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03-19-2020, 05:35 AM - 1 Like   #1
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HD Pentax-FA 35mm F2 vs SMC Pentax-DA 35mm F2.4 AL on FF Pentax (K-1)

This thread is intended as a long term thread for collecting info about the differences between the apsc and ff version of the Pentax 35mm lens. There is of course also the SMC Pentax-FA 35mm F2 AL which can be added into the mix as well.

This thread is concerned with the performance on full frame cameras such as the Pentax K-1.

The differences between the lenses are a bit unclear. Are there noticable optical differences on FF? The obvious differences in build quality are not of interest for this thread. I'm starting this thread out with a comparison of the lens diagrams. I don't know if the pentax lens diagrams are accurate enough for this type of comparison but it's a starting point for further discussion.

From the simple scaling and overlaying of the two lens diagrams it seems the glass elements are larger on the 2/35. I will have a look at comparing photographs at a later point.

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Last edited by house; 03-19-2020 at 06:06 AM.
03-19-2020, 06:40 AM   #2
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this article discusses the DA 35mm F2.4 and the full frame sensor

QuoteQuote:
Full Frame Coverage of DA Lenses: Comprehensive Test
Which DA lenses cover the full 24x36 mm frame?
By PF Staff in Gear Guides on Jan 7, 2019
. . . Evaluation Criteria
We have evaluated the performance at select F-stop/focal length combinations per the following scale:
A: Full coverage
B: Full coverage, but with slightly soft corners
C: With reservations. Heavy vignetting and/or soft corners
D: With reservations. Very soft corners and vignetting
F: Fail, no coverage (black corners)
We used these F-stops: WO (wide open), F5.6, and F11. We also used three focus setting at each focal length and aperture: near, moderate, and far. The evaluation is thus condensed into a single rating that accounts for all focus settings. . . .
[ two stars ] DA 35mm F2.4 35
WO C
F5.6 B
F11 A
Read more at: Full Frame Coverage of DA Lenses: Comprehensive Test - Gear Guides | PentaxForums.com
03-19-2020, 07:02 AM   #3
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Scroll down on the following page, translated from swedish with google translate, to find independent mtf measurements for the Pentax SMC FA 35 mm f/2,0 AL (as well as several other Pentax ff lenses)

Objektivtest.se | Google Translate

The above linked MTF suggest the sharpness is decent across the field at f8.
03-19-2020, 07:39 AM - 1 Like   #4
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Comparing representations of lens designs is not enough to make a valid assessment. These design schematics are not detailed enough, plus you lack the information about the type of glass, stray rays management, a ton of other elements.

Pentaxforums already has an article covering the type of questions you ask, detailing for each Pentax lens (and a few third-party I believe) the results when using an APS-C lens on full frame. I believe this thread would be redundant.

03-19-2020, 07:47 AM   #5
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that article has already been linked to this thread

read the second post
03-19-2020, 07:59 AM   #6
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QuoteOriginally posted by bdery Quote
Comparing representations of lens designs is not enough to make a valid assessment. These design schematics are not detailed enough, plus you lack the information about the type of glass, stray rays management, a ton of other elements.
That's known and acknowledged in the first post.

QuoteOriginally posted by bdery Quote
Pentaxforums already has an article covering the type of questions you ask, detailing for each Pentax lens (and a few third-party I believe) the results when using an APS-C lens on full frame. I believe this thread would be redundant.
That page, as linked, is useful resource. The thing is that the da 35 (and to a lesser extent the da 50) are special cases that exist in apsc and ff versions. And it seems no one has looked at the difference in results between these two lenses on ff. The smc FA 35mm seems to perform well for it's age and size and was considered good enough to touch up for digital ff. Those factors make it an interesting lens both just out of curiosity but also personally as I'm thinking of buying the f2 version. If that happens a proper comparison will be made.
03-19-2020, 10:02 AM   #7
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One way to gain better insight would be to take the f2.4 and f2 lenses apart and measure each of the elements individually. Some kind of laser measurement jig might be useful so that there's no contact with measurement tools to possibly damage elements. At that point some real sense of differences in the DA vs FA designs could be possible.

How you non-destructively tell what materials were used to create each element, I don't know, although it's probably possible.

03-19-2020, 10:48 AM   #8
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This question has been discussed to death in the past. Unfortunately, the forum search feature is a bit too feeble to ferret this one out to the discussions being ancient history.

QuoteOriginally posted by house Quote
From the simple scaling and overlaying of the two lens diagrams it seems the glass elements are larger on the 2/35. I will have a look at comparing photographs at a later point.
This is obvious with both lenses in hand and has been noted in the past. They are similar in the same way that the DA 50/1.8 and various 50/1.7 lenses are similar. Quite simply, the HD DA 25/2.4 is not a less expensive version of the HD FA 35/2 on FF or on APS-C.

As for assessing the DA 50/1.8 on FF...I did so on 35mm film and it flunked. Similarly, multiple other users on this site have compared both the DA 50 and DA 35 "plastic fantastics" with same focal length equivalents with the general consensus being that that vignette and fall-off of image quality does not warrant use of either lens on FF except for novelty or if "cropping to quality" is the intent.

Since it appears from the comments above that you already own the DA 35/2.4 and are shooting FF, you could simply try it for yourself on that camera. If the results are subpar, the FA 35/2 or FA 31/1.8 Limited might be good options.


Steve

Last edited by stevebrot; 03-19-2020 at 11:32 AM. Reason: completeness
03-19-2020, 12:11 PM   #10
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I've owned both lenses, if that helps. My digital Pentax is ASP-C, but I've used the FA 35/2 on film, if that helps.

I'm not totally sure that I understand what your question is, but if my experience with the two helps, I'm happy to weigh in.
03-19-2020, 01:45 PM   #11
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QuoteOriginally posted by pres589 Quote
One way to gain better insight would be to take the f2.4 and f2 lenses apart and measure each of the elements individually. Some kind of laser measurement jig might be useful so that there's no contact with measurement tools to possibly damage elements. At that point some real sense of differences in the DA vs FA designs could be possible.
You'd be better off with a marble flat and vertical high-precision height gauge. But that would still be a nightmare to do, the kind of things that specialized labs charge thousands of dollars to do

QuoteOriginally posted by pres589 Quote
How you non-destructively tell what materials were used to create each element, I don't know, although it's probably possible.
By measuring the refractive index, you can perform a well-educated guess. But you'd need to know the geometrical properties of the lens element, from the previous point.

Feasible, but not by any amateur in his basement... I'm an optical designer by trade and apart from my time in university, I doubt I've had access to the required equipment to perform this kid of test.
03-20-2020, 10:02 AM   #12
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A very rough measurement of the front and back elements of the two lenses would be easy to do and might illustrate an obvious difference. You could perhaps even do it with photographs of both lenses adjusted to the same scale.
03-24-2020, 02:36 PM   #13
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I've done some initial test comparing the two now. Will upload some images when I find the time. Findings with my copies are as follows

- The da is MUCH sharper in the center wide open (keep in mind f2.4 vs f2 but still)
- The hd fa has much better corners sharpness at all apertures.
- The hd fa has significantly less vignetting wide open
- Center is about even at f4, hd get sharper by f5.6 to f8
- Da seems to have more ca
03-24-2020, 04:30 PM   #14
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I have found both the 35mm DA and 50mm DA plastic fantastics to be fine on the K-1, in FF mode.

I think people accustomed to shooting with APS-C don't understand that vignetting is common on 35mm format even in full frame lenses.

My guess is the DA*55 f.14 vignettes less than the so-called full frame Pentax FA 50 f1.4.

And I own the gigantic Sigma 35mm Art, speaking of the DA35/FA35 focal length. It was measured by Optical Limits as having 2.7 eV fall off in the corners.

That's not a misprint.

So, what would its rating be in the 'Full Frame Coverage of DA Lenses: Comprehensive Test' methodology?

Last edited by clackers; 03-24-2020 at 08:00 PM.
03-24-2020, 06:51 PM   #15
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QuoteOriginally posted by house Quote
I've done some initial test comparing the two now. Will upload some images when I find the time. Findings with my copies are as follows

- The da is MUCH sharper in the center wide open (keep in mind f2.4 vs f2 but still)
- The hd fa has much better corners sharpness at all apertures.
- The hd fa has significantly less vignetting wide open
- Center is about even at f4, hd get sharper by f5.6 to f8
- Da seems to have more ca
Check regarding bokeh wide open if you get the opportunity. And see how each does at smaller apertures, like f/11. Sometimes a lens is designed biased to hold better when stopped down as when doing some landscape shots having also a foreground subject.

---------- Post added 03-24-20 at 06:52 PM ----------

QuoteOriginally posted by mikesbike Quote
The hd fa has much better corners sharpness at all apertures.
- The hd fa has significantly less vignetting wide open
Looks like a better overall wide angle lens for FF. I presume if corners are much better, edges are somewhat better also.
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