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12-16-2014, 12:57 AM - 1 Like   #16
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QuoteOriginally posted by gofour3 Quote
+1 as well as the floating lens element made the FA31 a costly lens to produce.
The FA31mm f/1.8 Limited does not have any floating lens elements in its design* - the FA31 is a completely original design that doesn't borrow from any previous Pentax lens. The Pentax FA77 f/1.8 Limited however makes use of FREE - Fixed Rear Element Extension which is essentially identical to a floating element.

*Pentax did use a floating element in the K28mm f/2 - which, by the way is optically unrelated to the FA31.




Last edited by Digitalis; 12-16-2014 at 01:07 AM.
12-16-2014, 03:49 AM   #17
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I think the FA 31 is just more expensive to make (as others have said). Wider angles require larger/more complex glass elements than short telephoto like the FA 77.

Canon and Nikon sell their full frame 35mm f1.4 lenses for 1500 dollars, although Sigma (as a 3rd party maker) prices theirs at 900 dollars.
12-16-2014, 09:34 AM   #18
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QuoteOriginally posted by Digitalis Quote
The FA31mm f/1.8 Limited does not have any floating lens elements in its design*
Check out the Ricoh/Pentax web site for this lens and Bojidar Dimitrov's site:

smc PENTAX-FA31mmF1.8AL Limited / Wide-Angle Lenses / K-mount Lenses / Lenses / Products | RICOH IMAGING

FA 31/1.8 AL ED Limited

Phil.
12-16-2014, 10:02 AM   #19
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I have read that, and both of those blurbs are also inclusive of the other other FA limited lenses and points out the features without distinction. Does the FA43 Limited also have an aspherical lens and a floating element in its design? - No, it doesn't*.

From my first hand experience, the FA31 isn't particularly well corrected at close focusing distances (the FA77 is). I'm prepared to admit that there may be some elements that may be moving more than others as the FA31 focuses - it is hard do discern from appearances without complete disassembly of the lens with a close study of the internals.



This image from Bojidar Dimitrov's site adds to the confusion:


Amongst all the other features being highlighted, Where are the floating groups?

*there was a rumor that the early FA43s had an aspherical element - but that has been incontrovertibly proven to be a hoax.


Last edited by Digitalis; 12-16-2014 at 10:34 AM.
02-02-2015, 03:39 PM   #20
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QuoteOriginally posted by Digitalis Quote
I'm not sure of the extra 0.4 of a stop difference between them makes all that much of a difference.


Pentax K5IIs - Pentax SMCP-FA77mm f/1.8 Limited @ f/1.8
@Digitalis, was this taken at Glenrowan or Jerilderie?
02-02-2015, 05:28 PM   #21
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QuoteOriginally posted by hjoseph7 Quote
I was wondering, why does the Pentax 31mm f1.8 cost more than the 77mm f1.8 since according to reviews the 77mm is the better lens ?
They are supposed to use in different situations due to a huge difference in focal length. So an argument for which one is better is typically invalid and depends on personally preference. Pentax 31mm is a complete design comparing to 43 and 77. It is heard that originally 31 can only be made for f2.4 in the similar design of 77. So part of 31 price can be attributed by the change of design from f2.4 to f1.8....
02-02-2015, 06:10 PM   #22
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QuoteOriginally posted by starjedi Quote
It is heard that originally 31 can only be made for f2.4 in the similar design of 77
Citations needed. I'd like to know the origins of this utter rubbish.


Last edited by Digitalis; 02-03-2015 at 04:46 AM.
02-03-2015, 08:43 PM   #23
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I have both lenses and I can't say I prefer one or the other, but the 31mm gives me more and I mean way more shooting opportunities.
Here is one taken with the 31mm. I would have had to back up to Georgia to get this shot with the 77mm.
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04-21-2017, 07:45 AM   #24
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Parts diagram for 31mm shows cam focusing for floating element design: http://www.pentax-manuals.com/manuals/service/fa31.pdf

Current Ricoh promotional material says it's floating element: "and the floating lens system allows proper imaging performance at all shooting distances for providing exceptional image quality for every situation."
04-21-2017, 08:12 AM   #25
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QuoteOriginally posted by John Shriver Quote
Parts diagram for 31mm shows cam focusing for floating element design
I've seen this diagram before, those cams show the elements are moving in parallel. Do you know how floating elements are supposed to work?
04-21-2017, 10:35 AM   #26
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Not much between them

I prefer the FL of 31

There's perhaps a marginal extra sprinkling of magic in the 77 shots (in terms of rendering) IMO - this is a subjective thing though... let your eyes be the judge

If you need these approximate focal lengths... there can be no compromise - you need these *

---------

EDIT: Then rampant LBA should perhaps continue with the DA* 300 another insanely good lens..
and if crop lens size doesn't bother you add a 50-135 in there
and also the....
04-23-2017, 05:24 AM   #27
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There would not be two cams to move the two groups at the same rate -- they would be one group. But I would not expect the difference to be very dramatic -- this isn't a zoom ring.

It's easy to tell experimentally. Put the 31/1.8 face down on a table. Hold a small stick vertically against the ring around the rear element. Focus the lens. Note that the stick moves up and down. The rear group is not moving at the same rate as the front group.
04-23-2017, 06:47 AM   #28
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Pentax doesn't have much in the way of nomenclature for designating lenses with floating elements. The best current example among the FA limited lenses is the FA77: which as according to pentax, uses FREE* which does manage to compensate for loss of contrast at close focusing distances. Pentax hasn't given the FA31 any particular mark for having floating elements. They mention it in later marketing material. I'm willing to accept the rear cell might move at a different rate however from what I can see the overall degree of displacement between the rear and front optical cells is identical - the differential in movement is so small it doesn't appear to effectively compensating for the measurable loss of contrast an minimum focus distance**.

To settle this question without a shadow of a doubt, disassembly of the lens would be required.

You may recall there was a silly rumor when the FA43 was released, that the early versions of the lens had an aspherical element in its design***. I suspect this is another piece of disinformation that started in a similar vein.

Anecdotally: I did a comparative review of the FA31 against the Sigma 18-35mm f/1.8 ART**** and the FA31 performed worse than the more complex sigma lens at close focusing tests. Resolution was practically identical however, the level of contrast between the two lenses was drastically different. Only when the FA31 was stopped down it delivered performance equal to if not superior to the sigma ART zoom lens.



*Fixed Rear Element Extension

**This is evident with multiple copies tested on my optics bench. The FA31 needs to be stopped down to f/4 to compensate for lost contrast.

*** This has been thoroughly debunked, as it would require a re-design of the lens itself to accommodate aspherical corrective optics.

**** After the Sigma ART lens lens was expertly re- aligned.

Last edited by Digitalis; 04-23-2017 at 07:02 AM.
04-23-2017, 07:20 AM   #29
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QuoteOriginally posted by hjoseph7 Quote
I was wondering, why does the Pentax 31mm f1.8 cost more than the 77mm f1.8 since according to reviews the 77mm is the better lens ?
Why does a bus cost more than a dodge viper although the viper is faster.
Why is the 1.8/50 even less expensive?
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