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LENSES: Facts and Fallacies - Part VII
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PostPosted: Tue Apr 15, 2014 2:11 pm    Post subject: LENSES: Facts and Fallacies - Part VII Reply with quote
In the photographic universe, the wide angle lenses are unique. Unlike a telephoto lens, which allow isolation of the subject being photographed through the narrow angle of vision and shallow depth of field, a wide angle lenses often includes an excessive number of elements in the composition. From an artistic point of view is not easy to use efficiently a wide angle lens, what once led a famous photographer to say that wide angle lenses "separated the men from the boys."

Many photographers complain that wide angle lenses suffer from "distortion", when in fact they are talking about the exaggerated perspective which is characteristic of this type of lens. The exaggerated perspective is not a distortion from the technical point of view. Distortion is the bending of straight lines that do not pass through the center of the image. Distortion is undesirable in rectilinear wide angle lenses, but it is the hallmark of fisheye lenses. It may be mentioned here that it is possible to correct the distortion of a fisheye lens through a post-processing called "defishing ". This way the fisheye lens works effectively as a rectilinear wide angle lens.





to be continued...

Last edited by Gerald on Sun Mar 05, 2017 5:39 pm; edited 1 time in total


PostPosted: Sat Apr 19, 2014 9:06 pm    Post subject: Reply with quote
Perspective distortion depends upon the camera-subject distance. It is not caused by the focal length of the lens.

See : http://en.wikipedia.org/wiki/Perspective_distortion_%28photography%29


PostPosted: Sat Apr 19, 2014 9:47 pm    Post subject: Reply with quote
sichko wrote:
Perspective distortion depends upon the camera-subject distance. It is not caused by the focal length of the lens.

See : http://en.wikipedia.org/wiki/Perspective_distortion_%28photography%29


I'm afraid you're making a confusion between perspective distortion and perspective change.


PostPosted: Sat Apr 19, 2014 11:48 pm    Post subject: Reply with quote
Gerald wrote:
sichko wrote:
Perspective distortion depends upon the camera-subject distance. It is not caused by the focal length of the lens.

See : http://en.wikipedia.org/wiki/Perspective_distortion_%28photography%29


I'm afraid you're making a confusion between perspective distortion and perspective change.


Perhaps you could explain the difference.

The Wiki article introduces perspective distortion and then talks about perspective change. I don't see a contradiction. Distortion is a particular type of change. Some dictionaries describe one as a synonym of the other. It's quite common for a good author to use one word and then to replace it by another - simply to avoid too much repetition.

I've seen both distortion and change used in other places - to describe the same phenomenon - although distortion is much more common.


PostPosted: Sun Apr 20, 2014 1:11 pm    Post subject: Reply with quote
Perspective distortion is when you use a lens other than the normal. The perspective is exaggerated (wide angle lens) or compressed (telephoto lens). The perspective distortion is a visual effect.

Perspective change is when you change the vantage point. It is a real change of perspective. The perspective change has nothing to do with the focal length of the lens.

In the pictures of the boy with binoculars, the author used perspective distortion (he used a wide angle lens), and perspective change as he went closer to the boy to keep unchanged the size of the front lens of the binoculars.


PostPosted: Sun Apr 20, 2014 1:41 pm    Post subject: Reply with quote
Gerald wrote:
Perspective distortion is when you use a lens other than the normal. The perspective is exaggerated (wide angle lens) or compressed (telephoto lens). The perspective distortion is a visual effect.

Perspective change is when you change the vantage point. It is a real change of perspective. The perspective change has nothing to do with the focal length of the lens.

In the pictures of the boy with binoculars, the author used perspective distortion (he used a wide angle lens), and perspective change as he went closer to the boy to keep unchanged the size of the front lens of the binoculars.


I understand that perfectly and is how I would have described it.

Its like the word affect and effect, people get them very mixed up at times.


PostPosted: Sun Apr 20, 2014 6:14 pm    Post subject: Reply with quote
[edited

Last edited by bernhardas on Tue May 17, 2016 7:20 am; edited 1 time in total


PostPosted: Mon Apr 21, 2014 1:04 pm    Post subject: Reply with quote
Gerald wrote:
Perspective distortion is when you use a lens other than the normal. The perspective is exaggerated (wide angle lens) or compressed (telephoto lens). The perspective distortion is a visual effect.

Perspective change is when you change the vantage point. It is a real change of perspective. The perspective change has nothing to do with the focal length of the lens.

In the pictures of the boy with binoculars, the author used perspective distortion (he used a wide angle lens), and perspective change as he went closer to the boy to keep unchanged the size of the front lens of the binoculars.


Thanks for your comments. I think I understand where we differ. There are two considerations …

(1) The changes which occur when the focal length of the lens is changed, and

(2) The changes which occur when the camera-subject distance is changed.

In connection with (1), and if I understand you correctly, you describe the changes which occur when the focal length of the lens is changed, from something which has a normal focal length to something which is other than normal, as perspective distortion :

Gerald wrote:
Perspective distortion is when you use a lens other than the normal.

and
Gerald wrote:
… the author used perspective distortion (he used a wide angle lens)...
.

I, on the other hand, I would not describe this type of change in terms of perspective. I would not use the term perspective distortion. I see it simply as a change in the angle or field of view.

The Wiki article ( http://en.wikipedia.org/wiki/Perspective_distortion_%28photography%29 ) which I quoted earlier shows pictures of a set of beanpoles (if that’s what they are) and says …

Wiki wrote:
Below, a series of three photos shows the same scene shot from the same position with three different lenses: a normal lens, a wide-angle lens, and a telephoto lens. Notice that because the proportions in the image do not change with the angle of view, if the photos made with the wide-angle lens and the normal lens were cropped to contain the same scene as the photo made with the telephoto lens, then the image content would be exactly the same. The apparent difference in proportions results solely from the content added around the edges of the frame in the normal lens photo and the wide-angle photo.

Now the shot taken with the wide angle lens includes more content than that taken with the normal lens – the angle of view is wider. Is that what you mean by exaggerated perspective ? I would argue, and in agreement with the Wiki article, that after cropping and scaling, distances and angles measured for designated points are the same for all photos. There is no change in perspective and the use of the term perspective distortion is potentially misleading.

Vincent Bockaert provides an entry on Perspective for the Dpreview Glossary ( http://www.dpreview.com/glossary/optical/perspective )

He shows a series of images taken with different focal lengths and at different camera subject distances and comments …

Bockaert wrote:
… that changing the focal length while keeping the subject distance constant has—just like cropping—no effect on perspective.
…
… a tele compresses perspective (makes subjects look closer to one another), while a wide angle exaggerates perspective (makes subjects look more separated) compared to the "normal" way we see things with the naked eye. … this change in perspective is a direct consequence of the change in subject distance and thus only an indirect consequence of the change in focal length.
…

(Note : my use of bold in the last quote)

Nasim Mansurov ( http://photographylife.com/what-is-distortion ) provides a discussion of distortion which includes an entry on what he calls perspective distortion. A brief quote …

Mansurov wrote:
This is the part that seems to confuse a lot of photographers – the relationship (or lack thereof) of focal length to perspective distortion. You might hear some photographers say that one should use longer focal lengths to photograph people, or they will get distorted due to the lens’ short focal length. This is a mostly false statement, because lenses have no perspective. Other than fisheye lenses, all lenses have the same perspective – it is the camera to subject distance that determines perspective, not the focal length. There is an illusion of different perspective of lenses, because with long focal lengths you have to stand further away from the subject to frame them the same way. If you were to stand at the same distance, the subject would appear exactly the same!

(Note, again - my use of bold)

So someone else who doesn’t believe that the focal length of a lens affects perspective. Why then use the term perspective disortion ? It’s misleading and it’s used for something else : my second consideration, (2) The changes which occur when the camera-subject distance is changed. You call these changes Perspective Change. I call them Perspective Distortion - as do Mansurov and others, so I am not alone. However, I think that we are both talking about the same thing – whatever we call it.


PostPosted: Mon Apr 21, 2014 2:53 pm    Post subject: Reply with quote
Perspective is a mapping from 3D to a 2D space. The concept of perspective predates the invention of photography. Modern photographic lens produces exactly the same perspective as the pinhohe in a camera obscura (a fisheye lens is an exception). If you know how three-dimensional objects project through a pinhole, you know everything you need to understand the perspective created by a photographic lens.


sichko wrote:
So someone else who doesn’t believe that the focal length of a lens affects perspective. Why then use the term perspective disortion?


Perhaps the best explanation is through an example. Suppose you have a camera whose sensor has dimensions 24 x 36 mm.

Experiment 1
The lens is a 50mm.
You take a picture and print a copy with size 5X larger than the sensor, i.e., with dimensions 12 x 18 cm.
You watch the picture at a distance equal to 25 cm, which is 5X the lens focal length.
Result: You will have the same visual sensation of watching the scene with your own eyes, so you say the 50mm is a "normal" lens.

Experiment 2
The lens is a 20mm wide angle.
You take a picture and print a copy with size 5X larger than the sensor, i.e., with dimensions 12 x 18 cm.
You watch the picture at a distance equal to 25cm, which is 12.5X the lens focal length.
Result: You will have a very different visual sensation compared to watching the scene with your own eyes. You are experimenting what is called "perspective distortion".

Experiment 3
The lens is a 20mm wide angle.
You take a picture and print a copy with size 12.5X larger than the sensor, i.e., with dimensions 30 x 45 cm.
You watch the picture at a distance equal to 25 cm, which is 12.5x the lens focal length.
Result: You will have the same visual sensation of watching the scene with your own eyes. You are NOT experimenting what is called "perspective distortion". You even have the right to call the 20mm as the "normal" lens for this particular situation!


PostPosted: Mon Apr 21, 2014 3:05 pm    Post subject: Reply with quote
Gerald wrote:
Perspective is a mapping from 3D to a 2D space. The concept of perspective predates the invention of photography. Modern photographic lens produces exactly the same perspective as the pinhohe in a camera obscura (a fisheye lens is an exception). If you know how three-dimensional objects project through a pinhole, you know everything you need to understand the perspective created by a photographic lens.


sichko wrote:
So someone else who doesn’t believe that the focal length of a lens affects perspective. Why then use the term perspective disortion?


Perhaps the best explanation is through an example. Suppose you have a camera whose sensor has dimensions 24 x 36 mm.

Experiment 1
The lens is a 50mm.
You take a picture and print a copy with size 5X larger than the sensor, i.e., with dimensions 12 x 18 cm.
You watch the picture at a distance equal to 25 cm, which is 5X the lens focal length.
Result: You will have the same visual sensation of watching the scene with your own eyes, so you say the 50mm is a "normal" lens.

Experiment 2
The lens is a 20mm wide angle.
You take a picture and print a copy with size 5X larger than the sensor, i.e., with dimensions 12 x 18 cm.
You watch the picture at a distance equal to 25cm, which is 12.5X the lens focal length.
Result: You will have a very different visual sensation compared to watching the scene with your own eyes. You are experimenting what is called "perspective distortion".

Experiment 3
The lens is a 20mm wide angle.
You take a picture and print a copy with size 12.5X larger than the sensor, i.e., with dimensions 30 x 45 cm.
You watch the picture at a distance equal to 25 cm, which is 12.5x the lens focal length.
Result: You will have the same visual sensation of watching the scene with your own eyes. You are NOT experimenting what is called "perspective distortion". You even have the right to call the 20mm as the "normal" lens for this particular situation!


Could you give me the camera-subject distances for Experiments 1-3 ?

Back later - the sun is shining and I have some work in the garden.


PostPosted: Mon Apr 21, 2014 3:30 pm    Post subject: Reply with quote
The distance camera-subject doesn't matter for those experiments.


PostPosted: Mon Apr 21, 2014 4:06 pm    Post subject: Reply with quote
Gerald wrote:
The distance camera-subject doesn't matter for those experiments.


OK. I'll try your experiments. But I can't take any pictures until tomorrow afternoon. So It will be a couple of days before I get back to you.


PostPosted: Tue Apr 22, 2014 9:21 pm    Post subject: Reply with quote
Gerald wrote:
Perspective is a mapping from 3D to a 2D space. The concept of perspective predates the invention of photography. Modern photographic lens produces exactly the same perspective as the pinhohe in a camera obscura (a fisheye lens is an exception). If you know how three-dimensional objects project through a pinhole, you know everything you need to understand the perspective created by a photographic lens.


sichko wrote:
So someone else who doesn’t believe that the focal length of a lens affects perspective. Why then use the term perspective disortion?


Perhaps the best explanation is through an example. Suppose you have a camera whose sensor has dimensions 24 x 36 mm.

Experiment 1
The lens is a 50mm.
You take a picture and print a copy with size 5X larger than the sensor, i.e., with dimensions 12 x 18 cm.
You watch the picture at a distance equal to 25 cm, which is 5X the lens focal length.
Result: You will have the same visual sensation of watching the scene with your own eyes, so you say the 50mm is a "normal" lens.

Experiment 2
The lens is a 20mm wide angle.
You take a picture and print a copy with size 5X larger than the sensor, i.e., with dimensions 12 x 18 cm.
You watch the picture at a distance equal to 25cm, which is 12.5X the lens focal length.
Result: You will have a very different visual sensation compared to watching the scene with your own eyes. You are experimenting what is called "perspective distortion".

Experiment 3
The lens is a 20mm wide angle.
You take a picture and print a copy with size 12.5X larger than the sensor, i.e., with dimensions 30 x 45 cm.
You watch the picture at a distance equal to 25 cm, which is 12.5x the lens focal length.
Result: You will have the same visual sensation of watching the scene with your own eyes. You are NOT experimenting what is called "perspective distortion". You even have the right to call the 20mm as the "normal" lens for this particular situation!


So you are using the term perspective distortion as defined here : http://www.idigitalphoto.com/dictionary/perspective_distortion ?


PostPosted: Tue Apr 22, 2014 9:27 pm    Post subject: Reply with quote
sichko wrote:
So you are using the term perspective distortion as defined here : http://www.idigitalphoto.com/dictionary/perspective_distortion ?

Yes, exactly!
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