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09-17-2018, 03:39 PM   #16
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The DA20-40 is the only lens I have returned because of unacceptable image quality. There are enough fans to convince me that mine was a bad copy, but I have not felt the need to try another.

@Jomar, I wouldn't give up on your K-01 or your 18-55 if you've been happy with the images. Better to get another lens to extend your horizons. A fast prime, an ultra wide, or a telephoto zoom perhaps....

09-17-2018, 10:37 PM   #17
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Thanks for this reference, Mike - yes indeed, probably a case of not knowing what you donít know! Itís not an effect that Iíve consciously seen with the 20-40 so the concept of field curvature has never come up; or maybe itís just that I donít pixel peep enough away from the centre of the frame, or that I tend to shoot it at the wider end.

Regarding stopping down - hereís another quote on field curvature from photographylife.com; ĎLike other optical aberrations such as chromatic aberration, the best way to reduce field curvature is to stop down the lens. If a lens has small to moderate amount of field curvature, stopping down a little will significantly reduce it. But if a lens suffers from heavy field curvature, then you might have to stop down to f/8 or f/11 to reduce it.í

Surely it doesnít matter whether or not any curvature effect is hidden by depth of field, as in practical terms (non-geeky, in Cicalaís own terms) it may turn out not to be visible. Iím still going to investigate my 20-40 further just to confirm that I donít know what I donít know. Er, or now that I know what I donít know... aw rats, whereís the vodka... 😜

Last edited by microlight; 09-17-2018 at 11:18 PM.
09-18-2018, 03:20 AM   #18
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QuoteOriginally posted by microlight Quote
Regarding stopping down - hereís another quote on field curvature from photographylife.com; ĎLike other optical aberrations such as chromatic aberration, the best way to reduce field curvature is to stop down the lens. If a lens has small to moderate amount of field curvature, stopping down a little will significantly reduce it. But if a lens suffers from heavy field curvature, then you might have to stop down to f/8 or f/11 to reduce it.í
I think that quote is potentially a little misleading... Instead, I'd say "the best way to reduce the effect of field curvature is to stop down". Stopping down doesn't reduce the field curvature - it simply increases the depth of field, so that more of the image at the focus distance is within acceptable focus (acceptable - not perfect).

QuoteOriginally posted by microlight Quote
Surely it doesnít matter whether or not any curvature effect is hidden by depth of field, as in practical terms (non-geeky, in Cicalaís own terms) it may turn out not to be visible. Iím still going to investigate my 20-40 further just to confirm that I donít know what I donít know. Er, or now that I know what I donít know... aw rats, whereís the vodka... 😜
It depends on the scene or subject you're photographing, and your personal tolerance for acceptable focus. Most lenses will have some field curvature unless they're specifically designed to have a flat field, but in many cases it's relatively minor and reasonable depth of field will keep most of the scene acceptably sharp at the focused distance. But when the field curvature is severe, even stopping down considerably may not be enough to bring all of the scene into acceptable focus. But, "acceptable focus" is - to some extent - subjective.

With the 20-40, I only notice this effect at the long end and especially when focused at long distances. It hasn't impacted my photographs at shorter distances and/or other focal lengths, but as @superdave suggested, it can be a pain with landscapes... One of those things that, once you've noticed it, you can't un-notice it
09-18-2018, 04:54 AM   #19
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I can only offer examples, so here are a couple that I took in New Zealand earlier this year. I had to actually look quite hard for photos taken at 40mm; it seems that my favourite range for landscapes with this lens is 20-30! I've uploaded these at full resolution.

If you look at them and think 'this bloke's blind- can't he see the clear field curvature' then be gentle with me! But for the life of me, I can't see anything obviously out of focus.

Lake Tekapo, 40mm f7.1 1/500 sec ISO 100


Curio Bay, 40mm, f11, 1/200 sec ISO 200




Last edited by microlight; 09-18-2018 at 05:02 AM.
09-18-2018, 05:15 AM   #20
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QuoteOriginally posted by microlight Quote
I can only offer examples, so here are a couple that I took in New Zealand earlier this year. I had to actually look quite hard for photos taken at 40mm; it seems that my favourite range for landscapes with this lens is 20-30! I've uploaded these at full resolution.

If you look at them and think 'this bloke's blind- can't he see the clear field curvature' then be gentle with me! But for the life of me, I can't see anything obviously out of focus.

Lake Tekapo, 40mm f7.1 1/500 sec ISO 100


Curio Bay, 40mm, f11, 1/200 sec ISO 200
I haven't looked at the second photo yet, but the first one is quite a good example. You're at 40mm, f/7.1, and I assume you're focused on a fairly distance point - either the tree-line, or possibly a little bit in front of that. Zoom into the image and look at the trees in the centre. They look accurately focused. Now, move to the left hand side of the image and notice how the trees tend to lose definition and look rather "painterly". Those trees are slightly further away than those in the centre of the frame, but not by much. Next, look at the right side of the image. The trees are also less defined, but not so badly; they are, I believe, slightly closer than those in the centre. Finally, still on the right hand side of the frame, look at the thin tree branches in the near foreground... These are perfectly sharp - yet at 40mm, f/7.1, focused into the distance, they should be quite blurred. That - in my opinion - is due to the "runway style" field curvature.

Despite this, it's a beautiful photograph, and - viewed at reasonable size and distance - not significantly affected by the field curvature. But the effect is there...
09-18-2018, 07:26 AM   #21
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Thanks for the feedback, Mike. If you want gorgeous scenery, NZ is the place to go. As Tekapo is a glacial lake, that's actually the colour of the water - stunning.


I have to say that if that's what the effect is, then it's no wonder I haven't seen it before. I rarely pixel-peep and look for the overall effect of the photo rather than poring over details - and I tend to use the wide end for landscapes anyway. Yes, the trees and shore on the right were closer than the left, which is the direction that the road in the distance goes away into. Actual point of focus? No real idea, although it may even have been the bushes in the lake itself, as they're well-defined. I tend to pick a feature to focus on, compose, focus and re-compose, so the centre or very distance is rarely the point of focus for me.


As for the foreground bush being in focus: I have no idea how far away it was, but I was standing on a bank above it. My trusty DoF calculator says that at 40mm and f7.1, everything from 13 feet six inches to infinity should be in focus, and if you laid me end to end twice (oooh matron!) and added a tiny bit, I'm estimating that's at least as far away that the bush was from me, so I'm not surprised that the twigs are well defined. If I did focus on the bushes in the lake rather than on the mountains (or even closer - and I can't remember, so just guessing here), then the point of closest focus per DoF could be even closer such that at f7.1, infinity on the lens might not be covered by the DoF so that the most distant mountain (the one on the very far left) would be out of focus, which it is, slightly. So many variables!

Maybe it was the pixie dust interfering with field uniformity! At the end of the day, all lenses of all makes (especially wider ones) have field curvature to some degree, even those into four or even five figures maybe. Some have other foibles such as being prone to flare or chromatic aberration, and many people here say that the 15mm Limited has poor edge sharpness. For me, if there is a small effect with the 20-40 then in the general scheme of things with this lens it really doesn't bother me, so I'm sure that I will indeed un-notice it!
09-18-2018, 07:43 AM   #22
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QuoteOriginally posted by microlight Quote
Thanks for the feedback, Mike. If you want gorgeous scenery, NZ is the place to go. As Tekapo is a glacial lake, that's actually the colour of the water - stunning.
It is beautiful indeed, Martyn. One for the bucket list, methinks

QuoteOriginally posted by microlight Quote
I have to say that if that's what the effect is, then it's no wonder I haven't seen it before. I rarely pixel-peep and look for the overall effect of the photo rather than poring over details
I'm generally the same (one of the reasons I enjoy vintage lenses so much). However, I noticed the effect sufficiently on a few of my photos when I first bought my 20-40 that it surprised me - especially when compared to my Sigma 17-50 f/2.8. I tend to do my noise processing and raw sharpening at 100% reproduction, and I imagine that's where I noticed it most of all, but I could see the effect at 50% and even smaller on a couple of images, as I recall. It was enough to put me off the lens for a while.

QuoteOriginally posted by microlight Quote
For me, if there is a small effect with the 20-40 then in the general scheme of things with this lens it really doesn't bother me, so I'm sure that I will indeed un-notice it!
And that's what counts. If you're happy with the images you're getting, then the lens is working for you. As I said, I'm starting to like my own copy more... and gradually weaning myself away from looking for the field curvature, as much of the time it really doesn't matter
09-18-2018, 07:56 AM   #23
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Looks like we're in violent agreement! Did you look at the second photo? Not for any lens effects but for the scenery - this is more or less as far south as you can go on the south island and is just stunning.

Like you I always do my noise processing and sharpening at 100 or even 200% (in ACR) but I always head straight for sky, or areas of uniform colour and then out again, so that's probably another reason why I've never noticed anything. With how the lens renders, I'm too focussed (sorry) on the overall effect which is often, yes, painterly. It's the pixie dust, I'm sure.

09-18-2018, 07:59 AM   #24
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QuoteOriginally posted by microlight Quote
My trusty DoF calculator says that at 40mm and f7.1, everything from 13 feet six inches to infinity should be in focus
Just on this... as a point of interest, and by no means challenge... it depends, of course, on the distance you were focused at (amongst other factors). If you were focused on a point roughly 37 feet from your camera, then everything between approximately 18ft and infinity should be more-or-less acceptable (assuming a flat field), depending on your personal tolerance. But I suspect you were focused considerably further away than 40 feet. At 200ft focus distance, you'd be looking at approximately 31ft to infinity.

But this is immaterial, so long as you get the results you want - and clearly that's the case
09-18-2018, 08:03 AM   #25
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QuoteOriginally posted by microlight Quote
Looks like we're in violent agreement! Did you look at the second photo? Not for any lens effects but for the scenery - this is more or less as far south as you can go on the south island and is just stunning.
I did. It's a gorgeous scene, and very well captured. My kind of scenery!
09-18-2018, 08:29 AM - 2 Likes   #26
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Hahahahaha! I read '31ft to infinity' as '31ft to insanity'! Many a true word spoken in the Evening Argus.

Yes, I have no idea where I actually focussed. One of the vagaries of on-the-hoof shooting. One more before I stop hijacking this thread. With the PLM this time, twilight near Dunedin...
09-18-2018, 11:13 AM   #27
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QuoteOriginally posted by microlight Quote
One more before I stop hijacking this thread. With the PLM this time, twilight near Dunedin...
That's a stunner. I love the composition... Actually, I love everything about that shot. Nice work
09-18-2018, 12:40 PM   #28
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There's a story. We went to Cape Saunders on the Otago peninsula as part of a group to watch yellow-eyed penguins coming in from fishing. The previous photo was the second one we saw (ISO 350, TAv); the first one was trying to take his accustomed track back to his burrow, except that there were three big sea lions in the way! So the penguin stopped about a third of the way up the beach, and then took a big detour beneath our hide and snuck past the sea lions - this picture is of that; look at the penguin's expression! This was later in the evening and TAv mode selected ISO 5000!

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