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Author Topic: progessive stretch  (Read 13686 times)
Diapositivo
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« Reply #60 on: August 01, 2010, 10:47:23 AM »
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Quote from: JonathanBenoit
Are you saying that a building doesn't have straight sides? When I look at a building I see the sides as parallel to each other. Even if my eyes showed convergence, in reality we know that the sides are parallel. It's the job of a photographer to capture reality - or at least it's the architectural photographer's job.

Jonathan, what is reality? Are rails of a railway "parallel" or "converging"? They are parallel but, for a well-known law of nature, they appear to be converging. In reality they are (visually) converging and not parallel. Axonometry does not exist in nature, all that lives sees the world according to the laws of perspective.

Railways are not different from corridors, and corridors are not different from skyscrapers seen from below. If we had the habit of walking upward or if we could fly, it would seem to us as meaningless to try to make building sides "parallel" as it would seem absurd to try to make corridors walls "parallel". If swallows took pictures of buildings, they would never use PC lenses...
 
Do this experiment: put yourself in front of a convex corner of inside your house, at a distance of let's say 1,5 metres. Now tilt your head up and down with movements of around half a second each (in one second you will have gone from floor to ceiling to floor again). You will clearly see that your eyes record the vertical line between floor and ceiling as being curve (it will come "toward" you at its nearest point to your eyes).

Now, guess what: that line IS curve to your eyes and cannot be otherwise. You know, and I know, that from an "engineeristic" point of view that is a straight line. But from a visual point of view, that cannot be a straight line because where the line intersects the floor, and the ceiling, are points which are further than the nearest distance between you and the wall.

Now repeat it with any other element. Put yourself under a beam of your house. Look upward, and swing your head along the beam. See how curved the beam is? To your eyes, it is curved. Physically it is not. Visually it certainly is.

Now you might say: OK when I am near the corner, and look fast upward and backward, the effect is quite apparent but in my normal experience this effect is so small and my brain automatically makes me not to notice it.

Well, yes and no. I can be very easily aware that the sides of the inside of any builiding are NOT parallel when I look at anything. I can go round my house and be perfectly aware that my eyes look at it as if I were in, so to speak, a spheric environment. We all look at the world like a toad does, only with a less extreme perspective. It is a difference of degree, not of nature.

IMO our problem is how to make a certain picture look "natural". If you take pictures of a tall building, looking up, parallel lines will not necessarily look "natural". If you repeat your experiment with outside buildings and if they are tall enough that the perspective is evident, you will clearly see that the buildings don't appear to have parallel lines at all if you go enough under them and look at them upward.

Up to a certain extent our brain "stretches" buildings sides, but this "mental" effect is always only partial and up to a certain extent our mind is at the same time always aware of the perspective surrounding us.

Cheers
Fabrizio
« Last Edit: August 01, 2010, 10:48:34 AM by Diapositivo » Logged
JonathanBenoit
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« Reply #61 on: August 01, 2010, 11:49:19 AM »
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Fabrizio,

I understand your point. You must already know that the perspective shown with railroad tracks and a corridor are very different than the convergence of vertical lines. You will not be able to convince an architect or an architectural photographer that those vertical lines should be left alone. It's our job to capture the building as it is designed. If you think a photograph looks better uncorrected than that's fine. I guess it's all a matter of opinion.
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David Klepacki
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« Reply #62 on: August 01, 2010, 03:28:25 PM »
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Quote from: Ray
There is nothing in front of the fountain to cause confusion about its size and everything that is behind the fountain looks, to my eyes, appropriately diminished in size (approximately), indicating its greater distance from the viewer.
The perspective in your image is not "true perspective", as the relative scales of the near-far objects are not natural in the image (again, where natural refers to what the human eye perceives).  The chairs, tables and even doors of the central buildings are much too small to account for the ground distance, while the opposite is true for the fountain in the foreground.  Shooting the same scene with a larger format camera and an appropriately longer focal length lens would allow you to see this better.  Alternatively, you can try to simulate such perspective by digitally combining smaller format images.  For example, if you use a normal focal length lens and stitch images together to form the same field of view (and most likely do some focus blending as well), you would then be able to restore a more natural balance of scale.
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Ray
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« Reply #63 on: August 01, 2010, 07:07:20 PM »
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Quote from: David Klepacki
The perspective in your image is not "true perspective", as the relative scales of the near-far objects are not natural in the image (again, where natural refers to what the human eye perceives).  The chairs, tables and even doors of the central buildings are much too small to account for the ground distance, while the opposite is true for the fountain in the foreground.  Shooting the same scene with a larger format camera and an appropriately longer focal length lens would allow you to see this better.  Alternatively, you can try to simulate such perspective by digitally combining smaller format images.  For example, if you use a normal focal length lens and stitch images together to form the same field of view (and most likely do some focus blending as well), you would then be able to restore a more natural balance of scale.

I would certainly agree that my shot is not a true perspective as seen by the eye from the position I took the shot. It cannot be. I used a 14mm lens from a distance of about 1 to 2 metres from the statue (can't remember the precise distance, but I was very close). From that distance, the surrounding houses to the left and right would have been an unidentifiable blur. If I were to change the direction of my gaze, to the left or the right, so that I were able to see the houses clearly, the statues in front of me would become an unidentifiable blur. Hence my reference in an earlier post to the focal length of the human eye, which seems to be another complicated issue. I get the impression that the human eye is a bit like looking through a 50mm lens on a large format camera, which has atrociously bad edge resolution. So bad that the resolution begins to fall off just a short distance from the centre.

I don't agree with those who claim that the image looks more natural when uncorrected for perspective. It's true that parallel lines converge in the distance, and large objects appear smaller in the distance, and that's information the brain uses to guauge distance. However, buidings to the side of one do not appear as though they are about to topple because they are leaning so steeply. If I were actually able to see that brick building on the far right of the uncorrected photo, leaning to the degree it is, as I stood in front of the fountain, I would get very alarmed.

The problem as I see it, is how to correct the perspective so that the scene looks as though the shot were taken from a different perspective which more closely corresponds to the eye's rather narrow FoV. This other perspective must be from a greater distance. In my perspective-corrected shot of the St Martin's church, it's unlikely that the photographer was positioned 6 metres off the ground across the street, because that would mean that the fountain with statues would be unusually tall (not an impossibility, but unlikely). If the statues weren't unusually tall, then from that quite close position across the street, I would get a view of the top of the statues' heads, which I don't.

Therefore, in my opinion, as I try to make perspective corrections in this photo, I'm trying to create the impression that the shot was taken from a much greater distance than it actually was, using a much longer focal length of lens, perhaps a 200mm lens from a distance of 100 metres from the fountain, whatever.

So the question I would ask is, have I succeeded in creating this false impression, that I took the shot from a distance of 50-100 metres away using a fairly long focal length of lens? (on the basis of course that one doesn't have reference to the uncorrected image which gives the game away).




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elf
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« Reply #64 on: August 01, 2010, 08:46:10 PM »
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Quote from: David Klepacki
The perspective in your image is not "true perspective", as the relative scales of the near-far objects are not natural in the image (again, where natural refers to what the human eye perceives).  The chairs, tables and even doors of the central buildings are much too small to account for the ground distance, while the opposite is true for the fountain in the foreground.  Shooting the same scene with a larger format camera and an appropriately longer focal length lens would allow you to see this better.  Alternatively, you can try to simulate such perspective by digitally combining smaller format images.  For example, if you use a normal focal length lens and stitch images together to form the same field of view (and most likely do some focus blending as well), you would then be able to restore a more natural balance of scale.

Sorry, it just doesn't work this way.  You would need to change the postion of the camera to change the perspective.  Changing focal length or sensor/film size doesn't change perspective. (See multiple threads discussing this on this and nearly every other photographic forum)
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Ray
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« Reply #65 on: August 01, 2010, 11:55:15 PM »
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Since I'm in a creative mood on a high intellectual level (after a couple of glasses of wine   ) I'd like to elaborate on this perspective issue. I once made a remark on this site a couple of years ago, to the effect that we all know that wide-angle lenses can create an impression of a different perspective. All hell broke loose and 100 posters tried to convince me that I was dead wrong and that focal length had nothing to do with perspective, and that perspective was affected only by the distance to the subject.

Their empirical evidence for such a declaration was based upon the fact (with which I agree) if one crops a shot taken with a wide-angle lens to the same FoV as any shot taken from the same position and direction with any other focal length of lens, then the size relationships between obects in the foreground and objects in the background will be the same.

Of course, the claimants of this theory (Jack Flesher and Gabor spring to mind) seemed oblivious the fact that the choice of FL of lens determines in practice the 'impression' of the distance from the viewer (or photographer) to the subject.

For example, if I take a shot of an isolated figure on a hill top, who is immediately in front of a huge mountain in the background, it makes no difference to the relative sizes of the figure and the mountain, whether I use a normal lens or a telephoto lens. That's true and cannot be denied (discounting lens distortions, of course).

But what some people fail to realise, is that the choice of focal length may affect the viewer's impression of the distance to the nearest subject in the composition. The choice of focal length doesn't affect the spatial relationships between the nearest point and the furtherst point in any particular crop, but it does affect the sense of distance to the nearest point (in the composition) to the viewer.

Okay! Forgive my rant. You claim the chairs against the building on the left appear too small and that, therefore, the statues are unnaturally large in comparison.

In my defense, I'm posting crops of the statues and chairs from both the unadjusted and adjusted images previously posted. Now I agree that the first impression is that my perspective correction has made those two women appear too squat. That's a concern which I may correct, but I'll say that those two women caught my attention because they were such squat little dumplings, one of them obviously bemoaning the fact that they had yet another church after that grotesque monstrosity, the Gothic Cologne Cathedral (apologies to all Germans - just my esthetic, personal opinion).

Allowing for differences in stretching height, those chairs are the same size in both images, and would be if this shot were taken with a 600mm lens on a huge format camera or if several images were stitched from a smaller format camera, discounting effects of lens distortion.


[attachment=23439:crops_of...ted_0871.jpg]
« Last Edit: August 02, 2010, 04:04:01 AM by Ray » Logged
David Klepacki
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« Reply #66 on: August 02, 2010, 08:55:34 AM »
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Quote from: elf
Sorry, it just doesn't work this way.  You would need to change the postion of the camera to change the perspective.  Changing focal length or sensor/film size doesn't change perspective. (See multiple threads discussing this on this and nearly every other photographic forum)
Yes, you can change the position of the camera, but large format cameras also allow movements that change the relationship of the lens to the film plane in order to allow one to change perspective.
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JeffKohn
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« Reply #67 on: August 02, 2010, 09:01:48 AM »
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But what some people fail to realise, is that the choice of focal length may affect the viewer's impression of the distance to the nearest subject in the composition. The choice of focal length doesn't affect the spatial relationships between the nearest point and the furtherst point in any particular crop, but it does affect the sense of distance to the nearest point (in the composition) to the viewer.
This statement doesn't really make any sense. I'm not sure what you're trying to say when you talk about "impression of distance", but I'm guessing what you mean is that you can get closer to the foreground subject while still including more of the distant ground. But this isn't a function of focal length, it's a function of field of view. That's a significant distinction to make, although it's sometimes lost on people who don't have experience shooting multiple formats. Shooting a scene with 14mm FF DSLR and again with equilvalent FOV on 8x10 LF will give you the same results, focal length has got absolutely nothing to do with perspective or "impression" of perspective. The 14mm shot will probably have barrel distortion, but that has nothing to do with perspective and can be easily corrected. So elf's statement is correct.
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JeffKohn
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« Reply #68 on: August 02, 2010, 09:04:03 AM »
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Quote from: David Klepacki
Yes, you can change the position of the camera, but large format cameras also allow movements that change the relationship of the lens to the film plane in order to allow one to change perspective.
But this has nothing to do with the size relationship of foreground/background. Your previous statement implied that focal length played a role, and it doesn't.

And as far as managing perspective goes, you can do the same thing with a tilt/shift lens or with pano stitching software.
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David Klepacki
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« Reply #69 on: August 02, 2010, 09:11:55 AM »
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Quote from: Ray
Allowing for differences in stretching height, those chairs are the same size in both images,
Ray, the stretching height differences cannot be ignored.  I still see a difference of scale in the chairs and tables between your two crop images.  Also, the door and windows on the ground floor are at completely different scales in each image as well (what you refer to as stretching height), whereas the scale of the statue/fountain does not appear to change scale as much.  It is the perception of these scale differences that I see as being unnatural, but I guess it could be claimed to be subjective.
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David Klepacki
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« Reply #70 on: August 02, 2010, 09:26:38 AM »
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Quote from: JeffKohn
But this has nothing to do with the size relationship of foreground/background. Your previous statement implied that focal length played a role, and it doesn't.

And as far as managing perspective goes, you can do the same thing with a tilt/shift lens or with pano stitching software.
I only indicated that a longer focal length would need to be used for any larger format in order to achieve the same angle of view.

Even if using a tilt/shift lens, you would need a longer focal length in order to end up with the same angle of view in the final stitched image.   Tilt/shift lenses can achieve some perspective control, but typically the movements are confined to a single axis (e.g., typically they cannot tilt and swing at the same time).
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Ray
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« Reply #71 on: August 02, 2010, 12:00:50 PM »
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Quote from: JeffKohn
This statement doesn't really make any sense. I'm not sure what you're trying to say when you talk about "impression of distance", but I'm guessing what you mean is that you can get closer to the foreground subject while still including more of the distant ground. But this isn't a function of focal length, it's a function of field of view. That's a significant distinction to make, although it's sometimes lost on people who don't have experience shooting multiple formats. Shooting a scene with 14mm FF DSLR and again with equilvalent FOV on 8x10 LF will give you the same results, focal length has got absolutely nothing to do with perspective or "impression" of perspective. The 14mm shot will probably have barrel distortion, but that has nothing to do with perspective and can be easily corrected. So elf's statement is correct.

Okay, I'll try again to explain what I mean. Let's consider your following statement.

Quote
But this isn't a function of focal length, it's a function of field of view...........

Shooting a scene with 14mm FF DSLR and again with equilvalent FOV on 8x10 LF will give you the same results, focal length has got absolutely nothing to do with perspective or "impression" of perspective.

What gives you an equivalent FoV on 8x10 LF? Certainly not a 14mm lens. More like a 100mm lens. No?

Even if you have a variety of different formats of cameras, you often cannot use the same lens on a larger format in order to increase the FoV because the image circle may not be of sufficient diameter. I'd love to be able to use on my 5D one of those Zuko 9mm lenses designed for the Oly 4/3rds format, but I'm pretty sure even if I succeeded in fitting one to my 5D body I'd see very strong vignetting and resolution fall-off towards the edges of the 35mm frame.

Now I agree in an ideal world where we had a perfect lens of fixed focal length, with a huge image circle with 100% MTF at extremely high resolution across the entire image circle, and where such a lens could be used across the entire gamut of different formats of camera, from P&S to 11"x14" large format, then it might be true to say that FoV is not a function of focal length, period, until someone designed another lens of a different focal length which could provide either a wider or a narrower FoV than was previously possible.

Since photographers are making practical decisions every day to change FoV by changing the FL of the lens, why not recognise this practical reality?

However, I agree there is an important principle here. If two images taken from the same position are cropped to the same FoV, whether such cropping takes place in-camera or outside the camera, then the spatial relationships of the same objects within each image will be the same, whatever the focal length used, provided other factors such as focussing are also the same. I would hesitate to claim that an image with fuzzy rocks in the foreground, but sharp cliffs in the background, produces the same sense of perspective in the viewer as a similar scene with tack sharp rocks in the foreground, but fuzzy cliffs in the background, just as I would hesitate to claim that the following two images have the same perspective, even though I know they were both taken with the same lens, at the same time, from the same position.

[attachment=23441:Different_crops.jpg]

Jeff,
On reflection, perhaps I could have expressed this more succinctly and precisely. I'll put it this way. If one changes the focal length of lens but keeps all other image attributes the same, including FOV, focussing, DoF and shooting position, then the sense of perspective should also be the same. This is really self-evident and cannot be argued against. I merely point out that in practice, these other factors often change with changing focal length. The focal length of the lens may also affect the lens design limitations. Is there any wide-angle 100mm or 150mm lens for 8x10 LF that allows for focussing at such close distances as is possible with a 14mm on a DSLR?



« Last Edit: August 02, 2010, 07:47:18 PM by Ray » Logged
elf
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« Reply #72 on: August 03, 2010, 12:24:21 AM »
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Quote from: Ray
On reflection, perhaps I could have expressed this more succinctly and precisely. I'll put it this way. If one changes the focal length of lens but keeps all other image attributes the same, including FOV, focussing, DoF and shooting position, then the sense of perspective should also be the same. This is really self-evident and cannot be argued against. I merely point out that in practice, these other factors often change with changing focal length. The focal length of the lens may also affect the lens design limitations. Is there any wide-angle 100mm or 150mm lens for 8x10 LF that allows for focussing at such close distances as is possible with a 14mm on a DSLR?

Actually, the short answer to this is yes, nearly every lens on LF (even 20x24) can focus closer than a 14mm on a DSLR. The focus distance is controlled by the amount of bellows draw. I saw a for sale ad last year for an 8 foot long bellows setup for 8x10:)  

It's probably fair to say that most people don't stitch or crop to match another format and for that matter the vast majority of people don't stitch, so they don't see that it (edit: perspective) doesn't change except when the camera position is moved.  It's hard to explain perspective when 95% of the time perspective is described in terms of getting the same FOV from different formats.

p.s. I like the uncorrected version much better.  The leaning/distorted buildings give the squatty ladies something to be distressed about
« Last Edit: August 03, 2010, 12:25:41 AM by elf » Logged
JeffKohn
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« Reply #73 on: August 03, 2010, 11:11:47 AM »
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Since photographers are making practical decisions every day to change FoV by changing the FL of the lens, why not recognise this practical reality?
Ray,

What you say is effectively true in the context of a single format such as 35mm (unless you bring stitching into the picture).

My preference for talking about FOV rather than focal length might seem like splitting hairs from a practical standpoint. But the problem is that when you talk about focal lengths, some people get confused and start to ascribe characteristics of FOV and perspective to focal lengths. This can lead to people saying stuff like "I prefer full-frame over DX, because my 85mm lens on FF gives a more pleasing perspective for portraits than 60mm on DX"; or claiming that you can't get the same perspective as a wide-angle shot by stitching multiple shots with a longer lens.
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Ray
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« Reply #74 on: August 03, 2010, 05:55:26 PM »
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Quote from: elf
p.s. I like the uncorrected version much better.  The leaning/distorted buildings give the squatty ladies something to be distressed about

I like the idea.   I might make a large print of the uncorrected image. The more allusions the better. Photos should not only be about illusions, but also allusions.

There are now at least two things for these women to moan about. Yet another church which is going to increase their tax burden, and dangerously leaning buildings which are about to collapse any moment.

(I understand in Germany people have to pay additional taxes to support their churches, unless they take the trouble to inform the taxation office that they are not Christian. I might have got this wrong, but that's what I've heard.)
« Last Edit: August 03, 2010, 05:57:00 PM by Ray » Logged
Ray
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« Reply #75 on: August 03, 2010, 06:36:11 PM »
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Quote from: JeffKohn
Ray,

What you say is effectively true in the context of a single format such as 35mm (unless you bring stitching into the picture).

My preference for talking about FOV rather than focal length might seem like splitting hairs from a practical standpoint. But the problem is that when you talk about focal lengths, some people get confused and start to ascribe characteristics of FOV and perspective to focal lengths. This can lead to people saying stuff like "I prefer full-frame over DX, because my 85mm lens on FF gives a more pleasing perspective for portraits than 60mm on DX"; or claiming that you can't get the same perspective as a wide-angle shot by stitching multiple shots with a longer lens.

Yes, I understand that's why some of you are so adamant that FL has nothing to do with impressions of perspective when considered as an isolated factor. However, because most cameras produced nowadays are miniature P&S cameras with typical focal lengths ranging from 4mm to 15mm, with a 50mm lens acting like a long telephoto, there should be no confusion that focal length of itself can have any direct bearing on the sense of perspective in an image.

My point really is just that a camera of a specific format is always required to take a photo, in addition to a lens, and therefore such format of camera should always come into the equation because it may determine the selection of focal length of lens for a desired FoV. The FoV in the final image, which may affect the impression of perspective from the standpoint of the person viewing the image, cannot necessarily be claimed to have nothing to do with the focal length of lens, because the choice of FL of lens may have determined the FoV. They are related in practice.

Likewise, those photographers who insist on using one fixed lens for all their shots will find that it tends to change the perspective in at least some of their images because they have to use their legs more to get closer to the subject, instead of being lazy and using a zoom from a greater distance. There are indirect perspective effects flowing from the choice of lens focal length, just as there may be indirect perspective effects created by the greater FOV surrounding a particular subject as a result of the use of a wider angle of lens.

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