Correct way to set up camera?

#1

Hello,

What’s the correct way to set up camera properties if I want to mimic a real-world camera?

Mainly, I’m wondering about ccd/lens aperature - if my camera has an APS-C ccd, then should I use the aperature from EXIF or multiply that by 1.6?

My camera is an EOS 300D, with a ccd size of 22.7 mm × 15.1 mm and the lens I used for the pic is a Sigma 17-70mm, according to exif at 28mm.

I tried to make a camera in XSI with those values, focal lenght and film aperature (in inches of course) and it’s pretty close, but not a complete match. (I have the image as a rotoscope and try to match my model to the image)

What about optical shift? I left that alone for now.

Thanks!

#2

I don’t have a concrete answer for you but Softimage units are in centimeters not inches, I don’t know how this affect the Film Aperture (inches) setting. My only actual suggestion is setting up a simple test with your real world camera, measuring the distance to your subject with a tape measure, as well as the height/width of your subject, and rotoscoping the result into Softimage and proceed from there recreating the scene with some valuable real world data on hand. That way, if there is a difference between your real world setup and the 3D recreation of it, you will get a feel for the variation after a few tests and should be good to go.

#3

How close is it? It may not be possible (in an easy way) to get it 100% match in xsi because you would have to correct for lens distortion, which is different for lenses at various angles and distances, and may be significant. If your lens is supported by Canon’s “Digital Photo Professional” then you can correct your distortions in there.

#4

Jettatore,

I was a little surprised when I saw the film aperture setting use inches, too, but that’s how it is in the dialog. My native units are meters, so I converted the censor size from millimeters to inches and used that.

Mic_Ma,

Yes, I learned a little more about this lens distraction after I posted :slight_smile: Apparently this sort of thing cannot be done directly, I need to check out that Canon thingy you mentioned. I have 2 lenses currently, maybe one of them is easier to “fix” for this.

Oh, and here’s a capture of my shot at this:

Thanks to both of you!

#5

Hugin can also de-distort images but it may be a little more fiddly.

#6

See if this RealLens free plugin helps:

http://www.xsibase.com/tools/plugins.php?detail=1433

[I]RealLens

This plugin allows you to convert the angle of view from a real camera and lens system to an XSI camera and vice versa.
[/I]

I wont fix Lens Distortion because like Mic_ma said each Lens is unique, but its a help.

#7

Lens distortion doesn’t really change with distance, only with focal length.
There are several ways to address it, applications like boujou and other tracking softwares have tools to do it too, as do several vendor tools like those from Canon.
If you have a lens you use often it’s good practice to take a picture of a squares grid and checkerboard to undistort, and then save the undistortion parameters.

To reduce distortion you can use a long lense from the distance. If you push it too much (very far with a super zoom) you can incur in a pinch distortion, but that’s pretty extreme.

For reference pictures shooting with the longest lense you can from the distance that gives you the framing you need is the best you can do to reduce barrelling, if you shoot something close up with a fish-eye of course even the right gating and all the right settings in the world will not pull off a close match in 3Dworld’s undistorted lensing space :slight_smile:

#8

Oh ok, I wasn’t sure about that. The Canon software has a slider to adjust for distance when removing distortions. Perhaps it’s related to chromatic effects or something else.

#9

Camera ( real one ) got in their spec focal lentgh ( coming from the lens ) expressed in mm while the film back is in inch. That may change for Digital camera but for reel one they are expressed like this. ( Having a looking into the armerican cinamatogrpaher.

I don’t know why but I guess to remove any confusion between foacal length and film back.

If you use an Canon 300D ( like me ) you will notice in the EXIF you got 2 focale length. The first one is the actual one the second one is the what will be the focale lentgh if you were using a 35mm film.

So to setup in XSI, you use 22.7 x 15.1 mm ( 0.894 x 0.594 inches ) and your focale length of 28 mm

Now like Jacon mentioned you have to deal with lens distortion. You need to remove it from your plate. There is a few standalone that correct them for you accurately.
Have a look to ptLens or DXO labs.

Never never model something from a pic without correcting the lens distortion otherwise you will end up with discrepency in your model.

#10

There are aesthetics parameters that Canon undistortion software wants to serve, part of it is for things like beauty photography/portraits, and to try and keep framing and distributing things so that you don’t end up with underscan in your frame.

The general idea of undistortion for CG though is solely lens based, otherwise undistorting a plate when the shot has a moving camera and objects in it are at different distances over time would be impossible withouth a camera beacon and a lidar scan of the set/environment :slight_smile:

Barrelling/pinching is due to the curvature of the lens, which with a constant surface size and film back gives you a direct relationship to focal length, and that’s is what ends up being used as a metre since it comes in convenient and digestible numbers.

That’s also why different filmbacks, sizes, gates and all make focal lengths look so different between different types of camera. It’s a combination of those factors determining how curved the lens will need to be (and hence the distortion it will cause), not just the focal.

For a 18mm length to fully cover a 35mm gate set to a certain distance from the mount the lens will be curved a certain way (very curved, fish-eye in standard 35mm SLR), but if you change that to a much larger area with a gate set practically level with the mount, 18mm can accomodate a much flatter lens which will distort less.

#11

Saturn,

I don’t know how you dug up that second focal length. I use Photoshop Elements and I can only see one, which is 28mm. For the film back I used those inch-values and just assumed this will “fix” the crop factor (1.6).

And worry not, I didn’t use this picture as a reference :slight_smile: The fender is part of my ongoing car project, but I use “regular” rotoscopes for that - drawings, that is.

JacO,

Do you think I would be better of starting with a longer focal length, then? As to have less distortion to be programmatically fixed?

I have the Sigma 17-70 used for this and a Canon 70-300, but I guess 300 would be pretty much off too?

Gotta try those programs mentioned, this is a totally new science for me.

Thanks to everyone for your answers!

#12

I can’t seem to get my hands on the Canon’s Digital Photo Professional. I don’t know where my original cd for the 300d is and they only give updates, dang.

Is there any other software? Free? Easy? Boujou didn’t seem to be either :slight_smile:

#13

Usually the file EXIF will tell you the Lens Focal used (28mm), but this 28 mm are for the 35mm sized Film or Full Frame in Digital terms, since you are using a croped sensor (APS-C) this 28mm are not really like the Full Frame 28 mm.

You need to multiply this 28mm (or the Focal Lenght your Lens is telling you) by 1.6, and 28*1.6=44.8mm, so efectively its like if you used this 44.8mm Focal Lenght on a Full Frame camera or on a 35mm Film Camera.

Some Canon Lenses are already coming with that information on their lenses, something like:

70-200mm that equals to 122-320mm

So it is like this, using your Lens of 17-70mm on a Full Frame sensor camera you would get the 17-70mm, but since you are using it on a Croped Sensor camera this Focal Lenght will be the equivalent of a 27.2-112mm Lens.

Hope I didnt confused you more hehehe

#14

Thanks,

Nope, no more confusion there :slight_smile: I just thought the film back measures would some how fix that automatically. Anyways, I got to find a way to fix the distortion first…

#15

As I said, Hugin should be able to do it too.

http://hugin.sourceforge.net/tutorials/index.shtml

They have lens files as well somewhere and if you’re lucky your lens might just be listed.

Edit: Here’s an article with software links (DXO has a 2 week trial version, Gimp and PTlens are free) :

http://en.wikipedia.org/wiki/Image_distortion

#16

Thanks, I will check those out. I should try to get familiar with Gimp anyway :slight_smile:

#17

Yes, ideally you should.
In film you have to deal with undistortion because the camera and lens choices are dictated by storytelling and photography, so you get what looks good, you undistort, work on it, and then distort the final result back to the original or to something close to it that looks good.
When you’re shooting reference photography that aims to be ortographic (or anyway without distortion) for single objects, so you have no stage compression artistic issues to deal with, you’ll get better results starting far off with a long focal.

When and if you get pincushioning or moustache (mix of barrelling in the middle and pincushion outward) depends from many things, including the range of the lanse.

On a decent 70-300 going in the 200s would net you a pretty decent result.
A lot of building front icon photography is shot within the 210-250 range.

#18

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