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#1
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Perspective control using shifts
A view camera has a lens that produces an image circle much larger then
is needed for the film size. When shifts are used you are selecting what part of this circle gets captured to film. Shifts do not change the perspective of the image, only the aiming of the lens changes that. If we wish to have a photograph that does not have converging vertical lines then the lens needs to be pointed horizontally, we then shift the relationship between the lens and film to frame the shot. This image shows what this would look like if you could see the whole image circle. http://www.sewcon.com/temp/shift.jpg The rectangle represents one possible position for the film. Note that the camera is pretty much pointing towards the horizon, if I wanted just a bit of convergence it could be pointed up just a bit. It would be possible to build a camera that has no shifts but can correct for perspective, simply us a piece of film that is large enough to cover the image circle and then crop what post of the image you want. The shift part of the camera makes far better use of the film and allows for a much smaller camera. Scott |
#2
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Perspective control using shifts
I don't understand your argument. Covering the image circle is what
35mm does. You're suggesting just use a short enough focal length lens to capture the whole subject and crop it? Why not just use a 35mm film/digital tilt-shift lens for the right focal length and correct for convergence/divergence in the field? And you don't need a LF camera. |
#3
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Perspective control using shifts
"Scott W" wrote in message
ups.com... A view camera has a lens that produces an image circle much larger then is needed for the film size. When shifts are used you are selecting what part of this circle gets captured to film. Shifts do not change the perspective of the image, only the aiming of the lens changes that. If we wish to have a photograph that does not have converging vertical lines then the lens needs to be pointed horizontally, we then shift the relationship between the lens and film to frame the shot. Strictly, that may seem true if you are used to cameras where the relationship between film and lens is fixed, but actually what prevents converging verticals is not where the lens is pointed: it's having the film plane vertical. To be precise, for a shape - say, the rectangle of a building - to appear on the film as the same shape, with no lines that converge, angles changed, circles turned to elipses, or similar, the film plane must be parallel to the plane that that shape lies in. So verticals don't converge if the film back is vertical too. Shifting the lens (or the film relative to the lens) moves which bit of the image circle falls on the film and so allows you, given enough coverage, to keep the film parallel to the plane you are interested in but record a part of that plane that isn't directly in front of the camera: the classic example is shifting the lens up relative to the film in order to include the top of a tall building without tilting the film plane because to do so would make the verticals converge. Tilting the lens - 'aiming' it as you say - does not alter the geometry of shapes. So (to simplify a lot) if the film is vertical, and so parallel to a tall building, you can include the top of the building only by shifting the lens up (or the film down) so that the line ('ray') from the bottom of the film through the optical centre of the lens goes on to include the top of the building. Whether the lens is angled up or down makes no difference to which part of its image circle you are recording or to the shapes of the objects projected. What it does do is alter which part of the projected image cone the film intersects and so where the plane of sharpest focus lies through the field of view. ie. - shifts affect geometry, lens tilts affect focus. It is only when you tilt the film plane that you (significantly) affect both geometry and focus. To give an example: I am photographing a pool table from a position a foot above the end cushion. I can shift the lens down a little to improve the amount of the near end of the table that is in my field of view, without altering the geometry of what I see. I can also use tilts to make the plane of sharpest focus lie parallel to the table instead of perpendicular to it. If I tilt the lens forward I won't alter the geometry of the table in the image, but I can get my plane of focus aligned parallel to the table and so get all the balls easily within the DoF - however, such a tilt might, even with the downward shift ('fall') I have already applied to the lens, mean that the lens can no longer project a complete image onto the film because the film now lies partly outside its circle of coverage. The alternative is to leave the lens untilted and instead tilt the film backward a bit. This can again align the plane of focus where I want it, and can do so without moving the film outside the lens' circle of coverage. However, it will alter the apparent geometry of the image, making the near end of the table, and the balls on it, seem relatively larger than the far end. (If we think about it, this is obvious: the image of the far end of the table is projected at the bottom of the film plane: by tilting the film backward I have made the top of the film plane relatively further from the lens than the bottom, so the far end of the table is magnified less, and the near end more, in the projected image. For the same reason, the near end of the table will be a little less fully exposed than the far end, because the light has effectively been projected down a greater bellows extension, where it falls off according to the inverse square law.) In practice, for such a shot, I might want a bit of extra emphasis on the near end of the table and be happy to exaggerate the size of the balls there, so I might us some back (film plane) tilt, but I might use front (lens) tilt as well if using all back tilt made the effect more pronounced than I wanted. I could even 'over-tilt' the film to give extreme effects, and then correct the plane of focus with a little back tilt on the lens - any combination that gives the image I want is possible, so long as the film stays within the circle of projection of the lens. I hope that rather simplistic explanation makes sense: I just wanted to emphasise that it is not really where we 'point the lens' that controls the geometry of objects in the image, it is where we place the film that does that. Peter |
#4
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Perspective control using shifts
Bandicoot wrote: "Scott W" wrote in message ups.com... A view camera has a lens that produces an image circle much larger then is needed for the film size. When shifts are used you are selecting what part of this circle gets captured to film. Shifts do not change the perspective of the image, only the aiming of the lens changes that. If we wish to have a photograph that does not have converging vertical lines then the lens needs to be pointed horizontally, we then shift the relationship between the lens and film to frame the shot. . . . . . . . . . . I hope that rather simplistic explanation makes sense: I just wanted to emphasise that it is not really where we 'point the lens' that controls the geometry of objects in the image, it is where we place the film that does that. Peter Just to add to that great explanation, here is a shot using a simple shift lens on 35 mm: http://www.allgstudio.com/gallery/SFX/balls_SFX.jpg These are all the same round shape, but the camera position and a shift of the lens in a diagonal direction causes the shapes in the background to go more oblong, and appear slightly tilted. The shape in the foreground is actually very close to how these appear at that location. Front shift and rear shift on a view camera can function somewhat differently than a shift lens on smaller formats. I think part of that is altering the lens alignment to the subject/scene, while in other instances is changing the alignment of the film plane to the subject/scene. Ciao! Gordon Moat A G Studio http://www.allgstudio.com |
#5
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Perspective control using shifts
I think you missed something. The image on the film resulting
from a shift is from a different part of the image circle. Just using a larger sheet of film with a wider lens doesn't change that characteristic. The result is that nothing has changed. Collin |
#6
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Perspective control using shifts
"Scott W" wrote in message
ups.com... http://www.sewcon.com/temp/shift.jpg A Volkswagen and a Mini Cooper! You're obviously not in GM country! ;-) -- Regards, Matt Clara www.mattclara.com |
#7
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Inverse Law: was: Perspective control using shifts
Hi all,
Could somebody post the formula for the inverse law as it relates to bellows extension so that I can calculate the increase in f stops and compensate for the exposure. My archives got scrambled and I can't find it. Thanks. Regards, Bogdan Bandicoot wrote: snip IFor the same reason, the near end of the table will be a little less fully exposed than the far end, because the light has effectively been projected down a greater bellows extension, where it falls off according to the inverse square law.) snip -- __________________________________________________ ________________ Bogdan Karasek Montral, Qubec e-mail: Canada "Wovon man nicht sprechen kann, darber muss man schweigen" "What we cannot speak about we must pass over in silence" Ludwig Wittgenstein __________________________________________________ ______________ |
#8
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Perspective control using shifts
Scott W wrote:
A view camera has a lens that produces an image circle much larger then is needed for the film size. When shifts are used you are selecting what part of this circle gets captured to film. Shifts do not change the perspective of the image, only the aiming of the lens changes that. Front shift/lens shift changes perspective. Try this experiment. Set your camera up with the lens shifted all the way to the left. Compose a macro image (1:1) of a ruler, say the top 3 inches. Just behind the ruler, about 2" or so, place a page of text, so that the text is visible on both sides of the ruler. Use a small f/stop to insure DOF keeps the text readable. Make an exposure. Now without changing anything else, shift the lens all the way to the right. Make another exposure. Compare the two exposures, and you'll see the perspective between the ruler & text has changed. The ruler will appear to have moved in front of the text. Rear shift moves around the image circle. Front/lens shift changes perspective. Neither shift changes focus. |
#9
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Perspective control using shifts
no_name wrote: Scott W wrote: A view camera has a lens that produces an image circle much larger then is needed for the film size. When shifts are used you are selecting what part of this circle gets captured to film. Shifts do not change the perspective of the image, only the aiming of the lens changes that. Front shift/lens shift changes perspective. Try this experiment. Set your camera up with the lens shifted all the way to the left. Compose a macro image (1:1) of a ruler, say the top 3 inches. Just behind the ruler, about 2" or so, place a page of text, so that the text is visible on both sides of the ruler. Use a small f/stop to insure DOF keeps the text readable. Make an exposure. Now without changing anything else, shift the lens all the way to the right. Make another exposure. Compare the two exposures, and you'll see the perspective between the ruler & text has changed. The ruler will appear to have moved in front of the text. Rear shift moves around the image circle. Front/lens shift changes perspective. Front shift changes the point of view and the framing but it does not change the vanishing points in the image. Scott |
#10
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Inverse Law: was: Perspective control using shifts
Bogdan Karasek wrote:
Hi all, Could somebody post the formula for the inverse law as it relates to bellows extension so that I can calculate the increase in f stops and compensate for the exposure. My archives got scrambled and I can't find it. Thanks. Regards, Bogdan Bandicoot wrote: snip IFor the same reason, the near end of the table will be a little less fully exposed than the far end, because the light has effectively been projected down a greater bellows extension, where it falls off according to the inverse square law.) snip BEF = Image Distance^2/focal length^2 (square of image distance divided by square of the focal length) Stated as 'x' 1x = 0 stops 2x = 1 stops 3x = 1-1/2 stops 4x = 2 stops 8x = 3 stops 16x = 4 stops 32x = 5 stops |
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