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Forum: Pentax SLR Lens Discussion 11-06-2017, 11:32 AM  
What (feasible) lens is missing from Pentax's lineup?
Posted By 52mm
Replies: 72
Views: 6,463
Speaking of 24, Pentax is really lacking a good "workhorse" normal to slight wideangle for crop. Something 24-28ish, maybe the size of a DA35 2.4, with image quality that's hard to fault, reasonably affordable but still solid enough. The closest thing would probably be the FA 31 Limited now, but the appeal of a Limited costing over a grand tends to be rather, umm... limited.
Forum: Pentax SLR Lens Discussion 11-04-2017, 06:03 PM  
Impact and importance of a lens hood (and question on designing one)
Posted By 52mm
Replies: 7
Views: 588
That's a very clever trick, actually. I presume it would only work up to about f/2.8 though, limited by the viewfinder? I can just about still make out the corner with the DA 35 f/2.4 on the K-50.

Expanding on the idea, I proceeded to roll up a piece of ordinary white paper (80 g/mē), make a tube to go around the lens and shine a smallish (LED) flashlight through the viewfinder. In a darkened room, it does give some decent-ish petal contours, more so on the inside than the outside. Seems workable enough for some pencil marks in any case. Cool. :cool:

EDIT: So for this combination of camera and lens I am getting:
Min distance 8-8.5 cm from mount
Side petals 11-11.5 cm
Long petals 15 cm
with a tube diameter of about 59 mm.
The lens itself is about 4.5 cm long, so from the front you'd be getting:
Min distance 3.5-4 cm
Side petals 6.5-7 cm
Long petals 10.5 cm
So my 35mm long, 58 mm dia. circular screw-in hood clears the corner with not much room to spare (seems right - I could already see that earlier). Those petals are quite impressively long, so I'm not surprised a circular hood is far from ideal.

Now with M 50 f/1.7. Boy, this is getting fuzzy.
Min distance ~9.5 cm from mount
Long petals 18 cm
Second try:
Min distance 9-11 cm
Long petals 20.5 cm
Lens length ~3.5 cm, so subtract that.

Now since I know that 70 mm long, 62 mm dia. in front of the lens already is a bit too tight, I guess minimum distance from the front should be around 6 cm, long petals maybe 12 then? This is getting pretty long.

Another data point for an Auto Revuenon (Chinon) 50mm f/1.4:
Min distance ~9 cm from mount
Long petals ~20 cm
Lens length ~3.8 cm

EDIT^2: I am thinking that instead of going with a super long petal hood, one may be better off complementing a circular hood with a rectangular window thingy, like what the 21 Limited has... This seems to work best on shorter lenses, since at short distances things are round... on a 135 I have to go maybe 20 cm away to see a decent rectangle (so that's why tele lens hoods are round, huh?), but a 35 plus 35 mm extension would be OK.
Forum: Pentax SLR Lens Discussion 11-04-2017, 05:13 PM  
Lens Aperture and Sensor Size
Posted By 52mm
Replies: 48
Views: 1,446
Sure, but it's still bigger rather than smaller, and slower to boot. If you wanted a 35mm f/1.25... well, just look at the various 35mm f/1.4s. And 35s still are quite moderate. Go to 28 or even 24, and things start spiralling out of control. I did find the odd vintage 35 that was at least worth using over the 18-55 kit... 28, not so much.

And the crop sensor doesn't even get a 33mm f/1.5 on the expected level if you interpolate between the two.

I think you two are saying the same, just from a different perspective.
25mm f/1.0 (M4/3) vs. 50mm f/2.0 (FF)
is entirely equivalent to
50mm f/2.0 (M4/3) vs. 100mm f/4 (FF).

The way things have been going in the last decade or two, we might still see "affordable" medium format digital cameras within our lifetime ("affordable" complete with quotes in the way that FFs are now)?
Forum: Pentax SLR Lens Discussion 11-03-2017, 10:36 PM  
Impact and importance of a lens hood (and question on designing one)
Posted By 52mm
Replies: 7
Views: 588
Please excuse me if I am preaching to the choir and beating a dead horse here, but I recently got some generic screw-in lens hoods in for my bunch of old manual glass and decided to do a bit of experimenting. Anyone can do this - a look into the back of the lens with a naked eye at various angles to a compact light source (with a hand for shading / light seal) will reveal stray light coming from places it's not supposed to (which is outside the aperture). If you can see it, chances are so will the sensor, leading to reduced contrast if it's severe enough. (The exact level still is up for debate.) We're generally talking angles well outside the field of view here, up to 30-60° off-axis. Doing it this way seems a lot easier and more insightful than staring at images taken under potentially less than ideally controlled and constant conditions.

My conclusions:
  1. For best contrast under adverse lighting, use a hood as long as possible without introducing vignetting. (Surprise!) That also means different hoods for crop vs. FF. Even if you can't resist the flair of a lens, you should be able to resist its flare. ;)

  2. Performance improves when stopping down, sometimes dramatically, depending on how good internal blackening is. (Lenses with manual aperture tend to prove more insightful than modern-day all-automatic ones as you have a manual readout.)

  3. There are some major performance differences in lenses at times. My worst 1.4/55 from the mid-'70s was 3 stops worse or needed a hood twice as long (into corner vignetting territory above f/2.8 for we are talking plain circular hoods) to become bulletproof at the same aperture as my best (if several years newer) 1.4/50s - or even its 1.7/50 cousin of same vintage for that matter. Somebody once called this 55 a "vampire lens" - do not subject to daylight, especially wide open. ;) This trait seems to be quite common on early fast lenses. My f/1.7s varied a lot less in general, none were straight terrible. The best result, incidentally, was turned in by an M 50/1.7, which would be decent even without a hood (and oddly enough, somewhat better than its A series counterpart).

  4. Integrated sliding lens hoods on 135s (as they were common around 1980) are a nice touch but generally way too short to do much of anything aside from keeping the worst reflections out of the front lens. We're talking something like 15 mm of effective extension when you could easily do with 100+ on crop assuming this is not getting too unwieldy for your tastes (I combined two hoods with filter threads for 70 mm total). Results on 135s seem less convincing in general, most of mine were rather meh. The K 135/3.5 did reasonably well, though the star of the show surprisingly was the CZJ "Zebra" Sonnar from the early '70s, complete with single coating and shiny metal aperture blades (if good internal blackening otherwise)! (Unsurprisingly, however, you don't actually want the sun in the frame with this one.)

(Writeup with all results at 135 and 50-55, respectively. Note that I am generally referring to crop, not FF.)

Now I've got a question left: There are some nifty websites that let you design print-out lens hoods, but you have to enter the measurements yourself - how do you actually arrive at the dimensions necessary? Is there a way to calculate them? Or is that way too lens-dependent, making it generally infeasible for anyone but the lens manufacturer and requiring an emprical approach instead?
Forum: Pentax SLR Lens Discussion 11-03-2017, 04:28 PM  
What (feasible) lens is missing from Pentax's lineup?
Posted By 52mm
Replies: 72
Views: 6,463
I have no idea why nobody makes good compact moderate telephoto primes these days. I mean, it's great that I can buy something like the Samyang 135mm f/2, but that is quite the beast (like telezooms) and for a rather compact camera like the K-50 I wouldn't mind something more the size and weight of a 135mm f/2.8 of old, maybe even the K 135 f/3.5. (I'm not even asking M 135 f/3.5 size, which is really petite.) Surely these days it should be possible to do better than these decades-old designs with their often lousy longi CA correction and other quirks? Superlatives may be better marketing-wise but in practice I would prefer something a little more.. practical.
Forum: Pentax SLR Lens Discussion 11-03-2017, 12:16 AM  
Lens Aperture and Sensor Size
Posted By 52mm
Replies: 48
Views: 1,446
And this is why traditional crop DSLRs are somewhat hampered on the wide end - along with the mount, they inherited the registration distance from their full frame film predecessors, hence anything below about 40 mm has to be a retrofocus lens, and those get big and heavy fast as you move towards shorter and faster lenses, not to mention aberrations being harder to control. You can't just downsize a 50mm to 33 and call it a day. (Might just about work for a 58 though?) In the end a 35mm f/2 ends up being rather bigger than a 50mm f/1.8 and usually not quite as sharp, and 35mm f/1.4s are quite chunky and heavy. And that's why they are not more popular. It would be too funny if a full frame camera with a standard 50 ended up being lighter than its crop equivalent.

I believe that some fast modern-day crop lenses are actually using an integrated "booster" (full frame converter) despite its potential shortcomings. The last four elements on the Samyang 16mm f/2 look suspiciously like that, for example. If all you need is the FOV of a 24, it sure makes a lot of sense to design a good 24mm f/2.8-ish first and then convert that. Things look even more extreme on their full frame capable 14mm f/2.8. I am guessing that if you were to play it straight, some elements would become really tiny, posing all kinds of optical and manufacturing problems (diffraction limiting, high and controlled surface smoothness etc.).
Forum: Contest Voting 10-09-2017, 08:55 AM  
Sunset at home
Posted By 52mm
Replies: 0
Views: 39
Took this one a while back, now there'd be a house in the way. Some NR and CA removal, WB = daylight, exposure adjusted, slightly cropped, that's it.
Forum: Pentax SLR Lens Discussion 06-15-2017, 05:13 AM  
DA 16-85: Two duds in a row or just no go with K-x?
Posted By 52mm
Replies: 12
Views: 850
They definitely were not the same, the replacement was sent out while the first one was still on its way back. Definitely a new unopened one (and seemingly from another distributor, too, as it had an Italian warranty sticker this time - heaven knows where Amazon gets their stuff).

Since said b'day is going to be relatively soon, I guess I'm going to wait until then, try it out on dad's K-5, and if it's just as bad there, it'll see a trip to authorized service, hopefully with no warranty complications.
Forum: Pentax SLR Lens Discussion 06-14-2017, 05:05 PM  
DA 16-85: Two duds in a row or just no go with K-x?
Posted By 52mm
Replies: 12
Views: 850
I wanted to give my dad a 16-85 for his birthday, he's got a K-5 that sees use with a 16-45 all the time. Reading reviews, I get the feeling that I better check it out to see whether there are any anomalies. So I get it, put it on my K-x, and the focus on the wide end is so far out it's not funny (severe backfocus at infinity). Things look great when focused manually though. Packaging looked a bit banged-up, so I thought maybe it took a bit of a hit, packed it back up and sent it back.

Well, today the replacement came in (packaging not as beat-up looking though hardly luxuriously padded either), and guess what, it does the exact same thing. In fact, LiveView AF has big trouble locking onto anything as well, even though the picture seems decently stabilized and focal length seems to transfer OK.

I have already cleaned the contacts on the lens mount for good measure even though I had already done that recently... that body has seen a fair few lens changes.

Now I'm stumped. Is the K-x just too old for this lens (the 1.03 f/w is from 2012-ish IIRC?), did I get two duds in a row, or was there a whole batch that does not play nice for some reason and needs a firmware upgrade or something?

Unfortunately I can't try the lens on the K-5 right now for obvious reasons.
Forum: Video and Pentax HDSLRs 03-18-2017, 09:11 PM  
How to keep K-x videos from starting to crackle when trying to transcode them
Posted By 52mm
Replies: 3
Views: 482
Oh, that's interesting. I'll have to grab a video from my dad's K-5 and test whether the same trick also works there. Really, once set up, the effort required is quite minimal. One may even be able to automate VLC (once you've found out which of the approximately 2 million command line options you need... this might help...) and add it to the conversion batch file...
Forum: Video and Pentax HDSLRs 03-18-2017, 04:04 PM  
How to keep K-x videos from starting to crackle when trying to transcode them
Posted By 52mm
Replies: 3
Views: 482
I never used the video mode on my K-x very much, but now I wanted to make some quick YT vids and figured 720p @ 24 fps and 32 kHz mono audio from the built-in omni mic would be just fine for that. Turns out one of them had a nasty crackle about once a second in the audio after transcoding via FFmpeg, even though VLC played it fine (MPC-HC shows the same crackle though). That's really unacceptable. To make a long story short, there seems to be something iffy about the files generated by the K-x, possibly the number of audio samples does not match the video frames or the playtime is saved wrong (always seems to be in whole seconds).

Salvaging the files turns out to be quite easy:
You can use VLC's converter function for this. (JFTR, version 2.2.4 here.) So open that and drag the file(s) there, right-click, Save.
I'd recommend creating a new converter preset. Encapsulation: AVI, enable audio, copy original audio stream, enable video, copy original video stream. Call it "Video - AVI, copy streams" or somesuch.
Give the output a fancy name.
Since there is nothing computing-intensive involved, this is super-fast, mostly limited by I/O.

The result may be cut about a second short at the end, but that's easily taken into account beforehand. (Trying to do a direct transcode in VLC resulted in the first second getting cut off, which is a lot more annoying.)

That's it! You should now have AVIs that are behaving themselves and transcode fine. This stupid problem cost me more than 2 hours to figure out.

That's what my transcoding batch file looks right now - I guess I should use the AAC LC encoder for audio once I figure out how to do that:





Code:

\bin\ffmpeg -i %1 -af aresample=resampler=soxr -ar 44100 -acodec libvorbis -q:a 7 -c:v libx264 -preset fast -profile:v high -movflags faststart %1.mp4



Forum: Pentax SLR Lens Discussion 05-23-2013, 01:08 PM  
M 35/2.8, bogus aperture scale?
Posted By 52mm
Replies: 6
Views: 1,045
True enough, the accuracy of the last stop can be off. Apertures may look pretty funny at this point. Several of my Chinon lenses also tend to become darker as you stop down and choose exposure time according to nominal f-stop.

The M 35mm 2.8 is a perfectly fine f/2.8 wide open, no complaints there (brightness at my usual f/2.8 exposure time is what it should be - I always use the same lamps in my test setup). It's just that the next step (on my sample) is about f/3.3, not f/4, so it actually goes
2.8 3.3 4 4.8 5.6 6.7 8 9.5 11 13 19
or thereabouts. Actually brightness creeps up a little as you stop down, being about +1/4 EV by the nominal f/16 position even with exposure time set for f/13. And lo and behold, what this lens calls f/22 corresponds to f/16 on the Chinon counterpart when looking at the physical aperture (or images when applying exposure compensation to make up for the different time settings). The Chinon lens becomes a little darker as you stop down, so with the half-stop scale offset to begin with they finally meet. I have to apply almost +1/3 EV to the Chinon's image at f/8 to get the same brightness as on the M 35/2.8 at a nominal f/9.5. (Both lenses give maximum file sizes / sharpness at this point.)

Guesstimating by eye, I'd say "f/11" is about as big as f/11 on a 50 (while in theory they should differ by exactly one stop), same for "f/8" and "f/5.6". "*" is between f/4.8 and f/4 on the 50, and wide open is a bit bigger than f/4.

I'd say they probably intended the aperture to work like on the M 28/2.8, but at least on my sample, this didn't work out.

TL;DR: My sample of the M 35/2.8 has a +1/2EV offset from the second aperture stop down (vs. what the scale says), and gains about an extra +1/4EV by minimum aperture. The Chinon counterpart (49mm MC, 0.3m min focus) seems broadly accurate but loses about 1/4EV by minimum aperture.

Does anyone else have an M 35/2.8 they could check?
Forum: Pentax SLR Lens Discussion 05-22-2013, 04:08 PM  
M 35/2.8, bogus aperture scale?
Posted By 52mm
Replies: 6
Views: 1,045
My "new" M 35mm 2.8 has nice equidistant aperture ring clicks with markings being as follows:
2.8 - * - (unmarked) - 5.6 - (unmarked) - 8 - (unmarked) - 11, etc.

* should be f/4 - but only one click after f/2.8? That struck me as odd. If at least there were twice the travel compared to full-stop clicks, as between f/16 and f/22, but no. The aperture also looked suspiciously large compared to what it should be.

So I consulted green button metering, and what do you know, it's nice half-stop steps from f/2.8 all the way down to f/16.

IOW, the scale appears to be bogus, and in fact should read:
2.8 - (unmarked) - 4 - (unmarked) - 5.6 - (unmarked) - 8 - (unmarked) - 11, etc.
Which makes an awful lot more sense, also considering that the lens already is quite usable wide open. Why would they not provide an f/3.3 stop then? That leaves the last two stops at f/13 and f/19 though.

Did anyone else notice that? Is it just my sample, maybe (with 30 year old lenses, you never know)? Otherwise I'd say somebody goofed at the time.

Performance is about on par with a (presumed to be newer) Chinon 5-element MC job, btw, with a bit of decentering.

BTW, has anyone ever determined true open aperture for the M 28mm 3.5? Honestly it meters like an f/4, though it's slightly brighter in the center compared to some other 28s at f/4 (but vignetting means the outer areas are darker). I'm inclined to think that f/3.5 sold better...
Forum: Pentax SLR Lens Discussion 05-22-2013, 03:08 PM  
Interesting Observation - Pentax vs "other" lenses
Posted By 52mm
Replies: 14
Views: 1,509
Color rendition is an interesting (if complex) subject. IMO it is mainly affected by three factors:
1. Transmission, or rather the variation thereof across the visible spectrum.
2. Contrast.
3. Exposure.

#1 is a function of glass and coatings. Unfortunately, with AWB in play, it is not easy to judge the effect of a certain observed tint by eye, though I suspect brownish tint might be more critical than greenish tint. (AWB can only shift the levels of the red, green and blue channels as a whole. It cannot compensate for any variation / tilt within the respective wavelength ranges. The sensor has already "binned" these with its primary color filters.)
I do know that my Sigma 70-300 DG OS (moderate brownish cast) takes its toll on blues, with the sky always looking a little muted. By contrast, the Pentax 55-300 (slight cast only, and very clear for a complex lens like this) seems to produce beautiful colors.
Oh, and there is a point beyond which AWB will refuse to fully correct a lens tint even in broad daylight, probably as a safety measure. I have at least one lens that you can identify by the slight greenish tint on pictures (old Tokina AT-X 28-85) - it does exhibit a rather heavy one when looking through!
Usually primes are a safer bet here, as in general there's simply less glass in them (along with fewer surfaces).

#2 is influenced by many things. Lens configuration, coatings and which ones are used where, internal blackening, aperture surface, use of a hood. (The hood comes into play when there is a tendency towards veiling flare and you've got a nice and bright blue sky.) It's not necessarily predictable. I've had a nominally single-coated lens whose contrast proved just fine, and a nominally multicoated one that struggled even on my indoor test target. (That one has the last few elements single-coated in amber, which is not a good combination with a DSLR sensor. By contrast, magenta single-coating seems to be uncritical.) There apparently is one uncoated element lurking in the cheapo Sigma 55-200 (a typical cost-cutting measure seen in inexpensive '90s/2000s zooms), but its contrast struck me as good.
BTW, contrast does not necessarily always differ between lenses - once it comfortably exceeds either the dynamics of the scene or the camera's capabilities, they will generally produce equivalent results.

#3 comes into play because of the camera's internal gradation curve, which is nonlinear. Simply put, a mere difference in exposure (due to inaccurate aperture calibration or whatnot) may yield a difference in contrast in the resulting image. Tweaking contrast by varying exposure should actually be familiar to seasoned film shooters. If, however, you're aiming to compare lenses fairly by image results, it means you won't get around shooting RAW and matching exposure very accurately afterwards (sub-1/6 EV if need be). Suffice to say, it's a PITA when you have basically well-performing lenses. A truly "bad apple" will be obvious without going to such extremes, but often it's subtleties that are harder to track down.

Fun, eh?

But it's true, Pentax lenses can be very good performers when it comes to transmission and contrast. Before I'd seen the 55-300, I had already been quite impressed by the old M 80-200, which does really well in transmission for such a complex old zoom (12 groups!). SMC sure was something special in those days. Well-performing coatings are much more generally available nowadays though. The issue with (some) Sigma lenses may simply be one of price pressure, as several of their models are essentially better-value alternatives - I'm thinking telezooms in particular (50/55-200/4-5.6, 70-300/4-5.6 et.al.). They have to cut corners somewhere.
Forum: Pentax SLR Lens Discussion 05-22-2013, 10:10 AM  
A Tamron 28-75 puzzle.
Posted By 52mm
Replies: 16
Views: 1,933
In lens disassembly tutorials you'll typically see a flexible PCB mounted on the inside of the barrel somewhere, with one or more chips on it. Basically you need something to decode a pattern that allows determining the focal length (think rotary encoder), and a couple hundred bytes or so worth of EEPROM. Nothing too fancy even by the standards of 20 years ago, it just has to work reliably.

Incidentally, I had the same issue of different AF performance on different camera lately. A Pentax 16-45 showed noticeable backfocus on my K-x, but was spot-on on a K-5. Go figure. All I can think of is that the AF sensor sees through a different part of the lens or somesuch, and a non-ideal lens alignment then causes these discrepancies. This sort of problem seems to be pretty much limited to complex zooms with multiple moving lens groups, probably because a lot can go wrong mechanically in those, in terms of tolerances and whatnot.
Forum: Pentax SLR Lens Discussion 05-22-2013, 09:48 AM  
So do M series 28s not focus to infinity or what?
Posted By 52mm
Replies: 7
Views: 1,228
Must be a sample / batch thing then. The S/N of my M 28/3.5 is 6626297.
The M 35/2.8 that I got in recently will focus to infinity just as much as any other of my primes. Maybe a hair short, nothing dramatic (like I said, that might be my K-x's fault).
Forum: Pentax SLR Lens Discussion 05-20-2013, 07:03 AM  
Sub-150$ wide angle M42 lens?
Posted By 52mm
Replies: 12
Views: 2,292
Basically, sub-24mm you might just as well stick with the lowly 18-55 kit when it comes to lenses that old. Old 28s currently don't have me terribly convinced either.
Forum: Pentax SLR Lens Discussion 05-20-2013, 06:46 AM  
Welmy 28mm F2.5 for Pentax K mount - Anyone heard of it?
Posted By 52mm
Replies: 11
Views: 2,961
You wouldn't have some pictures of the side and back of the lens? That may help IDing the actual manufacturer - it looks like another of these old designs "recycled" with multicoating at the time, pretty much a "no-name" cheapie. (The massive spherical overcorrection and resulting wacky bokeh tells an old construction from like the early '70s, as does the comparatively larger front element.) I have a similar Porst-branded 28/2.8, presumably made by Sun Optical. Big hunk o' glass (58mm filter thread), but nothing special performance wise - not an awful lot better than a little Korea special wide open and worse when stopped down.
Forum: Pentax SLR Lens Discussion 05-19-2013, 02:44 PM  
Prime Lens vice zooms
Posted By 52mm
Replies: 9
Views: 917
This. Primes can be small, light and unobtrusive for what they do. Look at a DA 35/2.4, one of the super small 40/2.8s (barely a body cap), the 50/1.8 or the 70/2.4 Limited, and then look at the Tamron 28-75. (And I suppose you don't even want a fast 300+ mm lens in anything else but prime form.) Not to mention that a lens on the camera (subjectively) tends to be multiple times as heavy as in the bag.

Plus, having a fixed focal length makes you shoot differently, more consciously. (Even more so for manual primes.) It's something for those occasions when you can / want to take your time, not so much when it's hectic.

Oh, and I while I certainly wouldn't want to drop a prime, I REALLY wouldn't want to drop a zoom.
Forum: Pentax SLR Lens Discussion 05-19-2013, 01:35 PM  
So do M series 28s not focus to infinity or what?
Posted By 52mm
Replies: 7
Views: 1,228
It's not like it's a super big deal to me, as I do have several 28s (though I don't find any of them to be an awful lot better than the lowly kit lens, and some worse; generally, none of them render fine detail at a distance with as much "bite" as the aforementioned Chinon 35mm job). But being a scientific type, I'm always interested in why things are the way they are.

My M28/3.5 does do fairly well on a test chart @ ~1m, sharpening up faster than, say, a Vivitar K03 or the kit lens. But that's obviously of no use if it fails where detail retrieval is needed most often, i.e. at a distance. After a little garden test shoot, I'd say maximum focus distance is at about 10m.

As far as field curvature goes, I pretty much do not have to refocus when turning the camera so the focused object moves from image center to edge, i.e. the focal plane is most definitely curved. It ain't a macro lens for sure! (Expectedly, it didn't do too well on an extension tube.) However, I can see why this property may actually be useful for street shooting and the like. Sure beats having a focal plane curved in exactly the opposite way. Besides, many of my lenses seem to be like that, though obviously a larger viewing angle (shorter focal length) will magnify the effect.

I am still looking for the right size tool for the front ring. So far, nothing too suitable has turned up. I'll keep looking.
Forum: Pentax SLR Lens Discussion 05-18-2013, 04:49 PM  
So do M series 28s not focus to infinity or what?
Posted By 52mm
Replies: 7
Views: 1,228
I have noticed that infinity focus seems a little tight on a number of my manual primes - enough of them, in fact, that it has me wondering about actual registration / flange distance on my K-x. (Even my DA L 18-55 is a hair short of true infinity at the short end.) My M 28/3.5 may be the worst offender here, it doesn't get a lot of AF confirm beep action for distant subjects at all, and several of my other 28s are noticeably sharper at 20-50 meters+. Now when reading reviews of this lens and its f/2.8 companion, you often find "it's nothing too exciting for distant subjects" or similar statements, including "corrected for intermediate distances". While I don't want to exclude the possibility of the latter, could it be that these lenses routinely do not reach infinity right from the factory?

Using a DOF calculator, I find hyperfocal distance for 35mm film even at f/5.6 to be less than 5 m, and in fact it would still be less than 8 m wide open. So possibly these lenses were adjusted for that instead? That wouldn't be good by modern-day pixel peeping standards on crop DSLRs. A slightly too big effective registration distance in the camera obviously wouldn't help either, seeing how little a 28 actually moves between 3 meters and infinity.

A little test shows that with my M 28/3.5 sample mounted on my K-x, focus indication at a measured distance of 2.3 m is 3 m, or very short of this at best. So it does look like calibration on this particular combination is in fact off. At 2.0 m measured, the 3 is about halfway between the red focus indicator's tip and the IR infinity indicator on the DOF scale, which I'd normally read to be about 2.5 m (taking a look at a 50, the 2m position should be right in the middle between 1.5m and 3m, and we're definitely closer to the 3m mark at this point). At the same time, an old Porst (Cosinon) Auto 50/1.7 M42 lens (using an adapter that retains infinity focus) is pretty much bang-on, a newer PK mount relative shows about 2.1m, and my M 50/1.7 even reads 1.9m or thereabouts. A battered Auto Revuenon (Chinon) MC 35/2.8 with some play in the lens barrel gives maybe 2.1-2.2m or so. None of them are very far out.

That leaves us with two questions:
1. Is this typical for M 28/3.5s or even M 28s in general (which I suspect to be the case), or did I just get a misadjusted one?
2. How would one go about adjusting infinity focus on this lens? Screws under the front ring?
Forum: Pentax SLR Lens Discussion 05-18-2013, 02:19 PM  
DA 35mm f2.4 AL vs M 50mm f1.7
Posted By 52mm
Replies: 23
Views: 7,598
Just to get this straight.... A 35 on 1.5x crop is equivalent to a 50 (or more precisely 52.5) on FF, at least as far as perspective goes. The resulting viewing angle (and only viewing angle) is what determines required distance and hence perspective distortion. You can get the same viewing angle with a large sensor and long focal length or with a tiny sensor and short focal length. For a classic 1/1.8" sensor (8.98 mm diagonal), the equivalent of 50mm on FF would be about 10.4 mm.

DOF is a different story, but even then the difference between APS-C and FF would be minor, and easily dominated by a change in distance.

And of course the focusing ring marks on a 35 and 50 will be different if both of them were built for the same sensor/film format. After all, the 35 would have had larger DOF due to not only being shorter but also having a significantly smaller maximum aperture, and hence required less accurate focusing / a shorter focus throw. That's why my M 28mm 3.5 gets by with a measly 90° turn for 0.3m to infinity while its 50mm 1.7 counterpart covers 0.45m to infinity in about 210° (which takes up maybe 60-70° on the 28/3.5 - seems about right for a factor of 2 in max aperture and 1.8 in focal length, or about 3.5 in total).

Unfortunately, there is one little problem in crop DSLRs: There is no 35 with truly equivalent performance to a 50 on FF. This is because they have inherited registration distance (or flange distance, or whatever you want to call it) from their FF ancestors along with the mounts, and hence the required backfocus distance for lenses. A lens can only stick so far into a SLR camera without getting into trouble with mirror clearance, and once the distance from the final element to the sensor (i.e. backfocus) exceeds desired focal length, you've got a problem. For M42 or PK, the limit is somewhere between 40mm and 35mm.
At this point, you have to resort to resort to retrofocus a.k.a inverted telephoto designs, which place at least one negative meniscus element in front of a "conventional" lens to achieve a lens that is physically longer than its focal length, including the desired increased backfocus distance (while regular telephoto lenses do the exact opposite). These meniscus elements can get pretty big, depending on how short and how fast you want to go.
This is why a 40mm 2.8 is a tiny pancake lens and a 35mm 2.8 is almost as long as a 50mm 1.7, and they get bigger and bigger as focal length is reduced further (hence why you're looking at ultra wide angle lenses with f/4 or so to keep size halfway reasonable). Retrofocus lenses generally also are not as sharp / need more stopping down.

That said, I have an M 35/2.8 on the way, let's see how that does vs. a lowly Chinon 5-element MC job that strikes me as rather decent for what it is. Seeing sharpness test results of that vs. the FA 35/2 (which should be much the same as the DA 35/2.4) I wonder whether I shouldn't have gone straight for the DA 35/2.4 though. That one seems to be rather good, even if the (fancier) Canon and Nikon 35/2 and 35/1.8 offerings seem to sharpen up faster according to DxOMark. (Small wonder, for they are using 7 elements in 5 groups and 8 elements in 6 groups, respectively, where the little Pentax lens has to make do with 6 elements in 5 groups. It's still good for what it is.)
Forum: Pentax SLR Lens Discussion 05-18-2013, 10:11 AM  
Replace FA50 f1.7 with a DA*55mm f1.4
Posted By 52mm
Replies: 18
Views: 2,027
FWIW, I have several 50/1.7s and a few 1.4s (all MF), and the 1.7s are the ones I actually use. It all depends on what you want. 1.7s are better for across-the-frame sharpness when stopped down, and their lower weight and smaller size goes nicely with my K-x (1.4s are a fair bit more chunky). If, however, you are shooting around f/2..f/2.8 more often, 1.4s have a clear advantage. Most of my 1.7s also have more or less edgy or otherwise funky aperture shapes.
Forum: Pentax SLR Lens Discussion 05-18-2013, 09:59 AM  
Agfa MC 50mm f1.9
Posted By 52mm
Replies: 2
Views: 1,178
Interesting, I didn't know these f/1.9 Chinons were made with MC as well. Usually you only see (nominally) single-coated versions.

Anyway, that lens is equivalent to an M series lens and is used accordingly, as outlined above. Go to Custom Settings to enable aperture ring usage, then manually set aperture on the lens and use stop-down metering (green button) in M mode.
Forum: Pentax SLR Lens Discussion 05-13-2013, 12:26 PM  
SMC Pentax M 1:2 50MM. Whats it good for??
Posted By 52mm
Replies: 32
Views: 3,729
EDIT: Seems I was kinda slow again. Whatever.

For landscape shots, you should be stopping down to at least f/5.6, preferably f/8. In good light, the basic ISO 200 sensitivity setting (I shoot with highlight correction on, which is recommended) should yield perfectly exposure times of 1/250 s or lower, which is enough to preserve full sharpness even by today's pixel peeping standards. Focus is usually at or near infinity at this point, and assuming the camera isn't way off, focus confirm should easily be sufficiently accurate. (Testing AF accuracy via one of the usual focus test charts once is recommended.)

TL;DR: Landscape should be easy. A 50 makes a light tele on a crop DSLR though (75 mm equivalent), so it's not exactly a wideangle. That also means that you should be able to use a tele lens hood without vignetting. At the very least, the standard "standard lens" job should keep a lot out already.

Closeups are more tricky. At least this lens will focus down to 0.45 m, which is good for a basic 50 (it is not unusual to see a MFD of 0.6 m there), though its performance at this point may be another matter. Often things will be moving around in the wind, and DOF can be ridiculously small at short distances. The former means you want fairly low exposure times, the latter means you want to stop down somewhat - around f/5.6 tends to be a good compromise. You'll probably have to increase sensitivity in many cases in order to match both criteria. Catch-in focus may also be useful. Further reduction of MFD is possible with either a closeup lens (the proven Raynox DCR-150 can be used with a step-up filter ring), an extension tube or a reverse adapter. (The latter two can also be combined for ridiculous extreme macro. You'll probably run into shading problems first though.)
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