Camera matching query...

[biglolly]

I assume a lot of us here that do architectural work, would be asked to create photomontages as a part of the job. For the photography part I normally use my 350D with a 10-22mm lens. My question is this...how can I make sure the camera focal length in 3D Max is identical to that of the photograph?

That might sound like a silly question, but what is throwing me off is the focal length multiplier of the 350d. It's 1.6x so to find the 35mm equivalent I multiply say 10mm by 1.6 to get 16mm. Is this the correct way of doing it? I've found sometimes that the Max camera doesn't look like it actually matches the image when I multiply by 1.6 and I end up just doing it manually by eye.

Some of our images are created for planning purposes and so we're often required to make sure they are as accurate as possible, so obviously if you're compositing 3D elements on a photograph you'd have to make sure the 3D camera is at the same focal length as the actual camera.

Can anyone offer any advice on this topic?

Thanks!

[SteveC]

Hi biglolly,

If a camera has a FLM of 1.6, the lens on the camera will have an angle
of view equivalent to a lens with 1.6x its focal length on a 35mm camera.

So the angle of view on a dslr camera with a 35mm lens has an angle of
view equivalent to a 56mm lens on a 35mm camera, 1.6x35=56mm.

So to replicate the field of view of a 35mm lens on a 35mm camera, you
would need to set you lens to approx 21mm on a dslr camera, 35/1.6=21.87mm

The focal length does'nt actually change, just the apparent angle of view
because of the cropping effect of the smaller image area on the dslr
cameras image sensor, I think this is why when you take a photo on a dslr
camera using a 35mm lens it looks different to the same photo taken with
a 35mm lens on a 35mm camera, the dslr photo is 35mm, but its cropped
because of the smaller image sensor, so to get the same image as the
35mm camera you need to use your FLM to get the same field of view.

I think this is right, somebody correct me if i'm wrong

steve

[GALACTUS]

Hey

I found this...

http://www.cgtechniques.com/goodies/doftoy/

But the interesting bit was this quote "I also have to point to a 3dsmax problem:
3dsmax calculates the lens from the horizontal Fov value, but with the wrong aspect ratio!!!
I now understand that 3dsmax will never be able to match a real photography with it's maxcamera.

Please understand that the formular to calculate the Fov uses the aspectratio of a 36mm slide which is 1.5 and NOT 1.33.

-> have a look at the render outputsettings when selecting a 36mm slide film and you will see on the right that they don't use 36mm for something they call "Aperature Size" . Using "Aperature size" is one joke , but then using the wrong value is really annoying!
anyway ..it's a blemish bug of a 5000euro Software! "

Is the above accurate?

Colin

[biglolly]

Cheers for the responses guys. I'm actually a bit more confused now because of the so called bug in max.

I understand the effect the FLM has on an image becuase I've seen say a 50mm f/1.8 lens on a Canon 5D and then on a 20D. The 20D one looks like it's a bit more 'zoomed in' but in reality it's just a cropped version of the 5D image (I think!).

So if I took an image on a 5D at 80mm and put an 50mm lens on a 20D would both images look identical? Would the perspective lines match exactly?

Anyhoo, it's the max thing I'm most concerned about. What I've done in the past is set the render size in max to match that of my 350d image output. Then I bring an image in to the viewport background, set the max camera for the focal length of the image, move the camera to roughly where it should be in reality. Is there anything special I should be doing in max to make sure the 3d camera and actual camera match up?

[dapeter]

That's how the focal length multiplier works - a 50mm (as marked) lens on your camera is actually 50x1.6 = 80mm. I'd also set the render resolution to be the actual pixel size image your camera took, then lock the image aspect ratio and change it if you want to do previews. It gets even more confusing when trying to match to a DV camcorder, the sensor is so small the focal length multiplier becomes very large.

Also, keep in mind the Max camera is a pinhole camera - if you want to match it up as close as possible you'd have to shoot a grid pattern with your digital camera and determine the lens distortion, then apply this distortion to the image itself. It also helps to have some sort of measurement to know approximately where the camera is in the real world (laser distancemeters work great for this).

[biglolly]

You mean like barrell/pincushion distortion? I have software that corrects this and in fact the 10-22 lens that I use so frequently barely suffers from this.

I've done all what you said, locking the aspect and such but still haven't found it to be accurate. I probably need to do some more tests. I found the camera to be too zoomed in I think.

Will experiment some more when I get a chance.

Thanks.

[joconnell]

Personally I haven't figured it out yet and I do a tonne of matchmoving stuff for vfx - get yourself a box that you can bring to the shoot, measure it and shoot that - you can optically match the perspective quicker than trying to calculate all of the factors (though personally I'd love a really accurate way of finding it out.)

[andyp7]

As above reply suggests I normally go to site with a tape measure and measuring wheel. I make a few real world measurments of curbs, pavements and walls with a small diagram. I find this the best tool for matching the camera, I would also like an accurate way to calculate it but I think there will always be an element of "eyeing it in".

[biglolly]

Cheers guys that sounds like sound advice.

Jconnell, what kind of matchmoving stuff do you do? I'm very interested in that kind of work, but there is so little of it being done in Northern Ireland to my knowledge.

We're thinking about using matchmover or boujou at work, to track HD footage so we can composite new buildings e.t.c on top of the footage instead of the same old photomontages.

[dellis]

Check out Syntheyes...way cheaper and just as capable!!!

[Clifton Santiago]

I don't do many accurate planning renderings, but a company that specializes in Verifiable Planning Renderings (or something like that), came and gave a presentation at our office one day. They do not try and match lens size, but instead they combine the site photography (medium format camera) with a site survey. They have like 20 or 30 measured reference points in the photo and they just adjust the Max camera until all lines up.

[joconnell]

Cheers guys that sounds like sound advice.

Jconnell, what kind of matchmoving stuff do you do? I'm very interested in that kind of work, but there is so little of it being done in Northern Ireland to my knowledge.

I'm working in screenscene in dublin and we do a lot of live action compositing jobs. Syntheyes is a really good app for tracking - fast an powerful and a fraction of the cost of the others.

[tricky]

Get a surveyor to measure exactly where your camera with respect to some real world object that you also have 3d information for.

(also, the crop multiplier is never exactly what it says it is - eg, it may say 1.6 but it could be 1.54, 1.57, 1.64 etc etc.

[dellis]

The new version of Syntheyes can import point data from a total station survey and use it in its tracking solution.