Perspective (yet again, sorry!)

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Tom Axford Veteran Member • Posts: 6,237
Perspective (yet again, sorry!)
3

I have an old copy of "The Manual of Photography", 1978 Edition (Focal Press) and with all the discussion recently about perspective, I thought I would look at what it had to say on the matter.

Here are two extracts from the section on perspective (in Chapter 4 "The Geometry of Image Formation"):

Perspective on taking a photograph

As we have already seen, the perspective obtained on taking a photograph - sometimes termed the true perspective - is governed solely by the viewpoint. If the viewpoint is fixed, there can be no change in perspective, even if we change to a lens of different focal length - although, as we have seen earlier, the image size will alter. If, however, the viewpoint is altered, so will be the perspective, and no change of lens will recreate the perspective obtained at the first viewpoint.

and

Perspective on viewing a photograph

The perspective obtained on viewing a print - sometimes termed the apparent perspective - depends, firstly, on the relative sizes of objects in the print - and hence on the perspective obtained in the negative on taking the photograph - and, secondly, on the distance at which the print is viewed.

Correct perspective is said to be obtained when a print is viewed in such a way that the apparent relation between objects as to their size, position, etc., is the same as in the original scene. This is achieved when the print is viewed at such a distance that it subtends at the eye the same angle as was subtended by the original scene at the lens. The eye will then be at the centre of perspective of the print, just as, at the moment of taking, the lens was at the centre of perspective of the scene.

I'm sorry to start yet another thread on perspective, but I thought these extracts from a very well respected book on photography from the era of film may be of some interest.

tony field Forum Pro • Posts: 10,224
Re: Perspective (yet again, sorry!)
3

Tom Axford wrote:

I have an old copy of "The Manual of Photography", 1978 Edition (Focal Press) and with all the discussion recently about perspective, I thought I would look at what it had to say on the matter.

Here are two extracts from the section on perspective (in Chapter 4 "The Geometry of Image Formation"):

Perspective on taking a photograph

As we have already seen, the perspective obtained on taking a photograph - sometimes termed the true perspective - is governed solely by the viewpoint. If the viewpoint is fixed, there can be no change in perspective, even if we change to a lens of different focal length - although, as we have seen earlier, the image size will alter. If, however, the viewpoint is altered, so will be the perspective, and no change of lens will recreate the perspective obtained at the first viewpoint.

and

Perspective on viewing a photograph

The perspective obtained on viewing a print - sometimes termed the apparent perspective - depends, firstly, on the relative sizes of objects in the print - and hence on the perspective obtained in the negative on taking the photograph - and, secondly, on the distance at which the print is viewed.

Correct perspective is said to be obtained when a print is viewed in such a way that the apparent relation between objects as to their size, position, etc., is the same as in the original scene. This is achieved when the print is viewed at such a distance that it subtends at the eye the same angle as was subtended by the original scene at the lens. The eye will then be at the centre of perspective of the print, just as, at the moment of taking, the lens was at the centre of perspective of the scene.

I'm sorry to start yet another thread on perspective, but I thought these extracts from a very well respected book on photography from the era of film may be of some interest.

a completely excellent working definition of perspective regarding photography.  As far as I'm concerned no more need be said.

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Charles Darwin: "ignorance more frequently begets confidence than does knowledge."
tony
http://www.tphoto.ca

WunWegWunDarWun Regular Member • Posts: 410
Re: Perspective (yet again, sorry!)

Tom Axford wrote:

I have an old copy of "The Manual of Photography", 1978 Edition (Focal Press) and with all the discussion recently about perspective, I thought I would look at what it had to say on the matter.

Here are two extracts from the section on perspective (in Chapter 4 "The Geometry of Image Formation"):

Perspective on taking a photograph

As we have already seen, the perspective obtained on taking a photograph - sometimes termed the true perspective - is governed solely by the viewpoint. If the viewpoint is fixed, there can be no change in perspective, even if we change to a lens of different focal length - although, as we have seen earlier, the image size will alter. If, however, the viewpoint is altered, so will be the perspective, and no change of lens will recreate the perspective obtained at the first viewpoint.

and

Perspective on viewing a photograph

The perspective obtained on viewing a print - sometimes termed the apparent perspective - depends, firstly, on the relative sizes of objects in the print - and hence on the perspective obtained in the negative on taking the photograph - and, secondly, on the distance at which the print is viewed. Correct perspective is said to be obtained when a print is viewed in such a way that the apparent relation between objects as to their size, position, etc., is the same as in the original scene.

Unless by "etc." he means their print-relative size and position that will be the case regardless of viewing distance.

This is achieved when the print is viewed at such a distance that it subtends at the eye the same angle as was subtended by the original scene at the lens. The eye will then be at the centre of perspective of the print, just as, at the moment of taking, the lens was at the centre of perspective of the scene.

Unless this is accompanied by geometric diagrams with parametrized distances, angles etc. that relate a scene to a print and the print to a viewing setting making it clear what he means by "such a distance that it subtends at the eye the same angle as was subtended by the original scene at the lens" I'm afraid this is just as handwavy as the attempted explanations in the previous 3-900 threads.

I'm sorry to start yet another thread on perspective, but I thought these extracts from a very well respected book on photography from the era of film may be of some interest.

OP Tom Axford Veteran Member • Posts: 6,237
Re: Perspective (yet again, sorry!)

WunWegWunDarWun wrote:

Tom Axford wrote:

I have an old copy of "The Manual of Photography", 1978 Edition (Focal Press) and with all the discussion recently about perspective, I thought I would look at what it had to say on the matter.

Here are two extracts from the section on perspective (in Chapter 4 "The Geometry of Image Formation"):

Perspective on taking a photograph

As we have already seen, the perspective obtained on taking a photograph - sometimes termed the true perspective - is governed solely by the viewpoint. If the viewpoint is fixed, there can be no change in perspective, even if we change to a lens of different focal length - although, as we have seen earlier, the image size will alter. If, however, the viewpoint is altered, so will be the perspective, and no change of lens will recreate the perspective obtained at the first viewpoint.

But does he explicitly address what happens to "true perspective" when changing the lens (to one with a different AoV) while maintaining the viewpoint ?

A later sentence says:

"Altering the focal length of the lens, without changing the viewpoint, alters image size only - leaving perspective unaltered."

and

Perspective on viewing a photograph

The perspective obtained on viewing a print - sometimes termed the apparent perspective - depends, firstly, on the relative sizes of objects in the print - and hence on the perspective obtained in the negative on taking the photograph - and, secondly, on the distance at which the print is viewed. Correct perspective is said to be obtained when a print is viewed in such a way that the apparent relation between objects as to their size, position, etc., is the same as in the original scene.

Unless by "etc." he means their print-relative size and position that will be the case regardless of viewing distance.

The apparent size of an object in the print depends on the viewing distance.

This is achieved when the print is viewed at such a distance that it subtends at the eye the same angle as was subtended by the original scene at the lens. The eye will then be at the centre of perspective of the print, just as, at the moment of taking, the lens was at the centre of perspective of the scene.

Unless this is accompanied by geometric diagrams with parametrized distances, angles etc. that relate a scene to a print and the print to a viewing setting making it clear what he means by "such a distance that it subtends at the eye the same angle as was subtended by the original scene at the lens" I'm afraid this is just as handwavy as the attempted explanations in the previous 3-900 threads.

I did not want to copy the whole chapter.  There is, indeed, much more explanation (including diagrams) that makes it clearer what he means.

A couple of later sentences extracted from this further explanation may help:

"For correct perspective, therefore, a contact print should be viewed at a distance equal to the focal length of the lens.  An enlargement should be viewed at a distance equal to the focal length multiplied by the degree of enlargement."

WunWegWunDarWun Regular Member • Posts: 410
Re: Perspective (yet again, sorry!)

Tom Axford wrote:

WunWegWunDarWun wrote:

Tom Axford wrote:

I have an old copy of "The Manual of Photography", 1978 Edition (Focal Press) and with all the discussion recently about perspective, I thought I would look at what it had to say on the matter.

Here are two extracts from the section on perspective (in Chapter 4 "The Geometry of Image Formation"):

Perspective on taking a photograph

As we have already seen, the perspective obtained on taking a photograph - sometimes termed the true perspective - is governed solely by the viewpoint. If the viewpoint is fixed, there can be no change in perspective, even if we change to a lens of different focal length - although, as we have seen earlier, the image size will alter. If, however, the viewpoint is altered, so will be the perspective, and no change of lens will recreate the perspective obtained at the first viewpoint.

But does he explicitly address what happens to "true perspective" when changing the lens (to one with a different AoV) while maintaining the viewpoint ?

A later sentence says:

"Altering the focal length of the lens, without changing the viewpoint, alters image size only - leaving perspective unaltered."

and

Perspective on viewing a photograph

The perspective obtained on viewing a print - sometimes termed the apparent perspective - depends, firstly, on the relative sizes of objects in the print - and hence on the perspective obtained in the negative on taking the photograph - and, secondly, on the distance at which the print is viewed. Correct perspective is said to be obtained when a print is viewed in such a way that the apparent relation between objects as to their size, position, etc., is the same as in the original scene.

Unless by "etc." he means their print-relative size and position that will be the case regardless of viewing distance.

The apparent size of an object in the print depends on the viewing distance.

But the apparent relation of sizes between objects in the print does not, their sizes relative to the size of the print do only if you stand close enough so that your field of vision is fully occupied by a part of the print.

This is achieved when the print is viewed at such a distance that it subtends at the eye the same angle as was subtended by the original scene at the lens. The eye will then be at the centre of perspective of the print, just as, at the moment of taking, the lens was at the centre of perspective of the scene.

Unless this is accompanied by geometric diagrams with parametrized distances, angles etc. that relate a scene to a print and the print to a viewing setting making it clear what he means by "such a distance that it subtends at the eye the same angle as was subtended by the original scene at the lens" I'm afraid this is just as handwavy as the attempted explanations in the previous 3-900 threads.

I did not want to copy the whole chapter. There is, indeed, much more explanation (including diagrams) that makes it clearer what he means.

A couple of later sentences extracted from this further explanation may help:

"For correct perspective, therefore, a contact print should be viewed at a distance equal to the focal length of the lens. An enlargement should be viewed at a distance equal to the focal length multiplied by the degree of enlargement."

I'm not sure what about this makes it "correct" or preferred, if anything I would expect a criteria that the print occupies the same area in the field of vision that the original scene did in the photographers field of vision, this would mean BIF photos would have to be viewed from across a huge hallway.

WunWegWunDarWun Regular Member • Posts: 410
Re: Perspective (yet again, sorry!)

WunWegWunDarWun wrote:

Tom Axford wrote:

WunWegWunDarWun wrote:

Tom Axford wrote:

I have an old copy of "The Manual of Photography", 1978 Edition (Focal Press) and with all the discussion recently about perspective, I thought I would look at what it had to say on the matter.

Here are two extracts from the section on perspective (in Chapter 4 "The Geometry of Image Formation"):

Perspective on taking a photograph

As we have already seen, the perspective obtained on taking a photograph - sometimes termed the true perspective - is governed solely by the viewpoint. If the viewpoint is fixed, there can be no change in perspective, even if we change to a lens of different focal length - although, as we have seen earlier, the image size will alter. If, however, the viewpoint is altered, so will be the perspective, and no change of lens will recreate the perspective obtained at the first viewpoint.

But does he explicitly address what happens to "true perspective" when changing the lens (to one with a different AoV) while maintaining the viewpoint ?

A later sentence says:

"Altering the focal length of the lens, without changing the viewpoint, alters image size only - leaving perspective unaltered."

and

Perspective on viewing a photograph

The perspective obtained on viewing a print - sometimes termed the apparent perspective - depends, firstly, on the relative sizes of objects in the print - and hence on the perspective obtained in the negative on taking the photograph - and, secondly, on the distance at which the print is viewed. Correct perspective is said to be obtained when a print is viewed in such a way that the apparent relation between objects as to their size, position, etc., is the same as in the original scene.

Unless by "etc." he means their print-relative size and position that will be the case regardless of viewing distance.

The apparent size of an object in the print depends on the viewing distance.

But the apparent relation of sizes between objects in the print does not, their sizes relative to the size of the print do only if you stand close enough so that your field of vision is fully occupied by a part of the print.

This is achieved when the print is viewed at such a distance that it subtends at the eye the same angle as was subtended by the original scene at the lens. The eye will then be at the centre of perspective of the print, just as, at the moment of taking, the lens was at the centre of perspective of the scene.

Unless this is accompanied by geometric diagrams with parametrized distances, angles etc. that relate a scene to a print and the print to a viewing setting making it clear what he means by "such a distance that it subtends at the eye the same angle as was subtended by the original scene at the lens" I'm afraid this is just as handwavy as the attempted explanations in the previous 3-900 threads.

I did not want to copy the whole chapter. There is, indeed, much more explanation (including diagrams) that makes it clearer what he means.

A couple of later sentences extracted from this further explanation may help:

"For correct perspective, therefore, a contact print should be viewed at a distance equal to the focal length of the lens. An enlargement should be viewed at a distance equal to the focal length multiplied by the degree of enlargement."

I'm not sure what about this makes it "correct" or preferred, if anything I would expect a criteria that the print occupies the same area in the field of vision that the original scene did in the photographers field of vision, this would mean BIF photos would have to be viewed from across a huge hallway.

Specifically this attempts recreate the setting of what the photographer is seeing in his TTL viewfinder or ground glass

OP Tom Axford Veteran Member • Posts: 6,237
Re: Perspective (yet again, sorry!)

WunWegWunDarWun wrote:

I'm not sure what about this makes it "correct" or preferred, if anything I would expect a criteria that the print occupies the same area in the field of vision that the original scene did in the photographers field of vision, this would mean BIF photos would have to be viewed from across a huge hallway.

To my reading, that is exactly what he means by "correct perspective".  I'm really puzzled why you should think otherwise.

WunWegWunDarWun Regular Member • Posts: 410
Re: Perspective (yet again, sorry!)

Tom Axford wrote:

WunWegWunDarWun wrote:

I'm not sure what about this makes it "correct" or preferred, if anything I would expect a criteria that the print occupies the same area in the field of vision that the original scene did in the photographers field of vision, this would mean BIF photos would have to be viewed from across a huge hallway.

To my reading, that is exactly what he means by "correct perspective". I'm really puzzled why you should think otherwise.

I'll clarify: this recreates what the photographer saw in his field of vision when looking through the TTL viewfinder of a camera or ground glass, not what a cameraless viewer would see with his eyes from the viewpoint of the camera, except maybe at whatever angle of view we decided human vision has.

OP Tom Axford Veteran Member • Posts: 6,237
Re: Perspective (yet again, sorry!)

WunWegWunDarWun wrote:

Tom Axford wrote:

WunWegWunDarWun wrote:

I'm not sure what about this makes it "correct" or preferred, if anything I would expect a criteria that the print occupies the same area in the field of vision that the original scene did in the photographers field of vision, this would mean BIF photos would have to be viewed from across a huge hallway.

To my reading, that is exactly what he means by "correct perspective". I'm really puzzled why you should think otherwise.

I'll clarify: this recreates what the photographer saw in his field of vision when looking through the TTL viewfinder of a camera or ground glass, not what a cameraless viewer would see with his eyes from the viewpoint of the camera, except maybe at whatever angle of view we decided human vision has.

The TTL viewfinder has a fixed eyepiece, it does not change when the camera lens is changed.  The passage I quoted from the Manual of Photography said that the contact print (essentially the same as the ground glass in a TTL viewfinder) must be viewed from a distance equal to the focal length of the taking lens.  To do this you would need to change the viewfinder eyepiece every time you changed the focal length of the camera lens.

WunWegWunDarWun Regular Member • Posts: 410
Re: Perspective (yet again, sorry!)

Tom Axford wrote:

WunWegWunDarWun wrote:

Tom Axford wrote:

WunWegWunDarWun wrote:

I'm not sure what about this makes it "correct" or preferred, if anything I would expect a criteria that the print occupies the same area in the field of vision that the original scene did in the photographers field of vision, this would mean BIF photos would have to be viewed from across a huge hallway.

To my reading, that is exactly what he means by "correct perspective". I'm really puzzled why you should think otherwise.

I'll clarify: this recreates what the photographer saw in his field of vision when looking through the TTL viewfinder of a camera or ground glass, not what a cameraless viewer would see with his eyes from the viewpoint of the camera, except maybe at whatever angle of view we decided human vision has.

The TTL viewfinder has a fixed eyepiece, it does not change when the camera lens is changed. The passage I quoted from the Manual of Photography said that the contact print (essentially the same as the ground glass in a TTL viewfinder) must be viewed from a distance equal to the focal length of the taking lens. To do this you would need to change the viewfinder eyepiece every time you changed the focal length of the camera lens.

Perhaps you were thinking of TLR, TTL means through-the-lens which encompasses [d]SLRs and mirrorless, and technically the ground glass is also a TTL viewfinder.

OP Tom Axford Veteran Member • Posts: 6,237
Re: Perspective (yet again, sorry!)

WunWegWunDarWun wrote:

Tom Axford wrote:

WunWegWunDarWun wrote:

Tom Axford wrote:

WunWegWunDarWun wrote:

I'm not sure what about this makes it "correct" or preferred, if anything I would expect a criteria that the print occupies the same area in the field of vision that the original scene did in the photographers field of vision, this would mean BIF photos would have to be viewed from across a huge hallway.

To my reading, that is exactly what he means by "correct perspective". I'm really puzzled why you should think otherwise.

I'll clarify: this recreates what the photographer saw in his field of vision when looking through the TTL viewfinder of a camera or ground glass, not what a cameraless viewer would see with his eyes from the viewpoint of the camera, except maybe at whatever angle of view we decided human vision has.

The TTL viewfinder has a fixed eyepiece, it does not change when the camera lens is changed. The passage I quoted from the Manual of Photography said that the contact print (essentially the same as the ground glass in a TTL viewfinder) must be viewed from a distance equal to the focal length of the taking lens. To do this you would need to change the viewfinder eyepiece every time you changed the focal length of the camera lens.

Perhaps you were thinking of TLR, TTL means through-the-lens which encompasses [d]SLRs and mirrorless, and technically the ground glass is also a TTL viewfinder.

I was meaning any viewfinder with a ground glass image produced by the camera lens, so the ground glass image is exactly the same size as the actual image on the film or the sensor and hence the same size as a contact print (as mentioned in quoted extract).

tko Forum Pro • Posts: 12,881
books are frequenty wrong
4

Tom Axford wrote:

I have an old copy of "The Manual of Photography", 1978 Edition (Focal Press) and with all the discussion recently about perspective, I thought I would look at what it had to say on the matter.

Here are two extracts from the section on perspective (in Chapter 4 "The Geometry of Image Formation"):

Perspective on taking a photograph

As we have already seen, the perspective obtained on taking a photograph - sometimes termed the true perspective - is governed solely by the viewpoint. If the viewpoint is fixed, there can be no change in perspective, even if we change to a lens of different focal length - although, as we have seen earlier, the image size will alter. If, however, the viewpoint is altered, so will be the perspective, and no change of lens will recreate the perspective obtained at the first viewpoint.

This is correct, thanks

and

Perspective on viewing a photograph

The perspective obtained on viewing a print - sometimes termed the apparent perspective - depends, firstly, on the relative sizes of objects in the print - and hence on the perspective obtained in the negative on taking the photograph - and, secondly, on the distance at which the print is viewed.

Correct perspective is said to be obtained when a print is viewed in such a way that the apparent relation between objects as to their size, position, etc., is the same as in the original scene. This is achieved when the print is viewed at such a distance that it subtends at the eye the same angle as was subtended by the original scene at the lens. The eye will then be at the centre of perspective of the print, just as, at the moment of taking, the lens was at the centre of perspective of the scene.

Unfortunately, this is wrong. The viewing point for a photo is forever fixed at the point from where the photo was taken. In the 3D world, moving closer and farther changes the relative sizes of objects that aren't the same distance from the lens. But once captured in a photo, the relative sizes are forever fixed.

Take the classic example of a portrait taken close up. The features are exaggerated. Try as hard as you can, no amount of moving the photo back and forth will alter this exaggeration. Printing the photo freezes the perspective, since there no "center of perspective" for a flat, perpendicular object

Take a photo. Now take a photo of the photo from far away and close up using a zoom the fill the frame. The two resulting photos will look identical.

Many classic works have serious  flaws. In this case, the flaw is somewhat laughable. Suppose you take a photo of a mountain from a mile away. Do you have to view the image from a mile away to have correct perspective? What about a macro? Do you have to glue the photo to your face?

Okay, technically, the edges of a photo might be smaller as you move the image closer, since they are now farther away. But most lenses and the human eye correct for this effect, which can't be categorized as "perspective" effect.

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tko Forum Pro • Posts: 12,881
Re: Perspective (yet again, sorry!)
1

Tom Axford wrote:

WunWegWunDarWun wrote:

I'm not sure what about this makes it "correct" or preferred, if anything I would expect a criteria that the print occupies the same area in the field of vision that the original scene did in the photographers field of vision, this would mean BIF photos would have to be viewed from across a huge hallway.

To my reading, that is exactly what he means by "correct perspective". I'm really puzzled why you should think otherwise.

I'm puzzled by your answer. What happens if you look at a BIF photo from 3 feet away? What is distorted? What changes? What is "incorrect?" Give me something tangeable besides "correct." I'm gonna guess 80% of the worlds photo are being viewed from a viewing distance that doesn't match the camera distance. And all these people notice nothing?

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no, I won't return to read your witty reply!
professional cynic and contrarian: don't take it personally
http://500px.com/omearak

OP Tom Axford Veteran Member • Posts: 6,237
Re: books are frequenty wrong

tko wrote:

Suppose you take a photo of a mountain from a mile away. Do you have to view the image from a mile away to have correct perspective?

Although I did not copy the whole chapter, a little later on he says:

"For correct perspective, therefore, a contact print should be viewed at a distance equal to the focal length of the lens. An enlargement should be viewed at a distance equal to the focal length multiplied by the degree of enlargement."

No need to go a mile away!!!

By the way, a contact print is a print made at the same size as the image on film (or on the sensor if you translate it to the digital age).  Of course, nobody makes prints that small these days.

Gandolphi Senior Member • Posts: 1,948
Re: Perspective (yet again, sorry!)

The “Manual of Photography” was originally “The Ilford Manual of Photography”.

Just a point of interest.

Tom Axford wrote:

I have an old copy of "The Manual of Photography", 1978 Edition (Focal Press) and with all the discussion recently about perspective, I thought I would look at what it had to say on the matter.

Here are two extracts from the section on perspective (in Chapter 4 "The Geometry of Image Formation"):

Perspective on taking a photograph

As we have already seen, the perspective obtained on taking a photograph - sometimes termed the true perspective - is governed solely by the viewpoint. If the viewpoint is fixed, there can be no change in perspective, even if we change to a lens of different focal length - although, as we have seen earlier, the image size will alter. If, however, the viewpoint is altered, so will be the perspective, and no change of lens will recreate the perspective obtained at the first viewpoint.

and

Perspective on viewing a photograph

The perspective obtained on viewing a print - sometimes termed the apparent perspective - depends, firstly, on the relative sizes of objects in the print - and hence on the perspective obtained in the negative on taking the photograph - and, secondly, on the distance at which the print is viewed.

Correct perspective is said to be obtained when a print is viewed in such a way that the apparent relation between objects as to their size, position, etc., is the same as in the original scene. This is achieved when the print is viewed at such a distance that it subtends at the eye the same angle as was subtended by the original scene at the lens. The eye will then be at the centre of perspective of the print, just as, at the moment of taking, the lens was at the centre of perspective of the scene.

I'm sorry to start yet another thread on perspective, but I thought these extracts from a very well respected book on photography from the era of film may be of some interest.

 Gandolphi's gear list:Gandolphi's gear list
Canon EOS-1Ds Mark III Leica M Typ 240 Leica SL (Typ 601) Leica SL 24-90mm F2.8-4 +1 more
OP Tom Axford Veteran Member • Posts: 6,237
Re: Perspective (yet again, sorry!)

tko wrote:

Tom Axford wrote:

WunWegWunDarWun wrote:

I'm not sure what about this makes it "correct" or preferred, if anything I would expect a criteria that the print occupies the same area in the field of vision that the original scene did in the photographers field of vision, this would mean BIF photos would have to be viewed from across a huge hallway.

To my reading, that is exactly what he means by "correct perspective". I'm really puzzled why you should think otherwise.

I'm puzzled by your answer. What happens if you look at a BIF photo from 3 feet away? What is distorted? What changes? What is "incorrect?" Give me something tangeable besides "correct." I'm gonna guess 80% of the worlds photo are being viewed from a viewing distance that doesn't match the camera distance.

Yes, of course that is true today when photographers use a very wide range of focal lengths.

And all these people notice nothing?

They do notice something.  They notice wide-angle perspective anamorphism or telephoto compression.   Both of those things occur because the photograph is not viewed with the "correct" perspective.  The viewer is either too far away from the image or too close to the image.

WunWegWunDarWun Regular Member • Posts: 410
Re: Perspective (yet again, sorry!)

tko wrote:

Tom Axford wrote:

WunWegWunDarWun wrote:

I'm not sure what about this makes it "correct" or preferred, if anything I would expect a criteria that the print occupies the same area in the field of vision that the original scene did in the photographers field of vision, this would mean BIF photos would have to be viewed from across a huge hallway.

To my reading, that is exactly what he means by "correct perspective". I'm really puzzled why you should think otherwise.

I'm puzzled by your answer. What happens if you look at a BIF photo from 3 feet away? What is distorted? What changes? What is "incorrect?" Give me something tangeable besides "correct." I'm gonna guess 80% of the worlds photo are being viewed from a viewing distance that doesn't match the camera distance. And all these people notice nothing?

I think "correct" here means that the object in the print will be perceived as being the same size as it would by a viewer looking at the object in the scene with his eyes - this also means that the focal length is being used here as a proxy for magnification and this wouldn't work for macro prints, but then again we don't print macro shots with the purpose to perceive objects at their true sizes.

OP Tom Axford Veteran Member • Posts: 6,237
Re: books are frequenty wrong

Tom Axford wrote:

tko wrote:

Suppose you take a photo of a mountain from a mile away. Do you have to view the image from a mile away to have correct perspective?

Although I did not copy the whole chapter, a little later on he says:

"For correct perspective, therefore, a contact print should be viewed at a distance equal to the focal length of the lens. An enlargement should be viewed at a distance equal to the focal length multiplied by the degree of enlargement."

No need to go a mile away!!!

By the way, a contact print is a print made at the same size as the image on film (or on the sensor if you translate it to the digital age). Of course, nobody makes prints that small these days.

This diagram (from the Manual of Photography) may help:

Just Tim 4
Just Tim 4 Contributing Member • Posts: 624
Re: Perspective (yet again, sorry!)
1

Tom Axford wrote:

I have an old copy of "The Manual of Photography", 1978 Edition (Focal Press) and with all the discussion recently about perspective, I thought I would look at what it had to say on the matter.

Here are two extracts from the section on perspective (in Chapter 4 "The Geometry of Image Formation"):

Perspective on taking a photograph

As we have already seen, the perspective obtained on taking a photograph - sometimes termed the true perspective - is governed solely by the viewpoint. If the viewpoint is fixed, there can be no change in perspective, even if we change to a lens of different focal length - although, as we have seen earlier, the image size will alter. If, however, the viewpoint is altered, so will be the perspective, and no change of lens will recreate the perspective obtained at the first viewpoint.

and

Perspective on viewing a photograph

The perspective obtained on viewing a print - sometimes termed the apparent perspective - depends, firstly, on the relative sizes of objects in the print - and hence on the perspective obtained in the negative on taking the photograph - and, secondly, on the distance at which the print is viewed.

And thirdly on our assumptions on the actual sizes of the objects depicted as governed by our experience. For instance your assumptions of the size of the half filled glass are not contained in the image but your assumptions of how big glasses are from your experience of them. They could be 5" tall or 8" tall and the point is that the assumption you make affects how you interpret the relative distance. This is the entire point of the illusion of the odd shaped room with the two people of the same size: We do not see reality in the images but make assumptions which conform to our experience of what is most likely to be correct, (it is more likely the room is square because that is our experience of rooms). The point is that there is no absolute reality in a 2D image, it is always interpreted by a human eye which makes assumptions modified by experience, (didn't I say that a while ago... ). The illusion also shows another important point, that a our assumption of distance between two objects is entirely dependant on our assumptions about the size of the two objects. It is not locked into an image by the relative sizes of objects but can change even though the relative size of object remain entirely fixed...

Correct perspective is said to be obtained when a print is viewed in such a way that the apparent relation between objects as to their size, position, etc., is the same as in the original scene. This is achieved when the print is viewed at such a distance that it subtends at the eye the same angle as was subtended by the original scene at the lens. The eye will then be at the centre of perspective of the print, just as, at the moment of taking, the lens was at the centre of perspective of the scene.

I'm sorry to start yet another thread on perspective, but I thought these extracts from a very well respected book on photography from the era of film may be of some interest.

Yes, agreed with the extra addition in bold above.

Also if you read on, (I actually own the same edition of the book... spooky. ):

If we view a print at a distance other than the correct one the perspective achieved will be distorted, (didn't I say that as well?).

The end result is not really of consequence as all we do is the same as the extra point I added:

The point is that there is no absolute reality in a 2D image, it is always interpreted by a human eye which makes assumptions modified by experience...

In effect we change our assumptions of relative scale and distance to suit an understanding that is in line with our experience and our position as the viewer of the print. This is the sole reason why wide angle images *appear* to exaggerate distance and telephoto one *appear* to foreshorten, the incorrect assumptions we make when we try to interpret an image when viewing it from the incorrect distance. View them fro the correct distance (change only your distance to the print) and the *apparent* distortions disappear, viv-a-vis they must be relative to your position as the viewer.

This even works with the UWA distortions of the tennis ball, they are in fact correct projections of the shape onto a 2D plane, (remember the plane of focus or sensor is not parallel to the object but an an angle and so spheres will appear elongated) viewed from the incorrect position. All circles do it but your eye pretty much cancels the effect if it can form a different assumption of perspective when viewing. No circle is ever depicted as anything other than an oval unless it is central and square to the camera but you never notice because you make the assumption of the correct shape when you view and just don't question these assumptions until the distortion becomes severe.

This is why artists dispensed with linear perspective a long time ago, because it is so dependant on the position of the viewer. There is only one viewing position for *correct linear perspective*, which we almost never achieve when viewing images, and that is dependant on the assumptions we make about the actual objects contained within the image based on our experience, (we recognise the object and relate it to an object we know, and the absolute size of the object we know - not the one in the image).

And so you can't talk about images in terms of a mathematically correct perspective that you never actually see, because you always interpret perspective in terms of you position and experience and never from the point of the correct position and knowledge of the exact sizes of the objects depicted.

It therefore seems pointless to me to try and understand perspective in mathematical terms, but rather understand it in terms relative to how you actually see it with your human eye and assumptions.

tony field Forum Pro • Posts: 10,224
Re: books are frequenty wrong

tko wrote:

Tom Axford wrote:

I have an old copy of "The Manual of Photography", 1978 Edition (Focal Press) and with all the discussion recently about perspective, I thought I would look at what it had to say on the matter.

Here are two extracts from the section on perspective (in Chapter 4 "The Geometry of Image Formation"):

Perspective on taking a photograph

As we have already seen, the perspective obtained on taking a photograph - sometimes termed the true perspective - is governed solely by the viewpoint. If the viewpoint is fixed, there can be no change in perspective, even if we change to a lens of different focal length - although, as we have seen earlier, the image size will alter. If, however, the viewpoint is altered, so will be the perspective, and no change of lens will recreate the perspective obtained at the first viewpoint.

This is correct, thanks

and

Perspective on viewing a photograph

The perspective obtained on viewing a print - sometimes termed the apparent perspective - depends, firstly, on the relative sizes of objects in the print - and hence on the perspective obtained in the negative on taking the photograph - and, secondly, on the distance at which the print is viewed.

Correct perspective is said to be obtained when a print is viewed in such a way that the apparent relation between objects as to their size, position, etc., is the same as in the original scene. This is achieved when the print is viewed at such a distance that it subtends at the eye the same angle as was subtended by the original scene at the lens. The eye will then be at the centre of perspective of the print, just as, at the moment of taking, the lens was at the centre of perspective of the scene.

Unfortunately, this is wrong

The book is correct. You are misinterpreting what it said. That is a superb and technically accurate book particularly so for the older editions. I wish I had a copy.

The viewing point for a photo is forever fixed at the point from where the photo was taken. In the 3D world, moving closer and farther changes the relative sizes of objects that aren't the same distance from the lens. But once captured in a photo, the relative sizes are forever fixed

Of course! That is the nature of projecting a 3D scene onto a 2d surface of the sensor

Take the classic example of a portrait taken close up. The features are exaggerated. Try as hard as you can, no amount of moving the photo back and forth will alter this exaggeration. Printing the photo freezes the perspective, since there no "center of perspective" for a flat, perpendicular object

Well that has to be said with care. Explain that this shot which is virtually full frame from about 1.5 and certainly less than 2 meter distance witha 28 mm lens. I do not see the that distortion you are describing

The perspective impression that you see on the 2D projection is a perceptual process and your perception might be surprisingly wrong in many cases because you misinterpret the visual clues.

Take a photo. Now take a photo of the photo from far away and close up using a zoom the fill the frame. The two resulting photos will look identical.

Since you have move far away from your original shooting point of course the actual perspective changes in the final 2d image projection. The images are not looking identical.

Many classic works have serious flaws. In this case, the flaw is somewhat laughable. Suppose you take a photo of a mountain from a mile away. Do you have to view the image from a mile away to have correct perspective?

Of course not you would not have the correct ocular point of view if you tried to do that.

What about a macro? Do you have to glue the photo to your face?

Okay, technically, the edges of a photo might be smaller as you move the image closer, since they are now farther away. But most lenses and the human eye correct for this effect, which can't be categorized as "perspective" effect.

Wrong again. Sorry.the correct ocular distance will always always give you a correct perspective effect.

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Charles Darwin: "ignorance more frequently begets confidence than does knowledge."
tony
http://www.tphoto.ca

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