[PenPusher]

I have been very remiss in not paying proper attention to the lens diagrams for the 50, 55 and 35mm lenses as this could explain why some lenses which look the same have different characteristics.

With the 50 and 55mm lenses Pentax seem to have reused the optical design where it has proved satisfactory, for instance there seems to be very little, if any, change in the design of the M, A, F and FA F1.7 lenses.

I am curious to know if there are any changes in the optics as otherwise the changes appear to be mostly cosmetic with the addition of automatic diaphragms and and autofocusing

Unfortunately the Forum specification notes do not give any details of the glass type used in each lens element.

Does any body know this or at least where to look for this information.

CD

[pschlute]

I think the most significant optical changes would be the coatings

[Digitalis]

I think the most significant optical changes would be the coatings

In many cases that is the only difference, and the advantages are sometimes questionable at best.

Unfortunately the Forum specification notes do not give any details of the glass type used in each lens element.

Pentax hasn't been in the habit of using many exotic glass types, getting documentation of the exact chemistry of each lens element used in a design is nearly impossible. Exotic glass and various types of aspherics only appear in some of the later A*,F*, FA* lenses*, and to an extent in FA lenses. More recent designs that have been developed with digital cameras in mind there has been a distinct shift to using more exotic glass types and increasing use of aspherics. Unfortunately documentation for the early lenses is scant, most of which is difficult to find and corroborate at best, and based upon rumor and speculation at worst.

Does any body know this or at least where to look for this information.

There are hundreds of sources for designs and documentation if you know where to look - Bojidar Dimitriovs website is a good place to start. He has most of the optical design schematics - though I think there are still some that are duplicates that may not be attributed to the correct lens.


*some late telephoto M* lenses for the Pentax 67 and A* 645 lenses incorporate ED glass into their designs.

[PenPusher]

Thank you

I am aware that the most publicised changes in recent times have been to the coatings but as far as I am aware coatings remained the same for some considerable time.

Those of us who glow in the dark are aware that there is radioactive glass in some lenses but no notion of where it is so they must have used some special glass somewhere.

I thought of Bos's site but I don't think he has any of this type of information.

CD

[Digitalis]

as far as I am aware coatings remained the same for some considerable time.

New coatings have been developed over the years: Aero Bright being one of the more recent ones that has continued to be somewhat of a mystery on exactly what it is and more pointedly, what it does. The current and more sophisticated Fujinon EBC coatings claim to deposit 14 layers of anti-reflective coatings to each lens surface, Pentax SMC uses 12 coatings on each surface.

there is radioactive glass in some lenses but no notion of where it is

It really doesn't matter, for all practical purposes the effects are easily reversible* there are only a handful of lenses** that were tainted by radioactive thorium and in certain lenses, lanthanum. With the lanthanum lenses being the more viciously radioactive of the two.

*Though the precise mechanism of the physical effects remain a matter of dispute.
**Of which the Pentax SMC Takumar 50mm f/1.4 is an exemplar

[bdery]

With the 50 and 55mm lenses Pentax seem to have reused the optical design where it has proved satisfactory, for instance there seems to be very little, if any, change in the design of the M, A, F and FA F1.7 lenses.

Pentax is not alone in this. The double gauss design is so commonplace and straightforward, most manufacturers used it exclusively until Sigma released the first of the "modern" 50s some 10 years ago. It's a proven, textbook design, not perfect but pretty darn good.

[stevebrot]

With the 50 and 55mm lenses Pentax seem to have reused the optical design where it has proved satisfactory, for instance there seems to be very little, if any, change in the design of the M, A, F and FA F1.7 lenses.

You are correct. The screw mount 55mm f/1.8 Takumars have essentially similar design that is shared with the Pentax-K 55/1.8. The Pentax-M, A, F, and FA 50/1.7 share the same optical design. (For all I know, the glass itself may be interchangeable.) The DA 50/1.8 is dissimilar, but still a Planar-derived double-Gauss design. FWIW, almost all Japanese fast 50 SLR lenses share a similar design heritage. The Wikipedia article on Double-Gauss is interesting and includes three charts showing a sampling of lens implementations spanning the period 1936-2010. Yes, the designs are that old.

Double-Gauss lens - Wikipedia


Steve

[Na Horuk]

I am aware that the most publicised changes in recent times have been to the coatings but as far as I am aware coatings remained the same for some considerable time.

Dunno. The SMC coatings got "silent" upgrades over the decades. And I think the DA lenses needed slightly different coatings due to digital sensors being different from film (more shiny). And then there are the HD and Aero coatings. You can see there might be differences in the colour of the light reflected off the coatings.

So, we are fairly certain that coatings have been changed throughout the years, but we don't know when the last update was, other than when SMC was changed to HD. Aperture blades on some of the newer lenses are also different, more rounded, possibly different materials than the old M and A versions.
The optical designs for the lenses OP mentions has been the same, but we don't know if there have been small tweaks, for example to optimize centre sharpness at the cost of edge sharpness - which makes sense for digital crop sensors. Or changing the angle of incidence of light, which might be important for digital sensors. The other possible change is change in machining and even glass materials. These things can change over the decades, because the machines might get upgrades, factories might be changed, etc. Materials can also have slight changes, because some might get more expensive or less expensive, or better combinations are developed, or some materials get restricted due to stricter environmental and safety standards..
The problem with the published diagrams is that they are not necessarily perfectly accurate in terms of sizes and distances.4

So the changes between M 50mm f1.7 (design here) and DA 50mm f1.8 (design here) might be really small (only aperture blade shape), or more drastic (coatings, small tweaks, differences in machining or materials). We just can't know for sure. But comparisons have shown the differences in optical output are really minimal. You can search for threads, as people have compared lenses from different generations

[Digitalis]

The problem with the published diagrams is that they are not necessarily perfectly accurate in terms of sizes and distances

Indeed, the only way to be sure is to get the CAD drawings from the lens designer directly. Good luck with that.

For all I know, the glass itself may be interchangeable.

It is funny you mentioned that: Many years ago I received a copy of a Pentax M 50mm f/1.7 which, due to careless handling suffered from a significant gouge on the rear element. Being the curious type, I unscrewed the entire rear cell and replaced it with the the rear cell from a Carl Zeiss Planar 50mm f/1.7 T* in Contax/yashica mount - the rear cell was a bit loose fitting [some teflon tape fixed this] and it needed some focus adjustment however, it worked*. So somewhere out there, is a Pentax M 50mm f/1.7 that is a half-Pentax half-Zeiss franken-lens.

*Though I recall there were a few marked changes in the performance of the lens after this modification: bokeh developed hard, outlined edges. Vignetting and coma handling was worse than a standard Pentax M 50mm f/1.7. Due to the adjustment needed for infinity focus MFD was also slightly longer than normal.

[PenPusher]

Indeed, the only way to be sure is to get the CAD drawings from the lens designer directly. Good luck with that.

A CAD drawing from the 1950's/60's would be interesting, back then it was either engraving on 110g paper with a 5H pencil to avoid smudging by sweaty hands or an early Graphos ink pen to produce a line of uniform thickness, if the humidity changed the paper expanded/contracted so the drawing moved and all the scales were out. Kodak, I think it was but I could be wrong, produced some kind of wonder plastic drawing film which was going to replace paper, it didn't work, after a few years it became brittle and shattered if you weren't careful and generally you had to redraw the whole thing on paper.

---------- Post added 09-08-2017 at 04:39 PM ----------

Dunno. The SMC coatings got "silent" upgrades over the decades. And I think the DA lenses needed slightly different coatings due to digital sensors being different from film (more shiny). And then there are the HD and Aero coatings. You can see there might be differences in the colour of the light reflected off the coatings.

......Or changing the angle of incidence of light, which might be important for digital sensors. The other possible change is change in machining and even glass materials. These things can change over the decades, because the machines might get upgrades, factories might be changed, etc. Materials can also have slight changes, because some might get more expensive or less expensive, or better combinations are developed, or some materials get restricted due to stricter environmental and safety standards..

The problem with the published diagrams is that they are not necessarily perfectly accurate in terms of sizes and distances.4

So the changes between M 50mm f1.7 (design here) and DA 50mm f1.8 (design here) might be really small (only aperture blade shape), or more drastic (coatings, small tweaks, differences in machining or materials). We just can't know for sure. But comparisons have shown the differences in optical output are really minimal. You can search for threads, as people have compared lenses from different generations

Machines wear out and the replacement is nearly always different from the original so there could be tiny changes in the matching process but what I'd like to know is did any of the elements in say the 50mm F1.7's or 1.4's etc get changed somewhere along the line for some different type of glass which might account for one model being lauded in reviews and another not, but to do that you need to know what the original was.

CD

[stevebrot]

So the changes between M 50mm f1.7 (design here) and DA 50mm f1.8 (design here) might be really small (only aperture blade shape), or more drastic (coatings, small tweaks, differences in machining or materials). We just can't know for sure.

I own both lenses. Short of disassembly, it is difficult to assess differences other than the DA 50/1.8 having a smaller diameter (~2mm) front element opening and front and rear element curvature different from the M 50/1.7 (fairly obvious on visual observation). The image circle on the DA 50/1.8 is smaller resulting in obvious vignette wide open on a 35mm film negative. My subjective impression is that the DA 50/1.8 compares poorly with the M 50/1.7 on 35mm film, but punches above its weight on APS-C digital to almost equal its predecessor. It is an enjoyable lens to use on my K-3 and is not a lens I plan to sell anytime soon.


Steve

[stevebrot]

...what I'd like to know is did any of the elements in say the 50mm F1.7's or 1.4's etc get changed somewhere along the line for some different type of glass which might account for one model being lauded in reviews and another not...

By reviews, I assume you mean user reviews on this and other Web sites. Other than possible improvements in coatings, there is no reason to choose one of the 50/1.7s over the other. Build, ergonomics, support for AF, and coatings are the differentiating factors. High prices for the F and FA variants reflect the relative scarcity of those lenses such that both have attained mild cult status. Of the 50/1.7 variants, the only model I will not recommend is the the A 50/1.7 due to pervasive aperture ring failure. The lower review scores reflect that.


Steve

[lytrytyr]

With the 50 and 55mm lenses Pentax seem to have reused the optical design where it has proved satisfactory

IIRC, the cemented join in the M 50/1.4 is a little flatter than on the K or A versions,
which supposedly makes the performance slightly better at infinity, and worse closer in.

[PenPusher]

. Of the 50/1.7 variants, the only model I will not recommend is the the A 50/1.7 due to pervasive aperture ring failure. The lower review scores reflect that.


Steve

Yes user reviews mainly on this web site. I now know about the A aperture ring failure the usual way so until I figure a way around it I now use the M 1.4 instead.

I was actually surprised at the difference between the F1.7 and the F2 designs, I would have suspected that manufacturing economics dictated some cunning work around where the same lens elements were fitted with a ring to reduce the aperture.

CD

[boriscleto]

I was actually surprised at the difference between the F1.7 and the F2 designs, I would have suspected that manufacturing economics dictated some cunning work around where the same lens elements were fitted with a ring to reduce the aperture.

They did that with the 55/1.8 & 55/2 for years...But I always wondered how they handled the years that the S-M-C 55/1.8 & Super 55/2 co-existed...