Does a photo give spatial information (the nose job)?

[fredjeang]

(1) The standard lens, 50mm for 35mm format, most closely matches the natural perspective of the human eye. Correct, but it is more a 70mm 35mm equivalent that matches closer the human eye. although it is admited the 50 as a reference.

(2) Shots from wide-angle lenses, uncropped, make distant objects appear further away and make close objects appear closer than they actually are in reality. in part correct: in fact the distance between the planes being bigger wich gives the sensation that what is close is closer.
If you measure with a ruler an object, let's say a canva and you take the picture in PS and choose a scale, when measure with the PS ruler, you'll have the same value as the real object you measured. In other words, dimensions are respected.
So a photo gives effectively a spacial information regardless of the lens used. Studdied carefully, it gives distances-dimensions informations that matches with the real world. The only thing you have to have is a reliable reference point: you have to know a real dimensions of at least one object in the scene. You can try that easily at home, shooting a room.

A software like Nuke is capable of creating a 3D space from a 2D image. It has to know the focal lenght used. It has a database of lens brands in wich it calculates the correspondant 3D space in wich the picture was shooted. Therefore, it has the capabilities to do stichings in 3D without 3D infos because the 2D are actually "containing the infos" as long as you know the lens used. Here is how it works: http://www.youtube.com/watch?v=vW7K-AwFAuM&feature=related

(3) Cropping any image, whether in camera or in post-processing, is effectively no different than using a longer focal length of lens that provides the same angle of view as the cropping. Correct, cropping in post is eactly the same as using a longuer focal at the exact same camera position. The only difference is that you'd have a smaller image dimension for the crop process.

(4) The effective focal length of any lens is a relationship between its actual focal length, the size of the sensor, and/or the degree of cropping in post-processing. Correct

(5) It's not the focal length of the lens per se that has any bearing on perspective, but the effective focal length. Don't really understand what you wrote here so can't opinate. The perspective has nothing to do with the lens and never has. It has all to do with the spacial point of view where the picture is taken from. If you don't move and you change your lenses, your perspective will be exactly the same. It only changes if you move in space, regardless of the lens.
That's what happens in tilt-shift as to correct-alter the perspective, a displacement has to occur, wether a physical displacement of the camera or the lens itself, wich is the case in view-tech cameras and tilt-shift lenses.
What yes changes when you change lens is the D.O.F but as long as you stay at the same place, the perspective is the same.(see your point 3)

[hjulenissen]

We encounter the same problem in this thread with AlfSollund's thesis that a photo cannot convey any spatial information.  I believe the very fact that he makes this statement is evidence that his definition of spatial information is quite different from the concept shared by the majority of photographers (and other observers).  However absent any precise definition of what Alf means by spatial information (I asked him, but have not yet received a clear answer), it becomes impossible to engage in any sort of a logical discussion.

A camera-sensor is a 2-d array of sensels. In that sense, there is not enough information to specify general 3-dimensional structures with certainty.

3-d structures (generally) tend to be a lot less general than one might think. If one use this assumption, you may be able to make well-educated "guesses" as to what the scene really looks like. Especially if you are satisfied with a "2.5D" luminance + distance estimate. A good example of when this breaks down is if you take a snap of a painting or a photograph. Every part of the in-camera jpeg file may suggest that it is a castle in southern France, but the true shape of the object is flat...

Using focus information may help the procedure. I believe that one of the focus stacking software packages can do faux 3-d models from a series of stacekd focus images.

-h

[Ray]

(2) Shots from wide-angle lenses, uncropped, make distant objects appear further away and make close objects appear closer than they actually are in reality. in part correct: in fact the distance between the planes being bigger wich gives the sensation that what is close is closer.
If you measure with a ruler an object, let's say a canva and you take the picture in PS and choose a scale, when measure with the PS ruler, you'll have the same value as the real object you measured. In other words, dimensions are respected.
So a photo gives effectively a spacial information regardless of the lens used. Studdied carefully, it gives distances-dimensions informations that matches with the real world. The only thing you have to have is a reliable reference point: you have to know a real dimensions of at least one object in the scene. You can try that easily at home, shooting a room.

Not sure I follow. Let's say I take a photo of a skyscraper that in reality is 700 metres away but appears to be about 2 kms away because I used a 14mm lens (on 35mm format). If there were a sufficient number of clues, that is objects of known dimension in the image, I can see how it would be possible to calculate all distances using a ruler. For example, if the image resolution were sufficient to enable me to count the number of floors in the skyscraper, I could determine the height of the skyscraper, approximately, provided I could see all the floors.

If there was a straight road leading from where I took the shot, filled with cars in a traffic jam, leading all the way to the base of the skyscraper, I could count the cars, if the resolution were sufficient, and provided the elevation of my perspective allowed me to see all the cars. I could estimate an average length for each car including the space between them, and get a rough estimate of the true distance of the skyscraper.

But these are all big 'ifs'. Supposing only part of the skyscraper were visible above a line of trees in the foreground which completely obscures everything behind the trees, apart from the tops of tall buildings?

In any case I'm not arguing that such relative sizes and distances could not be calculated if sufficient information were available, including I presume the focal length of the lens and type of camera, or lens equivalent.

A software like Nuke is capable of creating a 3D space from a 2D image. It has to know the focal lenght used. It has a database of lens brands in wich it calculates the correspondant 3D space in wich the picture was shooted. Therefore, it has the capabilities to do stichings in 3D without 3D infos because the 2D are actually "containing the infos" as long as you know the lens used. Here is how it works: http://www.youtube.com/watch?v=vW7K-AwFAuM&feature=related

The key point here is that lens and camera information is required. This software does not appear to work on the basis that perspective is unrelated to lens focal length.

The perspective has nothing to do with the lens and never has. It has all to do with the spacial point of view where the picture is taken from. If you don't move and you change your lenses, your perspective will be exactly the same. It only changes if you move in space, regardless of the lens.

How do you reconcile your statement above with your answer to my first question, ie.

Correct, but it is more a 70mm 35mm equivalent that matches closer the human eye. although it is admited the 50 as a reference.

Let's not quibble about whether it's closer to 70mm or 50mm. The point I would make is, if there is a particular focal length of lens (whatever it may be precisely) that matches the perspective as experienced by the human eye, then all focal lengths of lenses which are different to that will not match the natural perspective of the human eye. Therefore focal length of lens has a bearing on perspective, as experienced in a human observer.

[fredjeang]

Because:

The wide angle doesn't affect the distances, as no other lens in fact. It gives the sensation that it does affect and therefore that the distances between close and far are bigger but indeed that's not what is happening. It just affect the scale, but proportionally distances are the same.

To verify that fact, you need to put your camera on a tripod and do not change the location. Start with a 50mm-70mm to have a reference closed to the human eyes. Take a picture.
Then, put a wide in your camera without moving nothing. What do you see? the perspective is exactly the same, exactly. But, it recorded a wider angle of view. Your 50mm shot is contained within the wide shot and if you put it on a layer in PS you'd see that the 50mm shot is excactly a part of the wide angle shot with zero perspective change. As your angle of view is bigger, but the sensor that records it has not changed, it's the same size, the objects are appearing further BUT it's an ilusion, if you measure the distances, you will obtain the same than with the 50mm, than with a 1000mm, and it will match the real world.

If you have a painting wich size is 50cm x 40, it's 50x40 regardless of the lenses.

You can actually do this testing at home.

[Ray]

Because:

The wide angle doesn't affect the distances, as no other lens in fact. It gives the sensation that it does affect and therefore that the distances between close and far are bigger but indeed that's not what is happening. It just affect the scale, but proportionally distances are the same.

To verify that fact, you need to put your camera on a tripod and do not change the location. Start with a 50mm-70mm to have a reference closed to the human eyes. Take a picture.
Then, put a wide in your camera without moving nothing. What do you see? the perspective is exactly the same, exactly. But, it recorded a wider angle of view. Your 50mm shot is contained within the wide shot and if you put it on a layer in PS you'd see that the 50mm shot is excactly a part of the wide angle shot with zero perspective change. As your angle of view is bigger, but the sensor that records it has not changed, it's the same size, the objects are appearing further BUT it's an ilusion, if you measure the distances, you will obtain the same than with the 50mm, than with a 1000mm, and it will match the real world.

If you have a painting wich size is 50cm x 40, it's 50x40 regardless of the lenses.

You can actually do this testing at home.

Fred,
Surely you don't believe there is anyone who thinks for one moment that changing lenses on a camera changes the actual distances between an observer and the subject. You'd have to be stark raving bonkers to believe that. Only a massive earthquake could do that, and then only marginally.

I'm talking about the appearance of perspective in a photographic image, taken from the same position with different focal lengths of lenses.

An increase in the FL of lens, which you've agreed is the same as cropping, results in a different perspective, as in the two images below. One is taken with a 10mm equivalent by stitching 3 images taken with a 14mm lens, camera vertical. The other is taken with a 50mm lens; both on the full frame D700.

Since I was there and took the shots, I can tell you with complete certainty that the perspective in the 10mm equivalent stitch is quite different from what I see from exactly the same position I used the camera, whereas the perspective and relative sizes of objects in the 50mm shot is a very close match to what I see when I look through the window of the 6th floor apartment from where I took the shots.

Now, I understand perfectly if I crop the 10mm shot to the same angle of view as the 50mm shot, I get the same effect, apart from resolution. And why shouldn't I? Cropping is the same as using an actual lens with the same angle of view as the crop. We've already agreed on that.

If I crop the 10mm image to the same FoV as the 50mm images, and find the perspective is the same, all I have proved is that two 50mm lenses used from the same position provide the same sense of perspective. That's so obvious it hardly needs confirming.

[fredjeang]

While you do not change your point of view, (means no pshysical displacement of any means) the perspective is the same for all focal lenses, wide, tele, whatever. To alter the perspective you need to displace your point of view.
Then you have distortions that occurs (different according to the lens-brand-price etc...) but they have nothing to do with the perspective itself as long as you stay at the same place.

So, if your point of view is the same, your perspective will always be constant between a 50 and a 200mm, a 200 and a 20.

It's not that 2 50mm at the same position provide the same perspective, it's that any focal lengh at the same position provide the same perspective.

[Bart_van_der_Wolf]

In any case I'm not arguing that such relative sizes and distances could not be calculated if sufficient information were available, including I presume the focal length of the lens and type of camera, or lens equivalent.

All that's required is the focal length, as it determines the magnification factor. Magnification M = f/(s-f), where s is subject distance and f is focal length. That combined with an inkling of a clue of what we're looking at is enough for human vision to derive a sense of dimension, and hence perspective since only position determines perspective, but do read on.

What is probably confusing your concept of perspective, is that there is also something like projection distortion, which is what DaVinci and his contemporates were getting a grasp on. Unlike the human eye, a lens (or a pinhole) projects its image on a flat plane. That means that only when the output is looked at from the wrong viewing distance, things look distorted (the perspective of vanishing points seems distorted, e.g. a wide-angle or a tele 'look'). Since short focal lengths have their corner rays strike the flat plane at a much more oblique angle, that effect is even stronger (because we're usually looking at the result from 'too far away' to maintain the geometrical integrity). Objects may look stretched out, people look fat, noses look out of proportion.

When we look at the image on a flat plane from the proportionally scaled distance, the sense of perspective is identical for all focal lengths. Proportionally scaled is when we multiply the sensor size by a factor M, then we need to view it from a distance of focal length times M as well (which may get unpractical for ultra wide-angle and telelens shots).

The whole concept is formally called anamorphic projection distortion, or anamorphosis.

Cheers,
Bart

[fredjeang]

All that's required is the focal length, as it determines the magnification factor. Magnification M = f/(s-f), where s is subject distance and f is focal length. That combined with an inkling of a clue of what we're looking at is enough for human vision to derive a sense of dimension, and hence perspective since only position determines perspective, but do read on.

What is probably confusing your concept of perspective, is that there is also something like projection distortion, which is what DaVinci and his contemporates were getting a grasp on. Unlike the human eye, a lens (or a pinhole) projects its image on a flat plane. That means that only when the output is looked at from the wrong viewing distance, things look distorted (the perspective of vanishing points seems distorted, e.g. a wide-angle or a tele 'look'). Since short focal lengths have their corner rays strike the flat plane at a much more oblique angle, that effect is even stronger (because we're usually looking at the result from 'too far away' to maintain the geometrical integrity). Objects may look stretched out, people look fat, noses look out of proportion.

When we look at the image on a flat plane from the proportionally scaled distance, the sense of perspective is identical for all focal lengths. Proportionally scaled is when we multiply the sensor size by a factor M, then we need to view it from a distance of focal length times M as well (which may get unpractical for ultra wide-angle and telelens shots).

The whole concept is formally called anamorphic projection distortion, or anamorphosis.

Cheers,
Bart

Exactly, and well described.

When student, I used to work with this master: http://www.tjeerdalkema.net/ . He is a recognized specialist in anamorphosis. We where getting a little bit of money assisting him to mount the structures. Calculations where extremely complicated. He was calculating on the studio floor that was full of geometrical equations.

The confusion often comes from the distortions factors, but in fact the perspective is only affected by a change in the position.

[Ray]

All that's required is the focal length, as it determines the magnification factor. Magnification M = f/(s-f), where s is subject distance and f is focal length. That combined with an inkling of a clue of what we're looking at is enough for human vision to derive a sense of dimension, and hence perspective since only position determines perspective, but do read on.

Bart,
That doesn't really make sense to me. Surely a 360mm lens on 8"x10" format has the same degree of magnification as a 50mm on 35mm format. You need to know more than the FL of lens, don't you?

What is probably confusing your concept of perspective, is that there is also something like projection distortion, which is what DaVinci and his contemporates were getting a grasp on. Unlike the human eye, a lens (or a pinhole) projects its image on a flat plane. That means that only when the output is looked at from the wrong viewing distance, things look distorted (the perspective of vanishing points seems distorted, e.g. a wide-angle or a tele 'look'). Since short focal lengths have their corner rays strike the flat plane at a much more oblique angle, that effect is even stronger (because we're usually looking at the result from 'too far away' to maintain the geometrical integrity). Objects may look stretched out, people look fat, noses look out of proportion.

I don't believe so. I'm already aware of such differences, which is why I quoted the following on another thread, which explains these differences quite well. The bold type has been added by me.

This discrepancy between camera and physical eye is accounted for in part by the fact that in our eyes, light projects not onto a flat surface, but the curved inner surface of our eyeballs. Furthermore, a large portion of our perception comes from having two eyes that can triangulate relative depth (known as stereopsis, which is a form of parallax), and the ability to move our heads to accrete multiple views of a single object.

Parallels exist between the functioning of our vision and photography or linear perspective, but because our vision exists not only in the light that enters our eyes, but also the passage of time, and the interweaving of binocular pictures of the world by our brains in conjunction with our mental image of what we expect to see, the parallel becomes problematic. Artistic practice that developed contemporaneously with photography, such as impressionism and cubism, in many ways reflects this difference.

When we look at the image on a flat plane from the proportionally scaled distance, the sense of perspective is identical for all focal lengths. Proportionally scaled is when we multiply the sensor size by a factor M, then we need to view it from a distance of focal length times M as well (which may get unpractical for ultra wide-angle and telelens shots).

The whole concept is formally called anamorphic projection distortion, or anamorphosis.

This all sounds like excuses and rationalisation to me. It's like you are saying that lens focal length shouldn't have any effect on our sense of perspective from a given position, but unfortunately it does for the following reasons.

I would almost be prepared to accept a statement that in an ideal world using perfect lenses with no distortion, whether anamorphic or otherwise, focal length would have no effect on our sense of perspective. But even that seems to be plain wrong to me.

Ignoring the anamorphic problems of ultra-wide angle lenses, subjects taken with a telephoto lens look closer. How could anyone deny it? The same scenes shot with a wider-angle lens look different and are different. They contain more subject matter. Even if one uses DXO to reduce anamophisism, they are still significantly different images in content and appearance to a telephoto shot, and as a result impart a different sense of perspective to the viewer.

I'm just reporting on what I see. 

[AlfSollund]

Are you really contending that, in a photo, if one object partially obscures another, one cannot conclude that the partially obscured object is further from the camera?  Imagine for example, a photograph of a person standing in front of, and partially obscuring the lower part of a tree.  Viewing only the photo I suppose one could reach alternate conclusions:  The person is in front of the tree (closer to the camera) OR the tree is growing out of the persons head.  So granted, a photo cannot give any information about spatial relations in the real world if the observer has no experience with the real world.  In what galaxy do you reside?

Yes, I am. Viewing the photo you are correct in you conclusions. But there are other conclusions. I have already stated one; this is a photo of an photo. The object is only one, and equally close. Another; the object could have been manipulated in such a way to appear closer.

My point is that its no way of knowing for sure, since one photo is only a 2-D. And the experience from the real world makes it easier to construct the "illusion" of 3-D from a 2-D photo since we have learned that small objects are far away, that a person who are seemingly in front of a tree is so in 3-D and so on.

You are, I assume, the same AlfSollund who posted several times in a contemporary thread on the subject of perspective.  In that thread you were in firm agreement with the majority on the meaning of perspective and the fact that it depended on distance from observer to subject, and not on the focal length of the lens.  You posted:

I assume you are also in agreement with the common definition of perspective; that it involves visual distortions in a 2D image which provide information about the relative spatial positioning of objects in the original 3D scene.  Here again it is critical that the observer is cognizant of the significance of such visual distortions based on what he has seen and experienced in the real 3D world.

Now, in a separate thread, you are essentially arguing that perspective does not exist.  To paraphrase, you are arguing that "a photo cannot give any spatial information.  That we cannot derive from a photo any kind of spatial information about distances and objects spatial placement."

From this abrupt turnaround I can only conclude that you are badly confused, are unable to understand the meaning of the words (yours and others) that are being used or, perish the thought, you are simply trolling.

Yes I am Alf Sollund. And I agree "perspective and the fact that it depended on distance from observer to subject, and not on the focal length of the lens". And of course perspective exist. So if you go from 3-D to 2-D you get the perspective. Buts its no going the other way since one photo only contains 2-D information.

Or to put another way. I can create the same 2-D photo with a given perspective from various 3-D scenarios (some that I "fake"). But if I take one 3-D scenario I will get one 2-D perspective from a given distance. And another perspective from another distance for the same 3-D scenario. If you combine these two you will get some spatial information. Commonly used by rangefinders .

You might argue that in taking pictures in a real world situation with many objects this is difficult to "fake". This is not what I'm arguing, please read post again. And again; I'm not arguing that its possible to make educated guesses.

Im sorry that you take this for trolling, or in any way fells offended (but how I cannot know). You might assume anything you like based on my posts about my mental health or otherwise, but I dont think this will add much to discussion  

[AlfSollund]

I'm sorry I haven't been able to keep up with discussion and might have missed something. Thanks to all answers.  As far as I see hjulenissen has got it correct (but you still need more than one photo). Also thanks to BartvanderWolf for his "big noise" explanation, far better than I could have done.

I think this proves that we are bound by perceiving data from a 3-D + time reality. As soon as we capture this into a 2-D photo we loose some information, but our brains are remarkably adept of reconstructing and guessing  based on what we see. This of course makes it easier to create the illusion of 3-D from a photo. A knowledge all figurative artists have been and are using.

Also, you can use different 3-D situations and get the same 2-D photo. If you assume one 3-D situation you will get one 2-D perspective independent of focal length as long as you don't change position.

P.S.
I think some are using the word "perspective" about how we compare what we see the real world (3-D by our eyes/brain), and what we see in a 2-D photo? And perhaps claiming that there is a difference by these two, and this difference is depending on focal length given the same sensor size, i.e that we are comparing two different photo "perspectives" with realty? For me its not. Its one perspective from one distance, but you can take different crops of this with a tele and wide and compare. The perspective will be the same. The comparison with reality is another discussion.

[hjulenissen]

What is the "correct" 3-d interpretation of images such as this?

The answer of course is that several different physical "realities" can generate such an image, and humans (or dsp) cannot always reliably tell which from a single image. I think that we often can, though.

If we had information about the PDAF contrast of each of 9 or 81 AF sensors, we could be a lot more confident...

-h

[mouse]

(1) The standard lens, 50mm for 35mm format, most closely matches the natural perspective of the human eye.

This statement suggests to me that your concept of perspective remains different from that of mine, and of most others.  You will indeed find several references that claim a lens with a focal length approximately equal to the diagonal dimension of the sensor (or film, thus approx. 45mm on 35mm format) produces an image that most closely replicates the visual field of the human eye.  However visual field or angle of view is not synonymous with perspective.  Angle of view is a quality inherent in a lens; perspective is not.

[mouse]

And of course perspective exist. So if you go from 3-D to 2-D you get the perspective. Buts its no going the other way since one photo only contains 2-D information.

Sorry, I don't understand this statement.  Are you saying that perspective exists only when we are viewing the actual 3D scene, but that in a photo perspective no longer exists?

Im sorry that you take this for trolling, or in any way fells offended (but how I cannot know). You might assume anything you like based on my posts about my mental health or otherwise, but I dont think this will add much to discussion  

My apologies.  I had no intent to impugn your mental health.  I was merely expressing my frustration with our failure to communicate in a common language.  Which brings me again to your failure to provide a precise definition of spatial information.  Let me try to do this for you: 

If your underlying meaning is that, given only the information contained in a photograph of a 3D scene, one cannot reconstruct a precise model of that 3D scene, where the size and position of each object is an exact replicate of the original 3D scene; then I would certainly agree with you.  However this is so restrictive a definition of spatial information that I doubt you will find many who can accept it without qualification.

To my mind, the inability to construct such an exact model is a long way from simply being able to infer some spatial information about the objects in the 3D scene simply by looking at a photo.

[hjulenissen]

To my mind, the inability to construct such an exact model is a long way from simply being able to infer some spatial information about the objects in the 3D scene simply by looking at a photo.

A full 3-d model is clearly usually not possible - any occluded objects cannot be estimated from a single 2-d image.

You certainly can "guess" many things about distance from visually inspecting a 2D image. Let's call this 2.5d (luminance+distance in each pixel) And most of the time this guess may be fairly accurate. But the estimate will sometimes be very wrong - it is not bulletproof. This is easily shown by a number of "difficult cases".

Most people (non-photographers) would probably guess that the image below is a small scaled model because we are subconsciously trained that such apparent DOF only happens when you are doing macro work.
http://farm4.staticflickr.com/3008/2952613447_53906f2f6a_o.jpg

[Ray]

This statement suggests to me that your concept of perspective remains different from that of mine, and of most others.  

I don't thinks so. I believe I uderstand your concept of perspective. Let me try and describe it.

Your concept is based upon the very well-understood principle, if an object moves in relation to another object, the distance between those two objects has changed. For example, if I move my pencil closer to my notebook I can state categorically that the distance between my pencil and my notebook has been reduced.

If we imagine a robot eye that is so small it is effectively a point, smaller than a pin-head, manufactured say through nanotechnology, and imagine that robot constantly emits millions of rays in all directions as it moves around like a fly, measuring precise distances to all surrounding objects, we can state categorically that no movement equates to no change in distance and no change in perspective, and that any movement in any direction affects the distances between the robot eye and all surrounding objects, and therefore represents a change in perspective.

This is what you mean by perspective, isn't it? A totally theoretical, mathematical and abstract model which has excluded the experience of the human observer when peering through different focal lengths of lenses which results in a different 'view' of the world due to the effects of magnification or size-reduction in relation to the 'normal' perspective that a standard lens provides, or which our own eyes provide.

You will indeed find several references that claim a lens with a focal length approximately equal to the diagonal dimension of the sensor (or film, thus approx. 45mm on 35mm format) produces an image that most closely replicates the visual field of the human eye.

Hey! We almost agree on something . However, I'm not at all sure that the 'standard lens' actually replicates the visual field of the human eye. I think it has more to do with degree of magnification. The field of view of the human eye seems to be vastly greater than the FoV of a 50mm lens on a 35mm format camera. As I raise the camera to my eye to view the panoramic scene out my apartment window, using my 14mm Nikkor lens on my D700, I get the impression that my view with naked eye of the same scene with fixed gaze, standing in the same position and looking in the same direction, is actually wider.

Checking on the internet for the Field of View of the naked eye, I see there's some considerable variation in results, but all results are considerably wider than the 39 degrees horizontal FoV of a standard 50mm lens.

Wikipedia quotes the FoV for simultaneous visual perception, which presumably means without moving head or eyeballs, as 160 degrees x 175 degrees. I believe that's wider than 12mm.

If one wishes to make the point that the angle of focussed view is much narrower, it is indeed, much narrower than the FoV of a standard lens. I conclude therefore that the FoV of the standard lens does not match the angle of focussed view of the naked eye, nor does it match the total FoV of the eye which includes impressions of shapes and sizes, and some sense of color, but greatly lacks detail because focussing is so narrow.

What I conclude from all this is that the appearance of the size of objects as seen by the human observer greatly influences our sense of our own distance to the object viewed. If we increase that apparent size of any object, through magnification, through viewing the object directly through a telephoto lens, or viewing a print subsequently made, the object appears closer than it actually is. With a 50mm lens, it looks about right, enabling us to fairly accurately estimate the true distance from the photographer to the subject.

However visual field or angle of view is not synonymous with perspective. Angle of view is a quality inherent in a lens; perspective is not.

You're getting a little bit philosophical here, aren't you!    I can't disagree with this statement. Perspective is a quality, a sense, an experience, an impression that can exist only in the mind of a sentient observer. It's not a quality of the lens, which is an inanimate object. A lens doesn't have a perspective, but it can change our impression or sense of perspective, if it's not a standard lens.

Interestingly, even Nikon seem to agree with me. Here's what they have to say on the subject of perspective at this website:  http://imaging.nikon.com/history/basics/19/03.htm

I've highlighted key words to help avoid any confusion in the article.

Perspective

Perspective is a term for a visual effect that causes objects to appear smaller as their distance from the viewer increases. In photography, you can control perspective by changing the lens focal length. Lenses are typically described as being either wide (wide-angle lenses) or long (telephoto lenses). Wide-angle lenses have wide angles of view, increasing the difference in the apparent sizes of objects that are at different distances from the camera. In other words, nearby objects appear larger and distant objects smaller. Telephoto lenses, in contrast, have small angles of view, decreasing the difference in the apparent sizes of objects that are at different distances from the camera. This effect can be used to exaggerate or reduce the effects of perspective by changing the focal length of the lens.

You'll note the emphasis on appearance. No-one is claiming that changing focal length of lens changes the actual and real distance to any object. That would be absurd.

[AlfSollund]

Sorry, I don't understand this statement.  Are you saying that perspective exists only when we are viewing the actual 3D scene, but that in a photo perspective no longer exists?

My apologies.  I had no intent to impugn your mental health.  I was merely expressing my frustration with our failure to communicate in a common language.  Which brings me again to your failure to provide a precise definition of spatial information.  Let me try to do this for you: 

If your underlying meaning is that, given only the information contained in a photograph of a 3D scene, one cannot reconstruct a precise model of that 3D scene, where the size and position of each object is an exact replicate of the original 3D scene; then I would certainly agree with you.  However this is so restrictive a definition of spatial information that I doubt you will find many who can accept it without qualification.

To my mind, the inability to construct such an exact model is a long way from simply being able to infer some spatial information about the objects in the 3D scene simply by looking at a photo.

No offence taken, please forget this small interlude  .

Summary: One 3-D view translates to one 2-D perspective only. If you change position you get another 2-D perspective. But its not possible to translate the 2-D perspective back to one 3-D only, because there will be infinite 3-D models possible that led to the 2-D photo. In this sense the 2-D photo gives no spatial information. If you combine two ore more photos with different perspectives you get spatial information.

Im might not be to clear. What Im trying to say is that we see the perspective from a photo, but there is no way of translating this back to a 3-D. We can make good guesses, but its not possible to know for sure. Hjulenissen has given one example. Let me try another. let assume that I take a photo of a object, lets say a sphere towards a neutral background. From this photo it will not be possible to know if this was a sphere identical in proportions to the one in the photo but larger and placed further back.  So its possible to create the same photo perspective from these two simple scenarios. You might argue that in real life this is more difficult, but the principle still remains.

Another example. I take a photo of an mirror from an angle so that I don't show in the photo. All objects are at my back. How can you know from the photo?

So from one photo one can reconstruct infinite number of precise models of 3D scenes that corresponds to the photo.  And this is what we mentally do when seeing a photo; we create one 3-D model in our brain that fulfills what we think we see, but this is all. Its still just a guess  . And so how can one argue then that the photo contains spatial information? This is just what it is, if one remove one dimension this information is lost.

[Bart_van_der_Wolf]

What I conclude from all this is that the appearance of the size of objects as seen by the human observer greatly influences our sense of our own distance to the object viewed. If we increase that apparent size of any object, through magnification, through viewing the object directly through a telephoto lens, or viewing a print subsequently made, the object appears closer than it actually is. With a 50mm lens, it looks about right, enabling us to fairly accurately estimate the true distance from the photographer to the subject.

Hi Ray,

And he reason it looks about right with a 50mm lens is again due to anamorphosis. When we shoot with a 50mm lens on a 24x36mm sensor, then we should look at a print of 240x360mm at 500mm distance to get the exact same undistorted perspective 'look' as human vision perceives.

It's really more simple than people tend to realise ... Focal length equates to magnification, sensor dimensions equate to field of view (within the boundaries of the image circle), the position of the lens entrance pupil determines perspective, output viewing distance determines perspective distortion (with a planar projection, i.e. flat sensor to flat output).

Cheers,
Bart

[Ray]

Hi Ray,

And he reason it looks about right with a 50mm lens is again due to anamorphosis. When we shoot with a 50mm lens on a 24x36mm sensor, then we should look at a print of 240x360mm at 500mm distance to get the exact same undistorted perspective 'look' as human vision perceives.

It's really more simple than people tend to realise ... Focal length equates to magnification, sensor dimensions equate to field of view (within the boundaries of the image circle), lens entrance pupil determines perspective, output viewing distance determines perspective distortion (with a planar projection, i.e. flat sensor to flat output).

Cheers,
Bart

Hi Bart,
I wasn't aware that a 200mm lens (35mm format) has more anamorphosis than a 50mm lens. A 14mm lens certainly does. However the DXO converter does a reasonably good job correcting that. If one doesn't have the DXO converter with the appropriate lens module, one can simply select the problem areas with the rectangular marquee tool in Photoshop and use 'free transform'. This results in an alteration of the aspect ratio of the image, so instead of 3:2  it may become 4:3.

It's really more simple than people tend to realise ... Focal length equates to magnification, sensor dimensions equate to field of view (within the boundaries of the image circle), lens entrance pupil determines perspective, output viewing distance determines perspective distortion (with a planar projection, i.e. flat sensor to flat output).

There's some evidence that resolution and texture have a bearing on the sense of perspective. For example, if someone is presented with two images of same FoV and subject, but one of the images is a crop from a wide-angle lens and therefore low resolution and unclear, the appearance of fuzziness and lack of detail will tend to be interpreted as additional distance from viewer to subject, whereas the shot taken with an actual telephoto lens which is sharp and clear will tend to give the impression that it is closer to the viewer, even though all broad dimensions and angles are the same, indicating that the theoretical and abstract perspective is the same.

[mouse]

Summary: One 3-D view translates to one 2-D perspective only. If you change position you get another 2-D perspective. But its not possible to translate the 2-D perspective back to one 3-D only, because there will be infinite 3-D models possible that led to the 2-D photo. In this sense the 2-D photo gives no spatial information. If you combine two ore more photos with different perspectives you get spatial information.

What Im trying to say is that we see the perspective from a photo, but there is no way of translating this back to a 3-D. We can make good guesses, but its not possible to know for sure.

So from one photo one can reconstruct infinite number of precise models of 3D scenes that corresponds to the photo.  And this is what we mentally do when seeing a photo; we create one 3-D model in our brain that fulfills what we think we see, but this is all. Its still just a guess  . And so how can one argue then that the photo contains spatial information? This is just what it is, if one remove one dimension this information is lost.

Alf-

Thank you for this very clear reply.
This is a very clear statement of your meaning and I agree completely that, given your definition of spatial information, your conclusions are perfectly correct.  For you it's like pregnancy; either you are or you are not; there is no such thing as "a little bit pregnant".   

I think our difference now simply boils down to semantics.  I define spatial information less narrowly.  Any information in a photograph (perspective for example) which provides some clues that allow one to make an informed estimate (guess) about the relative position and/or size of the 3D objects, I would describe as spatial information.  Clearly it is not always guaranteed to be correct information, and certainly one can have photos which contain no spatial information (even by this looser definition). 

I have always loved fishing.  As a very young boy I learned a trick when being photographed with my catch.  Hold the fish at arms length straight out in front of my body, and close to the camera.  At first glance it makes the fish look much bigger, but it doesn't fool too many viewers.  Which brings us back to the original theme of this thread.  It is this same "trick" that makes a nose look much bigger in proportion to the face when the camera is positioned quite close.