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Forum: Pentax K-01
09-18-2018, 12:40 PM
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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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Forum: Pentax K-01
09-18-2018, 08:29 AM
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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...
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Forum: Pentax K-01
09-18-2018, 07:56 AM
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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.
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Forum: Pentax K-01
09-18-2018, 07:26 AM
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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! :D
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Forum: Pentax K-01
09-18-2018, 04:54 AM
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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
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Forum: Pentax K-01
09-17-2018, 10:37 PM
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Thanks for this reference, Mike - yes indeed, probably a case of not knowing what you don’t know! It’s not an effect that I’ve consciously seen with the 20-40 so the concept of field curvature has never come up; or maybe it’s just that I don’t pixel peep enough away from the centre of the frame, or that I tend to shoot it at the wider end.
Regarding stopping down - here’s another quote on field curvature from photographylife.com; ‘Like 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.’
Surely it doesn’t matter whether or not any curvature effect is hidden by depth of field, as in practical terms (non-geeky, in Cicala’s own terms) it may turn out not to be visible. I’m still going to investigate my 20-40 further just to confirm that I don’t know what I don’t know. Er, or now that I know what I don’t know... aw rats, where’s the vodka... 😜
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Forum: Pentax K-01
09-17-2018, 02:50 PM
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Thanks for the explanation - I think I understood! I thought that field curvature (which the majority of lenses suffer from to varying degrees, due to curved elements focusing on a flat plane rather than a curved one) was most noticeable at wide apertures and became less of an issue when you stop down - as long as you don’t hit diffraction. I’ll do some tests tomorrow and report back. 😀
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Forum: Pentax K-01
09-17-2018, 01:38 PM
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Mike, what do you mean by ‘runway style’ field curvature? I tend to use mine typically between f7.1 and f11 especially for landscapes, which may be why I haven’t noticed it.
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Forum: Pentax K-01
09-17-2018, 12:38 AM
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@superdave: sounds like you haven’t had good luck with the 20-40. My experience is the same as twilhelm.
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