field curvature: Pentax SMC K 35mm f3.5

StirlingBartholomew

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In 2015 a friend dumped all his Pentax gear on me to lighten his load moving back to Alabama. I already had some Pentax m42 gear which I used on my mu4/3 cameras. I have never been a fan of wide angle lenses. In the '70s my Nikkor 28mm f3.5 was rarely used but I did a series of rock formation shots with Tri-x at several locations inspired by Minor White and others. I never had any problems shooting with this Nikkor. I made some fine art prints of these and still have them on display. Mostly the Nikkor 28mm f3.5 added weight to my 35mm suite-case.

My digital experience with wide angle zooms hasn't been much different. I use them when I am shooting narrow spaces like Post Alley in Seattle or indoor architecture shots. I use medium to long lenses for everything where I have enough room.

During field testing, I measured the Pentax K 35mm F 3.5 performance against my Zuiko14-54mm f2.8-3.5 at 35mm and the Zuiko won the contest for shooting flat surfaces and landscapes. I complained on the Pentax forum about the lens and found out that it was an optical formula which was used with success for years and years the K version was the end of the line after decades. My copy of the lens was in pristine condition and I couldn't figure out why it didn't perform as well as the Zuiko 14-54mm. Yesterday I decided to revisit the issue. Did some in studio test shots of static subjects under controlled conditions. The results were passable but the depth of field was ambiguous and somewhat confusing.

Mulling this over I recalled in a conversation with a colleague 50 years ago (an industrial photographer) hearing him mention field curvature. Did some research online and found out that the Pentax K 35mm F 3.5 was performing like a wide angle lens in terms flat field focal plane. The Pentax isn't a bad lens. It is useful for subjects where flat field performance isn't critical. It is a special purpose lens. I shot some acceptable interior work with it yesterday for which I would normally use the Zuiko. Shooting 4/3 35mm is a bit long for indoor work. But the results were pleasing to look at and the field curvature issue wasn't apparent as it would be in flat field subjects. The Zuiko will still be used when I need a reliable result. It isn't flawless but it has served well over a decade and I know what it delivers.
 
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fortwodriver

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This reminds me of the Canon 20mm USM lens from the late 80s to early 90s. It was deliberately designed with a W shaped field curvature to allow the centre and the sides to be relatively sharp for close-up environmental portraits and near-far compositions without tiny apertures.

For years people who didn't know complained it was a terrible lens for brick-wall shots because it had that deliberate W-shaped sharpness zone across the field of view.

Could this be what you're seeing with that particular Pentax lens?
 

StirlingBartholomew

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This reminds me of the Canon 20mm USM lens from the late 80s to early 90s. It was deliberately designed with a W shaped field curvature to allow the centre and the sides to be relatively sharp for close-up environmental portraits and near-far compositions without tiny apertures.

For years people who didn't know complained it was a terrible lens for brick-wall shots because it had that deliberate W-shaped sharpness zone across the field of view.

Could this be what you're seeing with that particular Pentax lens?

Could be what I am seeing. I don't pretend to understand what was going on in the minds of Pentax optical engineers half a century ago. It might also be a design for landscape. You want everything on horizontal axis with your subject to get priority. I had my subject sitting on a wooden surface and I could map the shape of the in focus area by looking at the grain. It was the shape of an oval with sharpest part in the center. What was most notable was difference when testing 50-55mm primes form 1965-late 70s. With the Pentax M 50mm F1.7 after f 5.6 you could just forget about the problem. The oldest lens came with the Pentax S1A the 55mm f 1.8 (radioactive). This lens is soft wide open with dreamy bokeh. From f 2.8 on it is good. F5.6-F11 is not quite as good as the M 50mm F1.7. I used both for flat surfaces with a little fall of at corners but nothing to worry about. The 50mm f1.7 was in better mechanical condition so I used it more often. Saved the old one for soft focus shooting wide open.

I now have the 4/3 Zuiko 50mm 2.0 macro which I picked up in 2018. The vintage Pentax 50's have fallen into disuse.
 
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milaber22

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I can not help with technical details but this lens was ,far as i am concerned, mainly used for landscape.
That was what i have read while searching for it in order to buy it.
 

fortwodriver

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I can not help with technical details but this lens was ,far as i am concerned, mainly used for landscape.
That was what i have read while searching for it in order to buy it.

Funny, in the literature I have for that lens, it's listed as a "General Purpose" lens. There's no advertising commitment to landscape photography, although mine is actually the older m42 mount version. It's such a little jewel.

That said, a landscape lens doesn't need to have a flat field if you're taking photos of three-dimensional space. You want some curvature in its coverage to get more of the scene in focus.

Really the only lenses that should pass a 2-D sharpness are flat-field macro lenses and reproduction lenses (which are designed for copying a flat source.)

Roger Cicala seems to agree...
 

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