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09-04-2015, 05:10 PM   #1
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The True Nature of the DA* 55/1.4

Hello all,

I just got back into owning a DSLR after many years. I never was anything but a rank amateur, but now it's "game on." I picked a K-3 and got immersed in the whilrwind of learning about the actual particulars of lenses and their performance on actual cameras. It took a while, but I see a lot of information on-line, as a beginner looking for a clue, doesn't seem to be well-informed.

I assume the DA* 55/1.4 is a spherically under-corrected lens. As such, it should have soft contrast wide-open, lateral chromatic aberration issues, and should exhibit a decent amount of focus shift. These seem to be true from my use of it. It may also be a poor performer at infinity due to its design as a "portrait" lens, having a focus helicoid tuned to close-focus. But I'm not sure. I'm field testing it, but I'm gun-shy about the noted Pentax QA issues over recent years. I want to know that my copy is performing as designed. To do so, I need to know just what the design specifications are.

Pentax says it's the "spiritual successor to the FA* 85/1.4." Okay, so we can assume some gross performance similarities between the two. I imagine Jun Hirakawa MIGHT have tamed some of the wild-stallion properties of the FA* 85, to make the DA* 55 more "acceptable" by the average consumer in average use. Maybe not. If I were a Pro, I'd go out and shoot my lens and know everything about it and be comfortable in that knowledge. But nope! I want information!

I know Pentax doesn't publish MTF, and that MTF isn't the be-all measurement anyway. I, after many hours, looked up some of Jun Hirakawa's patents on his other lenses. It seems things like Point Spread Function plots and insightful design goals are provided in many lens patents. If I could get my hands on the patent for the DA* 55/1.4, I'd combine that with online test reports, anecdotal user reports and my own experience to A) know if I have a "good copy" and/or "good camera-lens combo" and B) be better able to maximize the potential of this lens while using it.

I know there's a lot to be said for practical photographic knowledge in testing and using lenses that were designed for particular uses.

I just want to get my hands on the goshdarn patent for this lens, and see if anyone out there has squirrled-away any documents measuring the design performance of this lens.

When I get into my weekend, I'll share how/where I've looked for patents, what I have found so far, and maybe share some general principles of lens design and performance that I myself didn't know when I started out. (Excellent resources like Lensrentals and a few excellent Zeiss whitepapers on MTF, for example.)

Please help! I'd like to see this thread turn into an effective reverse-engineering of the DA* 55/1.4 optical performance. I own it and so far I quite like it.

09-04-2015, 09:00 PM - 1 Like   #2
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QuoteOriginally posted by Nutox Quote
I own it and so far I quite like it.
I think this is the most important sentence....

To each their own. But I say, go take some more pictures! :-)
09-04-2015, 09:05 PM   #3
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QuoteOriginally posted by Nutox Quote
I assume the DA* 55/1.4 is a spherically under-corrected lens. As such, it should have soft contrast wide-open, lateral chromatic aberration issues, and should exhibit a decent amount of focus shift.
Welcome to the Pentax Forums!

That is an interesting assumption, though I am not sure it has been borne out in actual testing.

As for your concerns about Pentax QA and whether your lens is performing to spec, I suspect you are going to come up dry. There are a few members who may be able to afford and know how to use the Imatest suite, but probably even fewer who also own Imatest and the DA* 55/1.4 and are interested in doing the bench work.

That being said, I am sure you will get a number of opinions from DA*55/1.4 owners and lots of photo examples.

Cheers!


Steve
09-05-2015, 02:30 AM   #4
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My copy seems to be pretty sharp from f1.8 on, although it certainly continues to get sharper to f4 or so. I don't know about the patent at all, just what I've read on the forums. I mostly use mine for portraits, although I've been known to do some landscape shots with it as well.

at f1.8



at f4.5



09-05-2015, 06:05 AM   #5
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QuoteOriginally posted by stevebrot Quote
Welcome to the Pentax Forums!

That is an interesting assumption, though I am not sure it has been borne out in actual testing.
Thank you!

Also, I should have said "longitudinal chromatic aberration," not lateral. And I thought about it... yes, the lens is probably NOT under-corrected for spherical aberration, per se, may be it's just almost ALL 'fast fifties' suffer from their spherical aberrations wide-open or near wide-open. I doubt correcting for incidental spherical aberration issues in a fast lens, wide-open, is a realistic optical engineering goal. That would be something you pay thousands of dollars for. (How do the Zeiss Otuses perform in this regard?) Clearly this lens appears to perform to the highest marks when stopped down.

Loss of contrast, Long. CA, these together with focus shift are quoted as the hallmarks of spherical aberration by H. H. Nasse in his "How to Read MTF Curves" white papers written for Zeiss.
(I don't want to hot-link these papers. Search them up. They're fantastic reads for people who did okay in math and science in high school or had some college, and generally enlightening in their observations and conclusions, otherwise. Also, his "Depth of Field and Bokeh" seems like it would be an absolute smash hit among the various photography forums.) So I presumed all the reports of focus issues had something to do with this.

Most of the reports of autofocus problems with the DA* 55/1.4 seem to be with back focus and users dialing in corrections for that. That could indicate focus shift, which, according to the articles on diglloyd, almost always manifests as back focus. And he tested all the DA Limited primes (except the DA* 55) and found them all to exhibit focus shift. Which is kinda wild considering how slow they each are wide open. A compromise for the physical form factor of the lenses? Funky interactions with field curvature? Or maybe he's just got a severely critical eye, he is a militant wizard about sharpness. Then you have the highly visible reports of people trying a dozen DA* 55's and seeing all copies focus totally randomly, or having apparently tilted elements.

I think the biggest thing that triggers my curiosity, is the DA* 55/1.4 being marketed as specifically a "portrait lens" and being compared to the FA* 85/1.4. The FA* is reported to consistently front-focus and to "behave poorly" focused at infinity, though at what distances and what exactly that means, I don't know. And certainly it's tuned to be bokeh-licious, which would have something to do with engineered under-correction of spherical aberration, if you consider the writings of H.H. Nasse.

I also have a DA 20-40 Ltd., and I don't wonder about that lens, because it isn't marketed as being application-specific. It's just a solid all-around performer with no glaring weaknesses, a great "adjustable normal" walkaround lens, bomb-proof in handling flare, seemingly no loss in contrast working into the light, sharp as you need it to be stopped down, not horrendous wide open, just a little mushy on the edges like any other lens. I carry it in-hand on the street where it's easy to handle and I can bang it off lamp posts and not worry too much, spill some milk on it etc.

QuoteOriginally posted by stevebrot Quote
As for your concerns about Pentax QA and whether your lens is performing to spec, I suspect you are going to come up dry. There are a few members who may be able to afford and know how to use the Imatest suite, but probably even fewer who also own Imatest and the DA* 55/1.4 and are interested in doing the bench work.
Right! Not "in-spec" per se, we know how that goes with copy variation (thanks in no small part to the lensrentals blog) and factory service centers, I mean just performing in-character. Whaddaya mean, we don't yet have a forum group that's gone in together on the $420 Imatest "Lite" version (with printed test chart!) and gone all sortsa crazy testing Pentax lenses? That might have to change! (Though you have to pay megabucks for the full version that can test MTF other than MTF50. MTF10 and MTF90 at 10, 20, 40 lp/mm would make great fodder for arguments.)

Can someone enlighten me as to what is meant when it's said a lens "doesn't perform well at infinity?" Are we talking a nose-dive in the MTF, some visually apparent optical aberrations, crappy autofocus at the near-infinity mark, etc? It's not something you hear about much from people who spend actually-reasonable amounts of money on glass I guess.

I did some hand-held aperture series with a brick wall and an actual scene. I'll bust out the tripod to tighten things up, and set up my metal ruler under good light indoors and posts some test results myself. So far I at least don't appear to have any centering issues. I focused manually stopped-down in live view, I'll have to work a few variations of focus technique, too. Hit me up with those anecdotes and I'll come back with the patent search tomfoolery I've done.
09-05-2015, 08:00 AM   #6
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The 55 1.4 is a good lens. Hirakawa Jun is a good lens designer. Idk, maybe learn Japanese and shoot the guy an email?

edit: sorry, that was kinda dickish. I'm actually intrigued by the subject and have nothing of value to offer, but curious to hear what those that do have to say (and wouldn't it be neat if someone who knew Japanese actually contacted the designer?)

Last edited by ChatMechant; 09-05-2015 at 09:17 AM.
09-05-2015, 10:28 AM - 1 Like   #7
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09-05-2015, 11:27 AM   #8
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If you find front focus with many lenses you'll have to test several bodies also.

I can't help you on your theories but to know how a lens behave the best thing is to use it.

09-05-2015, 11:52 AM   #9
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Perhaps you could also rent a DA* 55 from Lens Rentals and compare with your copy? I'm not certain, but I think they test each copy of each lens they get and 'tune' them. And they return to manufacturer any outside of some sort of MTF bounds. If true, this would make it a great baseline for testing any further copies. If I was doing this kind of test, I'd probably send them an email and inquire!!

---------- Post added 09-05-15 at 01:59 PM ----------

QuoteOriginally posted by Nutox Quote
Then you have the highly visible reports of people trying a dozen DA* 55's and seeing all copies focus totally randomly, or having apparently tilted elements.
Also, completely anecdotal, however I noticed on my DA* 16-50 f/2.8 lens that, at 16mm aimed @ infinity at a bank of trees in the distance, it would focus seemingly totally randomly.. I could keep pressing the shutter release button down half way to refocus and it would give me a different level focus nearly each time... sometimes in focus and sometimes very much not. I'm not sure if it has to do with not using a lens hood (possibly) during these tests or if it is more an issue with the SDM somehow.

I converted the lens to screw drive and I haven't noticed this behavior though. I should probably go test now that I have a lens hood too just to refresh my mind!!

Last edited by mee; 09-05-2015 at 12:00 PM.
09-05-2015, 05:40 PM   #10
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QuoteOriginally posted by Nutox Quote
Then you have the highly visible reports of people trying a dozen DA* 55's and seeing all copies focus totally randomly, or having apparently tilted elements.
Those people must all be posting on "that other forum" They must have read the DPReview evaluation of the lens.

I have to admit to not having much personal interest in the DA* 55/1.4. I have a number of fast 50s (20+), including a few classics, and have little inclination to spend that much money for another one. That being said, I am not aware of any particular complaints regarding the lens performance or QA. The usual commentary is that if you don't need the AF or internal focus, you can do as well or better with manual focus glass. Similarly, the comment is often made that you can buy several DA 50/1.8 for the price of the * if you don't need silent AF or quick-shift. For the record, I have little interest in AF for portrait work. I am picky about focus point.

The claim regarding focus shift had me rolling my eyes a little. The subject comes up every once in awhile here where it generates a lot of talk, but no real evidence. I have done testing with the lenses in my collection vs. the split image finders on my film cameras as well as the Katz Eye on my dSLR. Of the fast primes that I own, only one has detectable focus shift (stopping down wide-open to f/8 and points in between). That is my Rokkor MC 58/1.4 which shows a shift of a few millimeters at 2' and none detectable at greater distance.

That did not surprise me since focus shift was part of the standard test suite for most photo magazines and fairly rare was the lens that displayed such. In addition, it is easy to test for with a split image finder so poor designs are readily exposed. Does it happen? Absolutely! I am familiar with the disturbing test done by diglloyd with the Canon 50/1.2L. Is it pervasive in modern glass? I dunno. Is it a problem with the DA* 55/1.4? I dunno, I suppose someone can test for it and post the results here.


Steve
09-08-2015, 05:50 PM   #11
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Oh, I'll shoot it, for sure!

I find this whole subject fun to learn about... anyone here fluent in Japanese, so I can write a letter?

In addition to my handheld brick wall test in full sunlight, I did a tripod aperture series with flash indoors, with a ruler taped up and some objects.

What I have learned:

I seem to have a good copy. I have noticed no unevennes of unsharpness, no misfocusing manually, no defects in construction or operation.
I shot from 18 inches, then 12 feet. Manual live-view 10x focus peaking. Tried to only come to focus from near focusing to far. (I wonder what sort of backlash these funny little piezoelectric legs have?) Focused wide open once, focus point was not altered through the series, shot stopped down. Shot a lot of infinity frames outdoors, but nothing formally processed and looked at like the two tripod indoor series were.
Shot RAW, Imported with LR 5, default adobe camera calibration profile, no lens correction profile, no tweaking, just the default import/export settings and a 100% crop.
I see zero, or at least almost zero - not noticeable - focus shift with this lens.
Longitudinal chromatic aberration in spades!
Resolves fine structure well, as well as would be expected in this class of lens, I assume.
Fine detail wide open is indeed masked by low contrast... classic spherical aberration is going on here, it seems. I tried cleaning up the f1.4 frame, to increase actuance, f1.4 cleaned up to almost f2.0 performance, except my un-educated hacking suggests sacrificing something significant in the color/contrast area... skills I don't yet have, to retrieve saturation and other stuff. F1.7 cleans up even easier and by f2.0, from a pp standpoint, just a little work and then we're waiting for the high-frequency resolution to come up with stopping down.
Mid-frame bokeh from christmas lights about 18 inches behind the setup showed strong border wide open, quite muted by f2.0 and gone at f2.8.
PhotoBone mentions off-hand "no noticeable field curvature with this lens," but that's all they say. I've never noticed they go into it, I should look around that site. Some tripod infinity work left, then it's just go shoot some fun and look critically at those images later for me. Except writing a thesis on the DA* 55/1.4 lens.

I don't want to do any lens tests with autofoucs. So far, this copy is certainly behaving like it justifies all the reviews that say it's great. I don't want to experience first-hand the harsh truths of camera bodies, not now.

Thank you all for the anecdotes, suggestions, observations and analysis! It's good to be given an idea of what's a reasonable expectation and just the general knowledge of how something is.
11-19-2015, 08:46 PM   #12
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I usually use Google translate....
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