I'm sure this is a really ignorant question, but it's something I've never understood. Suppose I set up a scene with the camera framing an object at, let's say, 50mm focal. If I now switch from 4x3 picture proportions to 3x4 (i.e. landscape to portrait) the object seems to leap away from the camera as if the focal length had been changed. This doesn't happen with a real camera if I turn it sideways. What is the explanation?

Never noticed that before. But it looks like they are just filling as much of the viewport with your image as possible for each aspect ratio, rather than have a black border all the way around your image.

I personally never apply arbitrary frames to images, since every image is different and it would be a great coincidence if the best cropping for the dynamics and composition of an image just happend to match some arbitray aspect ratio rule. Of course i'm not making images for TV or any other required industry standard aspect ratio.

_jc
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VERY weird - when i posted my answer half of it is cut. I see now. Renderosity is acting very strange these days. Extremely slow sometimes, extremely often with error messages. Now i even can't edit my comment. Therefore another try of the full thing.

Funny, i just looked a little closer to this last week. I always thought that the perspective changes too - but this was wrong. It is correct. I compared two images in Photoshop by simply overlaying them. The perspective is the same.

I don't really have an explanation to this - i just compared the focal lengthes of the camera(s). First thought is, that in fact the landscape position is basis for the image and therefore the image appears to be smaller on portrait ratios. But maybe i do a little more research on that and even look into some old computer graphics theory books to see how the perspective is calculated in relation to the ratio. The times of programming things like this are over a long time already. PS: I delete my first answer, looks too weird, cut like this.

Message edited on: 12/17/2005 14:23

Optical and virtual photography work "slightly" different. I'm not a specialist, but it seems to me, that Vue associates the horizontal viewing angle to it's focal length. If one changes the focal, one changes this horizontal angle. The focal distance and the horizontal opening angle don't change, when you switch ratio. But the vertical opening angle changes : switching to "portrait" opens the vertical angle, switching to panavision closes the vertical angle. Try it : put a cube in face of the cam so that it fills the image, then change ratio. Compared to optical photography, one does not "turn" the camera when switching from 4:3 to 3:4 or panavision, but one changes the heigth of the negative. Some optical cameras switch to "panoramic" this way : they dont change focal length to superwide angle, they just cut away the upper and lower parts of the photograph, and then enlarge the printing. In 3D, this does not mean loss of quality, because it's you who decides, how many dots you will render in that area. Well, that's the way I see it...

I've tried to do a simple diagram to illustrate what's happening. On the left the two frames are 4x3 and 3x4 ratio with the longest side the same length for each. On the right we see the effect of fitting the vertical frame into the same size viewport as you would have for the default horizontal frame. It's not that the size of the object has changed relative to the frame, but that the size of the frame has changed. (It's more of a scaling rather than a zoom) So if you render the same scene at the same focal length with the same longest length, the objects in your scene (those visible in both aspects) will be the same size. Not sure if this makes things any clearer. But the way I see it focal length controls the "physical" aspects of the camera (such as perspective) while the size of the viewport (or render) controls the scaling of the image (like blowing up a photo to a larger size) -- Mark

OK, I was going to render a couple of images to demonstrate this, but it turns out Vue isn't behaving quite the way I thought it was. It keeps the same horizontal framing. So the size of the object DOES change in the frame unlike with a real camera. -- Mark

Hi, Yggdrasil, I'm not alltogether ok about VUE changing things... 'cause of the different pixel width of those two renders : The left one is 320 wide, the right one makes only 240 wide. Look at your renderings : In both images, the ship fills the same (horizontal) percentage of the image width, so "focal" didn't change. The right one just "opened" the vertical angle of vision (added "negative" above and below),n and reduced the size of the "print". To compare, take comparable conditions : If you give the same ("negative film") WIDTH to both, let's say 640 * -480- to the "landscape", and 360 * -480- to the "portrait", the ship's size will be the same. I often haggle and argue about Vue's bugs, but think that in this case, Vue does it's job correctly... Gerhard

Gerhard, The argument (for me at least) isn't whether or not this is a bug, but whether the behaviour matches my expectations. For comparison the above image shows a couple of quick snaps I just took with my digital camera. The camera was locked off on a tripod so the distance and focal length are the same for both shots. Notice how unlike the Vue test in post 6, the size of the subject in the real photos doesn't change between landscape and portrait. Instead parts of the horizontal view is clipped while the vertical is expanded. In Vue that horizontal clipping does not occur, which for anyone who does a lot of photography is a bit counter-intuitive. Cheers Mark

Interesting observations. I suspect what is happening here is that Vue tries to maximise the viewing area, so as much detail as possible is shown. Vue, after all, is not a camera, it's a rendering program. The programmers must have decided that detail was more important than being a camera. Having been brought up in the pre-digital-camera age, I'm still having to get used to the fact that a) digital cameres dont behave like film cameras and b) rendering programs don't behave like either.. It makes life confusing at times... Cheers, Diolma

Hello, Yggdrasil,

as i wrote, above,
when you turn your real cam, the roll (or captor) turns with the cam.
When you change ratio in 3D virtual cam,
virtual captor does -not- turn,
but changes it's vertical size.

Vue squeezes that into the viewer window at 72 dpi.
Moreover, the pix widths and heights, which programs propose for rendering,
do not result from ratio and focal length calculus,
they are "most used" formats.
So, if one jest pushes the button, it's not astonishing that results don't look logic.

Look at these renderings of the same scene :
nothing in the scene changed, same position, same focal,
but :
Left render Standard PC Vertical at -320- pix wide (427 high),
right render Standard PC Horizontal at -320- pix wide, too (240 high).
See what I mean ?

I hope this helps understanding.

It seems like Vue only keeps the focal length horizontally - whereas when you actually hold a camera and rotate it to go vertical, the subject will never chage size. It's the same POV angle of degrees regardless of rotation. In the real world, it doesn't matter the size of your negative or CCD, and in a 3D program it doesn't matter what size your render will eventually be (800 pixels wide, or 2,000 pixels - it doesn't matter when you're just setting up the camera), it's an angle measurement in degrees from a starting point - the lens itself. It's a spherical projection onto a focal plane - so it doesn't matter if it's vertical or horizontal, or any angle in between. In Vue, it looks as if you will only maintain your horizontal angle degree POV (best example being the Viking ship example above by Yggdrasil) while adding more or less on the top or bottom depending on your chosen aspect ratio. Not necessarily a bug - maybe it was just a bit too much for the programmers to figure out in a real-world way, so they chose to stick to horizontal measurements (which might be logical as it's main use seems to still be landscapes)?

Well... I think that they DID it the real world way ! just "unscew" the virtual tripod (the "keep level") and yaw the camera for 90 degrees.

Oups, I forgot to turn back the image of the turned cam (in "real", one does that, without beeing conscient of doing it)

OK now that works as I expect. Not a bug, a programming choice. I guess the point here is that in a virtual environment the programmers have to make certain decisions about how to represent optics, and they appear to have chosen to make the angle of view associated with a given focal length a function of the width of the viewport. This is understandable enough, they've got to pick something. I just wish they'd chosen to make it a function of the longest axis of the current render settings as this would more closely mirror the real world. -- Mark

Thanks for all those replies, I found them very interesting and informative.