Although I'm a bit confused by the intent, (surely the building doesn't ever change size - and you can always use simple trig. to calculate how high it could/should be from a given distance away?) but either way, you want render to a print size, not a screen size, that way Revit won't adjust the output based on what's on screen (which is what I suspect is happening each time you're fiddling with the crop region)

Thanks for the input snowyweston. Sorry for not being more clear in describing what I'm trying to accomplish. I am working on a very tall building that I have to place on a site in an existing photo that shows the building site 6 miles away from the camra point of the photo. I placed the camra in Revit 6 miles from the building to do the rendering; at camra height of 6' and target height of half the height of the building. I used the Printer option for output and got 3 different building sizes depending on whether I checked the Region box or not and whether I resized the region or not. I was hoping there would be a standard procedure in Revit to accurately depit the visual size of the buiding at 6 miles away when I inserted it into the photograph (via Photoshop). I think your idea of using trig will give me the ability to see which rendering approach is more accurate. Thanks again for your help.
Norm1

What you are trying to do here is pretty tricky, but not impossible.

First of all, have a look at the properties in the photo you have - You´ll need to know the size (pixel x pixel) and the Focal Length. In the following, I´ll assume that the photo was taken with a regular digital camera with no zoom (28 mm) and that the photo is 3072 x 2304 pixels.

In Revit:

1. Set up a new camera, placed 6 miles from the building
2. Set the camera height to match the real camera position (usually 6')
3. Set the camera target height to to match the photo (if the horizon in the photo is centered, you´ll want to also set the target height to 6')
4. To match the 3072 x 2304 photo, you´ll set the physical size (Crop) of the view to 3072/300 x 2304/300 = 10.25" x 7.68" (Important to use "Scale" and NOT "Field of View" when you do this)
5. In the render setting choose 300 DPI (Notice this is the same value we just used to divide the pixels)

You should now be good to go, and the final render will fit exactly on the existing photo. If the render takes too long (due to the size) you can downsize both the render and the photo to accommodate this.

Now, it´s a different story if the photo is NOT taken with a standard 28 mm lense with no zoom... then you´ll need to involve some pretty serious math... and hopefully we can save that for later?

Way cool Klaus. Sounds like you've done this before. Am checking on the camra used as we speak but most likely just as you mentioned. I may be out to the site in a week or two (a long journey) and will take pictures with my own camra. Looks like I should use 28mm setting, no zoom so as not to complicate the issue. Once again, many thanks.
Norm1

I am just now getting used to this forum site and tried to send you a private message Klaus but may not have sent it properly so as to reach you. Thanks for the invite. I had success in accomplishing the rendering I was trying to do thanks to the help I received. When I checked on the pixel spread of the digital photo I was putting the structure into, it turned out to be 4,000X3,000 pixels. I have been using a viewr app. called IrfanVew for some years now and remembered I could change the pixel spread in the photo to 3072x2304 and it automatically changed the resolution to 300 dpi. After you open a digital image in IrfanView, there is an "Image" drop down menu and in that drop down pick the "Information" bar to see data in the image and be able to change it. Of course I also used 300 dpi in Revit Render. It's very very important that I show the correct relationship of the building to the photo/site. Thanks again.
Norm1

Hi Norm.

Glad you you got it working, but I´m gonna have to stress this:
If it´s paramount that you show the exact size, then Revit may not be the tool for you. Even the above mentioned is only as good as the Camera´s in Revit, and they are.... well, missing any physical camera settings. So when I claim that the standard Revit camera is equivalent to a 28 mm lense, I have no proof of that! It´t only what I got from doing several tests, and may NOT be exact enough...

I´d strongly advise you to use 3D Max, or any other software, where the cameras have real world physical settings (like focal length, etc.) when it´s as paramount as you describe it.

BTW, I did get your private message. But my Forum inbox is filling up faster than my Outlook inbox these days, so I´m not able to respond to everyone promptly

Hi Klaus
I did some comparision testing with objects in the photo with a known height and distance from my building and modeled a column of same height/distance along with my building in Revit. When I placed the new rendering of my building and colomn in the photo, the column matched almost excactly, if not exactly, with the known height of the reference feature I used.
I was wondering why I couldn't find camra settings in Revit. Will go to 3D Max and see what I get.

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