Perspective Myth and Maths

[dick]

What do you think affects perspective?

I had hoped that you could have answered each question, not just selected an option, but I cannot see an option to delete the poll, and do not know how to create a multiple-question poll?

 

[gandolfi]

Hello All,

Years and years ago when the silver halide still ruled the roost, when aeroplanes still had propellers and tweeting was something performed by sparrows, we were taught at college that..

"Perspective depends entirely on viewpoint"

Now that I am older and soooo much wiser I realise that this also could apply to life itself

However now that neutrinos can seemingly travel through Swiss mountainsides faster that the speed of light, I wonder if all polls regarding the nature of perspective are now redundant

Cheers,

Gandolfi.

 

[wentbackward]

Q3 ... front tilt or rear tilt? Why tilt and not swing?

 

[Shashin]

In regards to linear perspective:

Object distance effects "true perspective" in that it fixes the ratio of object sizes in the image. It is possible to have the same ratio of object sizes at different object distances because the ratio of object size is proportional to the ratio of object distances.

Viewing distance to the print/display effects "apparent perspective." Viewing distance is what makes it possible to see the effect of focal length/angle of view. If viewing distance is kept at the "correct" viewing distance, in relation to the talking distance which is proportion to the focal length and magnification of the image, then perspective will be the same regardless of focal length/angle of view. If the viewing distance is held constant, the usual condition, then apparent perspective changes.

Tilting or swinging the image plane effects perspective and the plane of focus.

Tilting or swinging the lens plane effects the plane of focus.

 

[Jack]

What do you think affects perspective?

I had hoped that you could have answered each question, not just selected an option, but I cannot see an option to delete the poll, and do not know how to create a multiple-question poll?

At the bottom of the poll screen is a small "advanced options" box where you can allow multiple selections. Unfortunately, once a poll has one response as originally set up, I cannot go back and edit the poll to accept multiples.

Answer:

Two things affect perspective: Lens position and sensor plane angle, PERIOD -- nothing else including format size, lens focal, aperture or focus distance will alter your perspective. If you alter the lens position in any fashion, moving the camera or adding rise or shift via the front standard, you've altered the lens position and hence altered perspective. Conversely, REAR sensor plane shifts or rise/fall will NOT alter perspective as long as the lens remains stationary. In some systems, lens tilts or swings actually alter the lens position and hence can also affect perspective. However, if the lens standard tilts and swings are true axial movements around the optical center of the lens (exit pupil), perspective will not be altered. Rear standard angle changes via rear standard tilts or swings alter perspective as they alter the sensor plane angle directly; camera angle changes (dramatically) alter perspective as they directly alter the sensor plane angle directly and (usually) move the optical center of the lens off its original position.

And yes, perspective is a confusing concept because of people misunderstanding the true meaning of the term as it relates to spatial 3-D to 2-D mapping...

 

[dick]

Q3 ... front tilt or rear tilt? Why tilt and not swing?

The options are limited to 100 characters, and, of course, tilt or swing would both affect the perpendicular image distance... the distance measured perpendicular to the image plane to the subject... I do not know if Wikipedia of Merklinger have a name for it.

 

[dick]

At the bottom of the poll screen is a small "advanced options" box where you can allow multiple selections. Unfortunately, once a poll has one response as originally set up, I cannot go back and edit the poll to accept multiples.

Thanks Jack.

I set up the poll without an end date - is it possible for you or I to end the poll so I could start again with a multiple question?

 

[Jack]

Thanks Jack.

I set up the poll without an end date - is it possible for you or I to end the poll so I could start again with a multiple question?

You have to restart a new thread with all the questions you want, and check the "allow multiple responses" box near the bottom of the poll creation page. I cannot edit the reply format, other than to delete or add questions, once a poll has been replied to.

 

[Shashin]

It is funny how folks are confused about perspective. If you look in a fairly mundane source like the Focal Encyclopedia of Photography, it lists the variables in linear perspective (page 550–52):

Object Distance and Image Size
Changing Object Distance and Focal Length (which is really a variation to the point above)
Viewing Distance

Here is a link to another photo book saying the same thing:

Optics in photography - Rudolf Kingslake - Google Books

Linear perspective is really a well studied technique. There are tons of books on it. And in order for a perspective to be preserved not only is the point you use to create the perspective important, but also the point it is view from--it is a projection system and the relative angular position something is viewed makes a difference. The pinhole view point was such a device and why when making large murals, the point the image was viewed from is very important to create the illusion of depth.

Today the popular myth is to say it is object distance only--where in the past folks recognized that lens choice could change the sense of space in an image (this is where viewing distance comes in). The "object distance only" view does not align to the the theories in linear perspective nor our perception of images. If perspective is simply object distance, how can we see, and see clearly, that an image has been taken with a wide angle or telephoto lens?

 

[dick]

Today the popular myth is to say it is object distance only--where in the past folks recognized that lens choice could change the sense of space in an image (this is where viewing distance comes in). The "object distance only" view does not align to the the theories in linear perspective nor our perception of images. If perspective is simply object distance, how can we see, and see clearly, that an image has been taken with a wide angle or telephoto lens?

Focal length does not affect perspective if you keep the object distance the same. This is irrelevant if you adjust the view point (subject distance) to get the same reproduction ratio (object image size).

If, using a high-res camera, you take two pictures of the same object from the same viewpoint (without rotating or panning the camera or back) with lenses of different focal lengths, and print them both at A4 or 10*8 (cropped so that the object fills the print) it should not be possible to tell them apart... if you use a low res camera and print big, then the greater enlargement would make the wider-angle shot look inferior... we usually subconsciously move the camera to fill the format, for example in a headshot, to make the most out of the res we have.

We could amuse ourselves by trying to think of how many ways we could change perspective:

Changing the (whole) camera viewpoint or image plane angle
Rear swings and tilts
digital perspective "correction" distortion, or analog base-board tilt
slide-projector lens axis not perpendicular to screen
viewing angle or distance

 

[Shashin]

Focal length does not affect perspective if you keep the object distance the same. This is irrelevant if you adjust the view point (subject distance) to get the same reproduction ratio (object image size).

I never said focal length changes perspective.

 

[dick]

I never said focal length changes perspective.

What were you trying to say or imply with:

If perspective is simply object distance, how can we see, and see clearly, that an image has been taken with a wide angle or telephoto lens?

The pictures in your link clearly show the effect on perspective of changing the viewpoint and changing the scaling in printing or post-production to keep the subject image of the girl the same size in the finished prints... but you can achieve the same effect by scaling (to keep the image of the girl the same size) by varying the focal length as you move the camera... and get a higher res finnished picture by not having to crop and scale up. ¿Is this complicated?

 

[Shashin]

What were you trying to say or imply with:



The pictures in your link clearly show the effect on perspective of changing the viewpoint and changing the scaling in printing or post-production to keep the subject image of the girl the same size in the finished prints... but you can achieve the same effect by scaling (to keep the image of the girl the same size) by varying the focal length as you move the camera... and get a higher res finnished picture by not having to crop and scale up. ¿Is this complicated?

And did you read the text explaining about viewing distance?

It is viewing distance that changes the apparent perspective. It is viewing distance that that allows us to perceive differences in the talking position and, by extension, the angle of view.

Here are some more resources for you about viewing distance:

The Focal Encyclopedia of Photography, Third Edition, page 552
View Camera Techniques, Fourth Edition, page 126
The International Center of Photography Encyclopedia of Photography, page 382
Photographic Materials and Processes, First Edition, page 158

It is really quite easy, if you have a projection made from a certain distance, you need to view the projected image from the same relative position to preserve the perspective in that image. With photographs, we don't do this; we view from a consistent viewing distance related to the print size rather than the taking position.

 

[dick]

And did you read the text explaining about viewing distance?

It is viewing distance that changes the apparent perspective. It is viewing distance that that allows us to perceive differences in the talking position and, by extension, the angle of view.

I do appreciate that this is true... all I am saying is that using a lens of a different focal length allows us to change the viewing distance without changing the subject image size... it is really not very complicated.

 

[Shashin]

I do appreciate that this is true... all I am saying is that using a lens of a different focal length allows us to change the viewing distance without changing the subject image size... it is really not very complicated.

No disagreement from me. I had that in my first post.

 

[timparkin]

An interesting point I discovered recently is that front tilt affects perspective as well as rear tilt. It's suggested that rear tilt causes looming and front tilt does not. However, the effect of looming is caused by tilt - the part of the sensor that is focused closer is magnified, an integral effect of having a lens further away from the sensor (i.e. focused closer). However, the effect of tilt on the front sensor is minimised by the associated perspective change caused by the fact that the image projected on the sensor comes from closer to the edge of the image circle.

 

[Jack]

An interesting point I discovered recently is that front tilt affects perspective as well as rear tilt. It's suggested that rear tilt causes looming and front tilt does not. However, the effect of looming is caused by tilt - the part of the sensor that is focused closer is magnified, an integral effect of having a lens further away from the sensor (i.e. focused closer). However, the effect of tilt on the front sensor is minimised by the associated perspective change caused by the fact that the image projected on the sensor comes from closer to the edge of the image circle.

Tim,

I indicated this very thing above and if your lens tilts altered perspective they were obviously not axial with the lens center (lens exit pupil):

Answer:

Two things affect perspective: Lens position and sensor plane angle, PERIOD -- ~~~ If you alter the lens position in any fashion, moving the camera or adding rise or shift via the front standard, you've altered the lens position and hence altered perspective. ~~~ In some systems, lens tilts or swings actually alter the lens position and hence can also affect perspective. However, if the lens standard tilts and swings are true axial movements around the optical center of the lens (exit pupil), perspective will not be altered.

 

[Bill Caulfeild-Browne]

In classes I've given in the past, I invited students to take a zoom lens like a 70-400 mm, on a tripod at a fixed distance from a model. Then I asked them to look through the viewfinder and tell me how the perspective changed as they zoomed from one extreme to the other.

They- usually! - told me that there was no perspective change, only field of view. And of course they were right.

 

[timparkin]

Tim,

I indicated this very thing above and if your lens tilts altered perspective they were obviously not axial with the lens center (lens exit pupil):

Yes but you will also 'distort' even if they *are* axial with the lens center. Adding tilt means that one end of the sensor is further away from the lens plane than the other end of the sensor. When you move the sensor further away from the lens plane, light diverges and you end up with the image being 'enlarged' (hence why your image circle gets bigger when focussed closer).

Hence if one side of the image is magnified more than the other, you have a distortion. The effect of this distortion is reduced by applying changes to the front standard because tilt on the front standard is the same as tilt+rise/fall on the rear standard.

Next time I get out with a camera I'll take a couple of pictures to demonstrate this.

Tim

 

[Shashin]

Yes but you will also 'distort' even if they *are* axial with the lens center. Adding tilt means that one end of the sensor is further away from the lens plane than the other end of the sensor. When you move the sensor further away from the lens plane, light diverges and you end up with the image being 'enlarged' (hence why your image circle gets bigger when focussed closer).

Hence if one side of the image is magnified more than the other, you have a distortion. The effect of this distortion is reduced by applying changes to the front standard because tilt on the front standard is the same as tilt+rise/fall on the rear standard.

Next time I get out with a camera I'll take a couple of pictures to demonstrate this.

Tim

Tilting/swing the lens around the rear nodal point does not change the distance from the lens to the sensor. It will change the plane of focus. Magnification is simply proportional to lens to sensor distance and tilts and swings properly set on the lens standard does not change that. If you are changing perspective with your tilt/swing, then you are not rotating the lens around the rear nodal point. The problem will be with your camera