Camera Focal Length

[beegee]

Cross post from Autodesk Revit ng:-

Leonid Raiz posted :-

The button In/Out of the Dynamic View dialog changes the focal length of the camera. However Revit does not provide focal length numeric value because of possible confusions. Focal length in photography usually relates to pictures taken with cameras that have 35 mm wide film. Internally cameras in Revit effectively shoot pictures on film that has width equal to height of crop region.

Digital camera focal lengths are different to traditional 35 mm cameras.

[Wes Macaulay]

Here's my cross posted answer as well!

See attached PDF.

PS: please deal with the fact that this document may use Canadian spelling. So guys, how do you spell aluminium? :twisted:

[Steve_Stafford]

Nice doc mister Meta...!

[gregcashen]

Very nice! Maybe if I did more renderings I would understand a word of it, but it looks great!

[sfaust]

very cool, and quite helpful. That brings up another tweak that would be nice for the wishlist, though. Several Rendering programs I've worked with (MAX,VIZ, and I think FormZ). Had buttons to simply set the camera to a specific focal length. would be nice to have that here...

[Wes Macaulay]

Leonid is theoretically correct (and if you're going to be right about anything, it's good to be right in theory)... while the 35mm camera is the standard, the numbers can be confusing. Sure they put all focal length figures for digital cameras in equivalent-to-35mm-film-camera-lenses, but I still wonder about them.

And you can't argue with the math. Well, OK, if you're a post-structuralist philosopher you can...

[Scott D Davis]

You don't want to argue anything mathematic with Leonid or Irwin. Both hold high degrees (doctorates?) in Mathematics.

[Steve_Stafford]

Mathematics?...which phase does that come in between, schematic and design dev or is it between master planning and preliminary design? Heck, I think we just skip over that phase usually? :twisted:

[Scott D Davis]

I had responded on the autodesk.revit NG, regarding the attachment above explaining how to calculate the focal length in a Revit view.

If is truely such a simple calcualtion, Revit should be able to do this for us, correct? One could set an initial 3D view, and then change a parameter for focal length, and Revit would update the view accordingly per the formula given. Conversly, if one created a view, and streched the view boundary on their own, and changed the camera position, Revit should be able to reverse-calculate the focal length, and report the actual value in the properties box.

Leonid? What are your thoughts?

[beegee]

If Revit could/can do that, then its only a hop-skip to get one of my favorite wishes (www.zoogdesign.com/forums/phpBB2/viewtopic.php?t=1671&highlight=camera).

For want of a better name, I call it " Align View"... and this is the way I would see it working :-

The designer could automatically reconstruct the position of a camera in the interior or on the street from a picture taken of it. This feature could be used both for inserting buildings into an existing street or designing an interior.
You need to know the height values of two vertical lines (four points in all) on the building and their locations on the photo. You place these "hotspots" on the imported photo and link their location to known points on your building or site plan.

You the enter the height of the hotspots in the appropriate fields in a dialog box. The upper points are located along the same vertical line as the lower points. After you click OK, a new camera will be placed on your site/floor plan.

The tool would adjust the image size of the scanned photo and the rendered image to match, keeping the proportions of the background image intact.

Well, thats the theory anyway. It may sound familar to users of another BIM program though, but I couldn't comment on that.

[LRaiz]

How about the following pieces of input -
Two vertical lines on the photo. For each line required input would be (A) real height and (B) horizontal distance to a line.

I think (have not verified yet) that the math would work and input is architecturally meaningful but does not involve specific coordinate system.

- LR

[beegee]

It may not be possible to know the horizontal distance to the height line. Presumable this is the real camera location.
Its relatively easy to know the height of a couple of specific elements on an existing structure in a photograph though.

[FK]

How about you make a model and then fit it to a photo? Could that work?

[aaronrumple]

...what is being discussed here is the Viz camera match tool. Works pretty well but requires more than just a few survey points for a good match.

Revit guys and the Viz guys need to talk more.

[beegee]

My post was actually based on the Archicad Align View tool which requires only two vertical lines of known height.