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inkpen3d
01-28-2008, 03:31 AM
I am currently working on an architectural visualisation job for a client and have generated a scene incorporating the landscape and existing houses from the supplied land survey plans. I also recently visited the site where the proposed build is to take place in order to take reference photographs, some of which will be used for photo-montage.

Within Layout with the above scene loaded I have imported one of the photographs of the existing houses as a backdrop image. I have set up a Real Lens Camera and placed it at the corresponding location/elevation to where the reference picture was taken and set the Layout camera to the same model/lens type (i.e. Nikon CoolPix 8800 with a standard lens) and focal length used - in this case 8.9mm (this value being derived from the EXIF data for the given image).

At a setting of 8.9mm the appearance in the Layout camera view-port is as if the scene is being looked at through a ultra-wide angle lens and there is a gross mismatch between the background image and the models in the scene. I have discovered that I can only get a correct match between the modelled houses and those in the background photograph if I change the focal length of the Layout Real Lens Camera to about 17.8mm - i.e. double that given in the EXIF data.

Does anyone know what's going on here to explain why there is this 2X discrepancy between the focal length used in the reference photograph and the LW Real Lens Camera? Is there some critical step that I'm missing out or doing incorrectly?

Thanks in advance.

Peter

kruemel_1
01-28-2008, 03:52 AM
What size is your frame size? It should be set according the chip-size of your camera (most compact cameras have 2/3" or 1/3" CCD-Size)

inkpen3d
01-28-2008, 05:47 AM
What size is your frame size? It should be set according the chip-size of your camera (most compact cameras have 2/3" or 1/3" CCD-Size)

Thanks for pointing that out. :thumbsup:

I had the frame size set to 0.5906", which I have now changed to 2/3" to match the CCD size of the Nikon CP8800. However, having reset the focal length to 8.9mm in the layout camera, although the match is much improved, it still not as good as I would have expected. I've checked over the sizes/position/elevations of the existing house models against the plans and they all seem okay as is the location of the camera.

Any further ideas/suggestions?

Regards
Peter

Captain Obvious
01-28-2008, 06:50 AM
Don't look at the focal length actually used by the camera. Instead, look in the EXIF data for the 35 mm equivalent focal length, and use that. Also, make sure to set your aperture height to 24 mm.

Your CoolPix 8800 has a really small sensor, so it becomes much narrower (cropped) at the same focal length by a camera with larger film/sensor.

inkpen3d
01-28-2008, 07:57 AM
Don't look at the focal length actually used by the camera. Instead, look in the EXIF data for the 35 mm equivalent focal length, and use that. Also, make sure to set your aperture height to 24 mm.

Your CoolPix 8800 has a really small sensor, so it becomes much narrower (cropped) at the same focal length by a camera with larger film/sensor.


Many thanks Captain. That made all the difference! :thumbsup:

The focal length of 8.9mm used by the Nikon camera lens was equivalent to a focal length of 35mm (the camera manual states that the lens is an 8.9mm - 89mm zoom, which is equivalent to a 35mm [135] camera format 35mm - 350mm zoom).

Having set the focal length to 35mm in the Real Lens Camera properties panel (set to the Nikon CP8800 standard lens), and then set the Frame to "35mm Full Frame" (on the main camera properties panel) I am now getting a much, much better match.

Just one question though, just to check that I've done everything correctly: When you mention setting "aperture height to 24 mm" - is that set up as I've done using the Frame setting, or is there some other place this should be set instead?

Regards
Peter

Captain Obvious
01-28-2008, 09:13 AM
Just one question though, just to check that I've done everything correctly: When you mention setting "aperture height to 24 mm" - is that set up as I've done using the Frame setting, or is there some other place this should be set instead?
Yes, that's the "Frame" setting. I guess they renamed it... Anyway, set it to 24 mm. 35 mm film has a frame size of 36 x 24 mm.


NEVER use inch or any other silly imperial measurement. Use metric, it makes stuff like this a lot easier.

inkpen3d
01-28-2008, 10:19 AM
Yes, that's the "Frame" setting. I guess they renamed it... Anyway, set it to 24 mm. 35 mm film has a frame size of 36 x 24 mm.


NEVER use inch or any other silly imperial measurement. Use metric, it makes stuff like this a lot easier.


Thanks for the quick response and for clarifying that for me (BTW, I never use imperial measurements [spit] unless forced to).

This all begs the question of why, when I'd set up the Real Lens Camera to the Nikon CP8800 and the actual physical focal length of 8.9mm and a CCD sensor size of 2/3", did the view still not match the actual photograph?

To put it another way, why did I have to switch to a 35mm equivalent focal length and 35mm Frame size?

Naively, I would have expected the "real-world" focal length (8.9mm) in conjunction with the "real world" sensor size (2/3") that I previously used in the Real Lens Camera settings to have produced the same result as using the 35mm equivalent ones (i.e. the choice of smaller sensor size "compensating" for the shorter focal length). Otherwise, what is the point in having the choice of sensor sizes in the Frame drop-down?

Regards
Peter

Captain Obvious
01-28-2008, 04:44 PM
This all begs the question of why, when I'd set up the Real Lens Camera to the Nikon CP8800 and the actual physical focal length of 8.9mm and a CCD sensor size of 2/3", did the view still not match the actual photograph?
The only explanation I can think of would be that the sensor size isn't actually 2/3".

inkpen3d
01-29-2008, 06:42 AM
The only explanation I can think of would be that the sensor size isn't actually 2/3".

Good idea! However, the manufacturer's specifications state that it is a 2/3" CCD. However....

I tried again today with a different photograph and, using the EXIF focal length (i.e. the "real world" focal length) and a Frame setting of 2/3" CCD, it all seemed to match reasonable well. In fact, when I tried using the 35mm equivalent values and a Frame setting of 35mm Full Frame, the match was way out!

Puzzled, I rechecked focal length of the lens used in the original photograph and, to my embarrassment, it wasn't 8.9mm but 15.0mm! So somehow I'd misread the focal length - arrgh! :foreheads Plugging this 15.0mm value into the Real Lens Camera with the Frame set at 2/3" resulted in a pretty good match.

It rather looks like when I used the 35mm equivalent values the first time round it was just pure luck that it appeared to give a good match.

So in summary, using the actual EXIF focal length in the Real Lens Camera panel in combination with the correct Frame (i.e. sensor) setting for the given camera appears to work as one would expect - that is, so long as you are not an idiot like me and misread the EXIF data!

Regards
Peter