Understanding entrance and exit pupils, camera design, and image distortion

[Theodoros]

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http://www.largeformatphotography.in...s-4x5-F-Metric

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I guess I'm not seeing the "distortion" to which is being referred. The need on a front standard is the ability to place the lens axis at any desired position, and at any desired 3-degree direction. The Arca, and similarly designed cameras, enable this capability.

I directly compared a Sinar Norma standard (base tilt above shift/swing) with a Sinar F (base tilt below shift/swing), and the worst I observe is that the latter rotates the lens a bit. But obviously, this does not cause distortion of the image, since lenses are axially symmetric. (Round.)

As for whether or not tilt is on axis, or at the base, it's always possible to use the focus knob in conjunction with other movements, to achieve the same position/orientation of the lens axis with base tilt that can be achieved with axis tilt.

Hi Neil... There is a certain point on the lens Axis around which all rotations of the Axis should happen (it being constant with respect to tilts/swings). This point is the exact position of the lens entrance pupil, the purpose of a view camera is to shift/side shift/swing/tilt with respect to the entrance pupil, not the axis of the lens. Think of it as a sphere that the image area intersects, if the entrance pupil is not exactly at the centre of the sphere, the shape of the intersection is not circular, but elliptical... This is not me saying... it is physics.

In other words, you will still have the axis of the lens penetrating the sphere by its centre, but while the image area plane will be an intersection by the centre of the sphere, the image projected by the lens will have a different centre (on the same axis) where the entrance pupil will be repositioned... This will cause the intersection not to be of circular shape (as it would be if the entrance pupil was positioned exactly at the centre of the where) but elliptical.

Let me give you a practical example (which will make it - I think - easier to understand). Say that you are using a recessed board on your camera, now by doing this, you retain the lens axis constant, but you've moved the entrance pupil of the lens backwards... If you now tilt the lens, the entrance pupil will drop with respect to the image area's centre... The image area will then record the intersection plane of a (hypothetical) sphere, the centre of which is where the original lens entrance pupil position would be, if no recessed board would be used. But the lens will project towards the image area a cone which is part of a different sphere, which now has a new centre, that being the position of the entrance pupil of the actual lens used on the recessed board. Thus the image area will intercept a plane that is of elliptical shape.

[B.S.Kumar]

Theodoros, just curious. Are all lenses designed so that their entrance pupils are at the correct position when they are mounted on flat boards from different manufacturers? Some are thicker than others, so would that make a difference? Also, concerning recessed boards, even Sinar makes such boards for wide angle lenses. Are Sinar boards designed to ensure that the advantage of using a P2 is not lost? Some lenses simply cannot be used with a flat board.

Kumar

[Theodoros]

Theodoros, just curious. Are all lenses designed so that their entrance pupils are at the correct position when they are mounted on flat boards from different manufacturers? Some are thicker than others, so would that make a difference? Also, concerning recessed boards, even Sinar makes such boards for wide angle lenses. Are Sinar boards designed to ensure that the advantage of using a P2 is not lost? Some lenses simply cannot be used with a flat board.

Kumar

Hi B.S., all good points that need to be discussed extensively.

Most (if not all) lenses that have been exclusively designed to be used on a view camera, meaning lenses that are of two cells (front and rear) despite if they are of symmetrical or of retrofocus design, have (to my knowledge) their entrance pupil positioned exactly where their mounting plane is. Most (if not all) other lenses that have been originally designed as to be used on a different camera, (meaning most lenses with focusing mechanism on them) have a variable entrance pupil (some people call it nodal point) the position of which varies with the focusing distance... If they are used on a view camera (like some use Mamiya RZ or Hasselblad V lenses) they are always focused to infinity and thus the entrance pupil doesn't vary with the focusing distance. However, the entrance pupil on these later lenses is never on their mounting plane and can be several centimetres from it... (from 3cm up to 7cm... even more sometimes).

The entrance pupil position of a lens, can be easily found by the user himself with a simple process... (put two bars 10m apart from each other, then the camera another 10m apart, and then swing the camera right and left and adjust the lens until the bar at 20m is invisible by the lens, hidden behind the 10m bar... the swinging point found if projected in the lens barrel, is the entrance pupil of the lens). So, a test can be perfumed once and for all lenses one may have, as to exactly know the issues involved with his (correct) use of equipment.

Now, as far as mechanical design of cameras are concerned, the tilt and swing mechanisms should be all applied by keeping the entrance pupil of the lens absolutely constant (meaning that the lens should rotate by its entrance pupil when swings and then tilt by it), while sifts and side shifts should move the entrance pupil (and hence the axis of the lens) horizontally or vertically. If all the before mentioned are taken care off, then one can be certain that his system performs to its maximum.
Therefore, it is obvious that if a shift mechanism is placed by a maker above the tilt mechanism, when one shifts, he will change the distance of the entrance pupil (and therefore the radius) with respect to the arc that the tilt mechanism has been designed to work... hence the entrance pupil won't be anymore at the centre of the circle of which the arc is part off and there fore the entrance pupil won't remain constant... Obviously, if the shift mechanism was below the swing mechanism, it would shift the tilt mechanism as well, thus keeping the entrance pupil & tilt mechanism geometry constant. Same thing would happen if side shift mechanism is above the swing mechanism... Imagine... you side shift your entrance pupil and then swing on the axis that it previously was! It moves on a circle doesn't it?

Now, (nearly) all modern cameras claim to be yaw free... (Yaw free means having always the entrance pupil constant when tilting or swinging the lens) but they "forget" to mention that if shift mechanisms are above the tilt/swing ones, "yaw free" is lost when one combines the two movements... Further more, L-type cameras and Arcas (Arca's Orbix does tilting exactly like an L-type of camera - by rotating the lens mount on an axis perpendicular to the lens axis) forget to mention that yaw free applies to the tilt mechanism only if the lens used has its entrance pupil positioned at the centre of the lens board... If a recessed board or a lens that has its entrance pupil in a different position than its mount is used, then... bye-bye "yaw free"!!! ...and in addition, the (geometrical) error caused is much more than monorails with arc type tilt mechanisms... simply because the tilting arc is by far more "tight" than them...

OTOH, the Arc type tilt mechanism monorails, they do tilting on a plane parallel (a few centimetres, but it is still not right) to the mounting board plane... This of course means that the entrance pupil of the lens (if a lens that has its entrance pupil on the lens board is used) is never on the centre of the circle of which the tilting arc is part of... and further more, swing mechanism axis is in most cases in the centre of the blog, rather than being a line on the same plane that the lens board is...

Now, add to this that all the above applies for the rear standard too, only difference being that instead of the lens entrance pupil, one has to consider the centre of the image area having the same (exactly) properties...

My conclusion then?

1. There are poor cameras, there are good cameras, there are better cameras, but there is no "perfect" camera....
2. It is better for somebody to chose one of the better cameras and convert it for being perfect...
3. Ideally a "perfect" camera should have its lens entrance pupil and image area centre point adjustable so that the user can position them for "yaw free" operation... Also, a perfect camera should be sturdy and have the shift mechanisms bellow the tilt/swing ones...

So why the Sinar P2 for me? Simply because the above criteria are only fulfilled by the Sinar P-series and the Arca Monolith cameras and out of the two, the Sinar is easier to convert and cheaper to find...
My conversion consists of new frames for the front and rear standard that can both slide back and forth on the standard (for one to position the entrance pupil and centre of the image area exactly for yaw free operation despite the lens used or the image area type) and - off course - new bellows as to avoid using recessed boards at all despite the mounting distance required for the lens. Needless to say that I'm "killing" Sinar's parallel plane off-axis system too.... the new frames are both centred on the blogs.
For one to convert the Sinar, he only has to bolt the new (sliding) bases of the frames on the existing threads and then slot the new frames in them and attach the bellows... It is as easy gets... especially if one will buy longer Arca type bases with the respective (longer) camera plates that slot into them and then bolt the "female" part on the blog and use the "male" part as frame base (so that the frame can move and adjusted back and forth). Care has to taken for the new frames centre to be retained at the centre of the tilt mechanism's arc (12cm radius for the Sinar)...

[Emmanuel BIGLER]

From Theodoros:
the lens should rotate by its entrance pupil when swings and then tilt by it

Well, I'm not sure that I clearly understand Theodoros' point (certainly other here do understand, not me for the moment).

In short my understanding is:
- I do not see why rotating the lens far from the entrance pupil could induce any image distorsion;
- rotating the lens itself or the whole camera around the entrance pupil keeps the center of perspective fixed but is not related to image projection of image distorsion;
- for lens tilts, rotating around the rear nodal point has the advantage to keep the image stationnary with respect to the subject, this is not related to the location of the entrance pupil;
- yaw effects used to be irritating to the LF studio photographers in the past, not sure that this is still a major issue today!
Doing mostly landscape shots with small tilts, yaw is basically a non-issue to me ...

Now in detail:

To me the only advantage or rotating any lens around its entrance pupil is when you want to make panoramic stitching by rotating the whole camera, so this is not related to the camera's tilts and shifts; or has the advantage that the view point and perspective center does not change when you tilt the lens, this is (IMHO) a very marginal advantage, except may be for close-up work??

Yaw effects, as far as I have read here,
http://www.largeformatphotography.info/yaw.html
refer to the fact that after combining movements, a vertical object is no longer projected parallel to the vertical image edges.
This can be very irritating and time-consuming when doing studio work, taking pictures of objects for which you have to combine all movements in order to get both the image sharp and the projected geometry right.
This yaw effect happens in LF cameras with axis tilts. For this reason several cameras have base tilts.

But may be Tuan Luong has to correct the above mentioned document, where no mention is made of the entrance pupil?

Regarding the entrance pupil: in the past, I am convinced that most professionals using large format cameras with film, never made any panoramic stiching, hence most view camera users never cared for the entrance pupil location!
But this is another issue, panoramic stitching by rotating the camera as a whole is not related to camera settings (tilts & shifts).

However there is some true and proven advantage of rotating the lens around the rear nodal point N' or image principal point H'; doing this you ensure that the image of a far-distant object does not move very much, i.e. remains stationnary on film with respect to the object after focusing, when you tilt the lens.
If you are doing a close-up at 1:1 ratio, in the 2f-2f focusing set-up, the best lens rotation axis to avoid image shift, for a symmetrical lens, is to rotate around the center of symmetry of the lens. Another rotation point!
So, regarding this important issue of keeping the image in focus or stationnary when rotating the lens with a fixed read standard, the entrance pupil plays, IMHO, no role.

Of course in non-symmetrical lenses, the entrance pupil is not located at N, N', H or H' [in telephotos, the entrance pupil can be located quite far from N=H or N'=H], so you have to choose, either you rotate the lens around the entrance pupil, to keep your center of perspective fixed, or you rotate around any other rotation axis optimized for image stationnarity, you can't get both.
In quasi-symmetrical lenses, with the iris is located close to the center of symmetry, the entrance pupil is always located very close to N or H; moreover the distance NN'=HH' is small (less the 3% of the focal length in apo ronars, trying to show that the entrance pupil is not located close to the front or rear nodal point is a real challenge for apo ronar users ;-) ), hence in most cases of quasi-symmetrical lenses you can consider that the entrance pupil is located close to the shutter blades, close to the lens board ... and even close to the rear nodal point N'. No mystery at all.

Base tilts allow a yaw-free operation for combined movements, but when you use a base tilt, you loose your focusing set-up.
And by rotating around any axis far from the exit nodal point N', your image will moves faster and faster when your axis is further away from N'.

This is my understanding, those issues are very subtle and not always visible to the LF practitioner doing landscape shots with small movements.

Yaw effects were important for studio photographers in an era when no digital post-processing of images was possible. Hence everything had to be perfectly right when depressing the shutter release.
Correcting a small amount of image rotation due to yaw does not seem to me as a major issue when you can perform digital image post-processing.
Sure you loose some image quality due to re-sampling ...

Another remark regarding projected geometry: in all our beloved quasi-symmetrical lenses with their pupils located in the principal (or nodal) planes, except for a small amount of intrinsic barrel or pincushion distorsion (this is very small for our modern lenses, and zero for symmetrica lenses used at 1:1 ) there is absolutely no difference in the projected geometry between a symmetrical lens and a pinhole camera, even for out of focus images.
Hence I do not see how camera movements, per se could induce any distorsion which is not described by classical laws of conical perspective, something well-known since Renaissance times!
And yes, there are some non-standard distorsion effects (i.e. not explained by basic laws of conical perspective) for out of focus images projected by non-symmetrical lenses (for which the pupils are located far from the principal planes), but, again, this is not at all related to camera movements.

[Theodoros]

Hi Emmanuel,

I'll try to explain better...
1. There is no way in photography where one can see (linear or angular or proportion) distortion through his lens and this distortion will not be transferred in the image, in photography what you see through the lens is what you get.
2. There are types of photography that the presence of distortion is more crucial than other kinds, or kind of photographers that don't care, or that only care for the more obvious of distortion... It is of course personal, but it doesn't change the fact that distortion is there, or that the same capture could have been done by eliminating it. In addition, when one stitches distortion presence doesn't only effects the geometry, but (significantly) the image quality too...
3. The description of yaw free you mention, is correct and is the same with mine... One describes the result, the other the cause behind it.
4. Digital distortion correction of images, can't correct for DOF or for perspective but only for some of the geometry. The geometry correction they can perform is with respect to the (wrong) position of entrance pupil that was applied during the capture, they can't correct for the distortion caused by the entrance pupil repositioning... Hence, there is linear distortion left in the final image (fatal for architectural work). In addition, digital "correction" of distortion causes further degradation of image quality.

Never the less, I can't see why one would use a view camera at all and use it (despite how demanding his work is for accuracy), if A.) He could have his camera set up right in the first place and yet he doesn't do so, B.) He could have used an SLR instead (even with a T-S lens) and "correct" things digitally later anyway...

I can see your position for considering things as being less critical than they are for the type of photography you do... But mind you... there is much more demanding photography for accuracy and then the use of a correctly set-up view camera is absolutely critical... Take my work for instance (which is what led me to analyse in depth the camera issues involved with it), I do (mostly) art reproduction for a living, which includes reproducing wall paintings in ancient byzantine monasteries and then internal and external architectural work for ancient monasteries and churches and even statues and architectural work with ancient monuments... Just try to capture and accurately reproduce then a four square meter wall painting masterpiece that is positioned higher up on a curved merging wall that extends towards the dome of the building... Or try to capture the front half part of the interior of an ancient Byzantine monastery (inc. half of the dome in height) and then (looking back) the rear half...

Sometimes it's best for one to admit "hey I didn't notice all this was happening all that time", than trying to pretend that there is no issue or that the issue is of limited extend instead... There is only one way for one to judge how crucial a camera design issue might or might not be and this is only if one compares equipment that has the issue, with other that doesn't.

[Bob Salomon]

Hi Emmanuel,

I'll try to explain better...
1. There is no way in photography where one can see (linear or angular or proportion) distortion through his lens and this distortion will not be transferred in the image, in photography what you see through the lens is what you get.
2. There are types of photography that the presence of distortion is more crucial than other kinds, or kind of photographers that don't care, or that only care for the more obvious of distortion... It is of course personal, but it doesn't change the fact that distortion is there, or that the same capture could have been done by eliminating it. In addition, when one stitches distortion presence doesn't only effects the geometry, but (significantly) the image quality too...
3. The description of yaw free you mention, is correct and is the same with mine... One describes the result, the other the cause behind it.
4. Digital distortion correction of images, can't correct for DOF or for perspective but only for some of the geometry. The geometry correction they can perform is with respect to the (wrong) position of entrance pupil that was applied during the capture, they can't correct for the distortion caused by the entrance pupil repositioning... Hence, there is linear distortion left in the final image (fatal for architectural work). In addition, digital "correction" of distortion causes further degradation of image quality.

Never the less, I can't see why one would use a view camera at all and use it (despite how demanding his work is for accuracy), if A.) He could have his camera set up right in the first place and yet he doesn't do so, B.) He could have used an SLR instead (even with a T-S lens) and "correct" things digitally later anyway...

I can see your position for considering things as being less critical than they are for the type of photography you do... But mind you... there is much more demanding photography for accuracy and then the use of a correctly set-up view camera is absolutely critical... Take my work for instance (which is what led me to analyse in depth the camera issues involved with it), I do (mostly) art reproduction for a living, which includes reproducing wall paintings in ancient byzantine monasteries and then internal and external architectural work for ancient monasteries and churches and even statues and architectural work with ancient monuments... Just try to capture and accurately reproduce then a four square meter wall painting masterpiece that is positioned higher up on a curved merging wall that extends towards the dome of the building... Or try to capture the front half part of the interior of an ancient Byzantine monastery (inc. half of the dome in height) and then (looking back) the rear half...

Sometimes it's best for one to admit "hey I didn't notice all this was happening all that time", than trying to pretend that there is no issue or that the issue is of limited extend instead... There is only one way for one to judge how crucial a camera design issue might or might not be and this is only if one compares equipment that has the issue, with other that doesn't.

Yaw occurs with any camera when the swing point is below the tilt point. To make a yaw free camera with either base or axis movements the modern solution is to have two tilt points on each standard, one below the swing point and the other above the swing point. Examples of axis tilt cameras with this feature are the Linhof GTL and for base tilt (in this case asymmetrical) the Sinar P system.

As an aside, any camera that is yaw prone, with any type of movements, becomes yaw free when it is rotated 90° so it's swing movement becomes the tilt movement. A Linhof TK or TKS even is equipped with an extra bubble level just for this purpose.

Lastly, regardless if a camera is yaw free or yaw prone, when equipped with the same lens the final result will be identical.

[Peter De Smidt]

Theodoros, by "distortion" do you mean the changing of the positions of near and far objects relative to each other in the image as movements are used?

[Theodoros]

Lastly, regardless if a camera is yaw free or yaw prone, when equipped with the same lens the final result will be identical.

True, but it can be totally misleading as to create the impression that one can use a yaw prone camera and end up having the same result as a (totally) yaw free system that one can even correct for the entrance pupil position of the lens and the centre of the image area...

Off course if an image area ends up to be at exactly the same position behind the same lens, the result would be identical... The question then should rather be if one can achieve that by using a yaw prone camera at all... "Hardly the case" is the only applicable answer... Unless if one uses a "perfect" camera, sets it up within seconds, then sets up another camera next to it, uses the same lens on it and then ....spends a couple of hours to recreate the exact same frame (for which the values used on that second camera will be totally different on both standards)...

If one uses a yaw prone camera in the first place, he will end up with a totally different result since he is unaware of the errors he creates and the magnitude of them... especially since he lucks the correct result to use as a reference...

[Theodoros]

Theodoros, by "distortion" do you mean the changing of the positions of near and far objects relative to each other in the image as movements are used?

Hi Peter,

I mean all direction distortions (of both linear or curved lines) distant or on a parallel plane.... including these you mention...

[Emmanuel BIGLER]

Thanks, Theodoros, for explaining your point. I have understood your points 1/ 2/ and 3/ and agree with them.
I'm happy if your definition of yaw-free is the same as explained here on this web site.

Now I have some comments on your point #4, quoted below :
4. Digital distortion correction of images, can't correct for DOF or for perspective but only for some of the geometry. The geometry correction they can perform is with respect to the (wrong) position of entrance pupil that was applied during the capture, they can't correct for the distortion caused by the entrance pupil repositioning... Hence, there is linear distortion left in the final image (fatal for architectural work). In addition, digital "correction" of distortion causes further degradation of image quality.

I agree that digital correction has hard times to re-create sharpness for blurred images, but I'll concentrate on geometry since I'm not sure that we speak about the same things.

I'm not sure that we use the term "perspective" in the same way, so we have to agree on the following.

If we neglect, or correct digitally, any distorsion (barrel or pincushion) generated by the lens, then the model for image projection is simply conical projection if we use a quasi-symemtrical lens, i.e., the same as for a pinhole camera.
And even if the images are blurred because we did not apply Scheimplug's rules properely, out of focus images obey the same rules as in classical conical projection.
If we do not agree on this, it will be hard to continue the exchange.
I assume that you do not use retrofocus lenses, they are affected (for out of focus images only) by a very special kind of distorsion unknown to symmetrical lenses. I can elaborate on this issue if required.

Hence with a symmetrical lens, provided that the entrance pupil is located where you want, all images projected on film through the lens exhibit a final geometry defined by basic laws of conical projection, exactly like through a simplified camera composed of an entrance pupil and an exit pupil of very small diameter separated by a small distance, ray tracing being the same as a pinhole camera, except for the separation between both pupils.

Of course, digital post-processing has hard times, or is totally unable to, re-create the desired image if the entrance pupil is located not where you want, because alignments as seen in object space differ if you move the entrance pupil sideways.
But if entrance pupil is properly located, and if the detector, behind the lens, is simply mis-oriented in any direction, and if the lens is quasi-symmetrical, since in-focus and out-of-focus images are projected like in a pinhole camera, I do not see any weird or peculiar distorsion there, that could not be modelled by classical conical projection.
And if easily modelled, that could not be digitally corrected by post-processing. At least for a small degree of blur, but for a pinhole like for a symmetrical lens, out of focus images are projected identically.

The major difference between a pinhole camera and a lens camera is of course that the price to pay for the extreme sharpness delivered by a modern lens is shallow DOF ... and in some circumstances, there is no other solution than digitally stacking images, a technique now widely used for macro work with a DSLR.