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09-06-2022, 01:15 PM   #1
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Curved Field Lenses for 645

The Pentax 645 33-55mm lens is a curved field lens. But are there others, like the 35mm or the 28-45mm.

Thanks!

09-07-2022, 09:48 AM   #2
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The FA 35 has field curvature that needs to be accounted for; probably the 45 as well. Whether or not it affects your photography is a different question, and only the user can answer that one. I know for some lenses---typically longer FL's or ones I'd use wide open, I don't care so much. I know that I've had all 3 35's; liked the A, traded "up" for the FA; sold the FA to get the DFA. Got rid of my 45 as well---although I sooooo wish Pentax would do a refresh of this lens. For what I used these lenses for, field curvature was something I didn't want to deal with (in the 45 it may just have been general corner/edge softness...). As always, YMMV.
09-09-2022, 02:35 AM   #3
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QuoteOriginally posted by texandrews Quote
The FA 35 has field curvature that needs to be accounted for; probably the 45 as well. Whether or not it affects your photography is a different question, and only the user can answer that one. I know for some lenses---typically longer FL's or ones I'd use wide open, I don't care so much. I know that I've had all 3 35's; liked the A, traded "up" for the FA; sold the FA to get the DFA. Got rid of my 45 as well---although I sooooo wish Pentax would do a refresh of this lens. For what I used these lenses for, field curvature was something I didn't want to deal with (in the 45 it may just have been general corner/edge softness...). As always, YMMV.
I totally agree with your assessment of the FA 45/2.8. A very nice lens in many respects, very sharp in the centre and free from fringing etc. BUT, the corners never really get critically sharp, regardless of aperture used. So not so good for (some) landscapes. I too am selling mine, and will stick to the superior FA 45-85mm zoom for that focal length.

As to the 35's, I have the A version and find it essentially flawless, which gives me no reason to "upgrade" to either of the FA versions. I have seen controlled comparative shots of the D-FA vs A 35's and saw no advantage to either, they are on a par. No experience with the FA version.

Marco

---------- Post added 09-09-22 at 02:37 AM ----------

QuoteOriginally posted by RICHARD L. Quote
Well, for landscape photography, the FA 35 mm f/3.5 produces magnificent images with a 645Z digital camera. I was apprehensive at first and I wanted the newer DFA 35 mm f/3.5 but I don't use this focal length much and used prices proved out of my financial capacity (1700 $ VS 500 $). I'm entirely satisfied with the results obtained up to now with the FA 35 mm. I work on a tripod, outside in good light and I vary my aperture settings between f/6.3 and f/32.
Honestly, you sound a bit unnecessarily defensive. Also, those shots you posted are nice, but useless in the context of this technical discussion, since at those sizes it is completely impossible to make any judgement one way or the other. Finally, as I'm sure you know, at f/32 you are well into significant diffraction territory, which does reduce ultimate sharpness no matter what lens is used.
09-09-2022, 07:55 AM   #4
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QuoteOriginally posted by marco_gea Quote
As to the 35's, I have the A version and find it essentially flawless, which gives me no reason to "upgrade" to either of the FA versions. I have seen controlled comparative shots of the D-FA vs A 35's and saw no advantage to either, they are on a par. No experience with the FA version.
And it's really too bad about the 45---dead useful FL, and the prime is so much lighter. Sigh. As far as the 35's are concerned, the only reason I traded in my A version was because I was doing a lot of shooting in the dark, on a ladder or on the ground dealing with a very tall tripod, and also in the cold with glasses fogging, & etc., and just couldn't deal with the manual focus focus confirmation with that FL, so found myself actually needing the AF. Otherwise I would have just kept the A.

09-09-2022, 08:07 PM - 1 Like   #5
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QuoteOriginally posted by RICHARD L. Quote
"Curvature of Field" never impacted negatively any of my landscape pictures taken with the FA 33-55 mm f/4.5 on a 645Z.








I'm not sure we're talking about the same thing. A curved-field lens is not some defect. Most lenses are flat-field, where if you pinned a map to the wall and photographed it, it would all be in focus even though each point on that map is a different distance from the lens. If focused on the center, a curve-field would only have one of the center in focus. The surface of focus is not a plane but curved, roughly circular. Curved field lenses work really well for landscape photography and allows you to use a smaller f-stop to take the same picture.
09-09-2022, 09:37 PM - 1 Like   #6
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QuoteOriginally posted by chicagonature Quote
A curved-field lens is not some defect.
When the desired mode of output from a camera is for view on 2D screens on in flat prints, field curvature (Petzval curvature) is in fact a pretty significant optical design issue.


QuoteOriginally posted by chicagonature Quote
Most lenses are flat-field, where if you pinned a map to the wall and photographed it, it would all be in focus even though each point on that map is a different distance from the lens.
Wrong. A huge majority of lenses even those labelled "planar" have focus fields that are approximately flat within a certain tolerance as defined by the manufacturer. Every lens with spherical elements has field curvature, it is simply a matter of whether it and other related optical aberrations are significant enough to impact Image quality, again this is at the discretion of the manufacturer.

QuoteOriginally posted by chicagonature Quote
The surface of focus is not a plane but curved, roughly circular. Curved field lenses work really well for landscape photography and allows you to use a smaller f-stop to take the same picture.
Yes field curvature can be exploited by distributing DOF in such a way to capture more of a scene with depth in focus, though you still have to stop down more to suppress astigmatism* which leads to increasing diffraction. For architectural photographers field curvature is a headache where corner to corner sharpness is not only required, it is demanded.

* Astigmatism is a 3rd order aberration that has two components: sagittal and tangential which can be either positive or negative in accordance with the Petzval curvature equation. Astigmatism occurs when the Tangential and Sagittal focus planes diverge and the magnitude of their divergence. If this trait is unconstrained, it invariably results in issues with corner softness. It is possible to construct lenses that have neither field curvature nor astigmatism, but it is difficult and complex endeavor frequently involving exotic glass types and aspheric elements.

Last edited by Digitalis; 09-09-2022 at 09:47 PM.
09-23-2022, 04:35 PM   #7
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The depth of field in most lenses is curved and only specially made lenses have a flat depth of field ( projection lenses). Curvature of field is considered an aberration at the focal plane and has to be corrected out in the design phase to eliminate it. In most cases, only a slight amount remains. It is easily masked by stopping down slightly. Some macro lenses have a flat depth of field but not many. Projection lenses have both a flat DOF and a flat focal plane.


Last edited by desertscape; 09-25-2022 at 04:26 PM.
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