focus, not so easy until you have an EVF.  For some commercial work that is worth a lot.  No questioning or praying for perfect focus.  Will just will be what I saw and shot.  If I'm shooting BW in the street, I get to see it that way when I'm shooting which I like.  Add image stabilization because it's there which also mitigates slower apertures.  

And if you wanna go nuts, use the Tech Art AF adapter and now you are taking this lens into the future.  But, I found I can't always trust the adapter, so if I have camera with me, I need to have an MF adapter also if I 100% to ensure I got every shot.  

For many, the real question is how does this compare to the Zeiss Batis 25/2, or ZF/ZE 25/2, etc.  Except, I don't really care.  If I needed a certain precision for different applications I could see the necessity of getting another lens.  For my use, I'm quite content.  Tests show the corners compare favorably with many new lenses.  I would love to have a 1.4 lens like this.  And Zeiss just launched a 25/1.4 Milvus, and it's incredible. I tried it at the PDN show in 2017.  It's also about 10 times larger, maybe more.  Justifiable, I'm sure.  But I'm not even sure the Otus 28mm will beat this ZM 25 in resolving power when closed down.  It certainly will make mincemeat of it at 1.4 - 2.8, of course.  And I look forward to that lens, too, one day.  But, again, very very different beasts for very different applications. 

For what this lens is and offers in top level Zeiss image quality, Zeiss build quality - all metal construction with no electronic gadgetry to go bad, and at its price - it's really incredible.  Potentially record setting resolving ability, sharpness, color, new perspective in food and other macro photography, landscapes, even portraits of the environmental nature, it has a lot to offer.  Combined with other methods to ensure image quality, such as studio lighting, tripod, knowledge and experience you can achieve images with this lens on par with just about anything.  I don't throw that out lightly.  

Because this lens is not new, not designed for Sony, not designed for digital sensors but actually for film, it's simply not a headline, nor even something many come across.  It doesn't stand out. It's not even on the radar of choices.   It is easily overlooked in favor of lenses that make so much more sense - the Batis 25, Loxia 21, Milvus 25, and perhaps there are other brands too!  ;P  But, few are relevant.  Voigtlander has the Skopar 25/4, which is far smaller - quite an achievement and asset, and I want one.  But, it's not a legendary lens for a reason and 2.8 is sort of my limit for what I can reasonably use.  I prefer faster lenses even though they tend to either suffer from far greater size, lesser image quality, higher costs - and those are not a Zeiss ZM.  The ZM lenses are something unique.  A joy to use.  Silky smooth focus feel, a small tab for it that allows for tab focusing using a single digit, or barrel turn focusing and muscle memory of the subject distance relative to your finger position.  In other words, for example, I know that when the tab is positioned at the bottom of the lens, I don't need to look at the lens to see what that distance is, nor even in viewfinder, as I already know that is 1 meter.  All the more easy if my aperture is dialed up giving me a greater zone of focus.  Granted, I still love to use the EVF to ensure it's really spot on, but the tab used as a general locator - like a compass for North, or marks on the F & J keys on a keyboard that orient us.  

In street shooting it's very easy to zone focus this lens - setting a slightly higher aperture and knowing that everything between, say 1.5 meters and 12 meters or so, will be in focus.  This is definitely a great way to shoot with this lens.  It's quick and easy to use on the street with little effort.  There is no struggle or awkwardness.  It's not like swinging a large rifle towards your subject arms extended.  Instead a quick turn of the hips and you're there with little attention brought to you.  This can be crucial in street shooting when you want to be inconspicuous to avoid altering the scene with people reacting to you point at them.  Focal length wise on the street it's that classic look of all in focus, should you choose, thus getting in an entire scene of players.  

If you are awe struck by textures and lighting making patters in the street, this lens excels there.  Textures are heightened due to resolving power and micro contrast, and dynamic range maximized through optimized and control of light transmissions from the lens coatings associated with the T* coatings Zeiss brought to their lenses in the 70s.  In fact they had started in the 30s with special coatings, perfected them with little to no fanfare in the 70s, but in the 90s realized it was of greater value to brand and market. 

For weddings, it's an excellent choice for dress shots, landscapes, full scenes of family and guests, and ideal for those editorial look multi-party portraits.  Ideal as well for those epic bride in dress shots for magazine covers or album spreads.  It's also nice for zone set focused portraits at events using off camera flash.  

My only question is why this lens is so overlooked.  As noted above, it's just not on the radar.  Every few months new lenses hit the market with best ever MTFs, casual reviews by those who never used, or only touched them at a trade show.   In fact, this is a large part of why people have asked me to write and show real world examples of lenses that work well and why, in various shooting environments. To me, the only real way to measure a lens for use and purchase is to see how it performs, especially in any way I would ideally use that lens.  Hopefully, the examples here can provide some sense of that, and even for uses people would find surprising, like commercial food work.  From my experience the Zeiss ZM range, particularly the Biogon designs,  would be considered almost exclusively for street work, and maybe some casual architecture shooting.  I find them valuable for so much more.  

In the ZM range, Zeiss also offers a quite similar 28mm, if not nearly identical except slight shift in focal length.  As it is, the lens is also noted for architecture due to near zero distortion.   Zeiss also offers a 35/2, a 21/2.8, and 18/4 that is no longer offered. Hoping to add a silver 18/4 to my collection one day.  Surprisingly inexpensive given what it is.  Zeiss also makes, believe it or not, a few "compact" versions - the C models.  These are even smaller, but at a cost of speed.  Those are the C 21/4.5, C 35/2.8 and the classic C 50/1.5 - a Sonnar design that, at the time of launch, was the fastest standard lens available.  For those unfamiliar with Zeiss heritage it is worthwhile to note they are the grand architects of many lens designs still in place by large camera companies.  They have lead optical engineering and many of its greatest feats that still stand today, and continually offer new legendary designs.  The Otus line being a prime example, and even the new ZM 35/1.4 - considered by many to be the finest lens ever produced for Leica camera bodies.  

 I understand this range of ZM Biogons to be unique in what they offer in tiny package, reasonable pricing, superb feel, build quality to last a lifetime, and image quality that will make your peers go "what the . . ", and looks to match (in silver anyway).  The Sony is not a perfect match, but, in actuality, the Sony sensor brings out more of what's possible than other platforms including the camera it was designed for, in many ways - particularly those that matter to me.  

Many shots presented here were not taken with the front end filter mentioned.  

In case I seem overly supportive, know I have zero affiliation with Zeiss at this time. Just a love for them based on experience.  
Besides, with all their new wide angle lenses, I'm not sure they really want me pushing the greatness of this at a lower price than their newest latest greatest.  

A new lens is a good way to go, of course.  But, you can save quite a bit by purchasing used at a reputable dealer like www.popflash.com, or reputable seller on eBay.  These lenses are solid.  Not much to worry about so long as it's a reputable seller.