photomatch - translating a compact digital camera into vray physical camera

[skogskalle]

Hi guys n gals,

im currently doing a photomontage of a building and Im having some trouble with the physical cam settings. (i know there are a few threads on this subject, but they didnt help me so here we go)

First of all, the photos were taken with a small compact camera and not a fancy SLR cam. Will this be a problem?

I know the exact location where the photos were taken, in relation to the new building etc, but I just dont know what values to use in my physical camera to get the correct perspective.

When i look at the cam-info in photoshop it gives me this:

Model: Canon DIGITAL IXUS 750
F-stop: f/7,1
Aperture Value: f/7,0
Max aperture value: f/2,7
Focal length: 7,7mm
Lens: .... nothing ...

When I look up the ixus 750 on the web I see that the lens is : 37-111mm (no zoom was used) F2,8-F4,9

So now what. How does the values above translate into focal length(mm) and film gate(mm) in my physical camera??

If all else failes (if the info above just isnt enough) Ill try to match it by eye. It doesnt have to be perfect so no biggie, but it would be nice to learn this stuff...

Thank you very much in advance!
K

[stef.thomas]

Here's a page to bookmark that shows focal length multipliers for a whole lot of cameras. http://jalbum.net/forum/servlet/Jive...ras.properties

Going by the value for yours the focal length of 7.7mm equates to 37.28mm in max, assuming your film gate has been left at the default 36mm. You don't need to worry so much about the other values, but the EXIF data should also give you the time of day which can be handy for setting up a daylight system.

[skogskalle]

Thanks!

the link doesnt work, could you please check it?

Ill try using 37,28mm and see how it goes...

[stef.thomas]

Thanks!

the link doesnt work, could you please check it?

Ill try using 37,28mm and see how it goes...

Hmm, works ok for me.

[stef.thomas]

I've copied and pasted the data into a notepad *.txt file. Had to zip it up though as it exceeded the file size limit for the forum.

[skogskalle]

thanks again - I will take a look

[skogskalle]

hmmm it seems to work. But why should the film gate be set to 36? Is this always the case with simpler cameras, or did you come up with that from looking at the exif-data?

really appreciate your help!
k

[stef.thomas]

Film gate refers to the width of the sensor in the camera. The focal length multiplier found in the list I posted allows you to calculate the focal length in it's 35mm equivalent which is the standard found in film cameras. The width of a standard full frame digital sensor is 36mm which is why this is the default value for the Vray camera.

Alternatively you could keep your focal length set to 7.7mm and adjust the film gate to the width of the sensor found in your camera and this will achieve the same result. Most people are more comfortable working in 35mm equivalent though as these values are more commonly understood in terms of what's a wide angle or telephoto lens.

[skogskalle]

cool - it all makes a little more sense now Thanks again!

[Dave_Wortley]

You need to adjust your focal length if you're using the camera in Portrait orientation, I did write a post on cgtalk about it if you're interested.

[skogskalle]

sure - Ill see if I can find it!
thanks
/k

[Dave_Wortley]

http://forums.cgsociety.org/showthread.php?t=854243

Found it....

If you're trying to perform a camera match on a standard 35mm camera and the picture has been shot in portrait rather than landscape, here's an easy tip to allow you to find out what lens value you need to use within 3dsmax to match this.

If you create a camera and set your settings within the render dialog to be portrait proportions say 1024x1536, then if you set your camera aperture width to be 35mm and then set your camera to be 35mm you'll notice that your camera is wrong, this is because it's a portrait camera the FOV angle of the width of a portrait shot is different to the FOV angle of a landscape shot.

To calculate what it should be you can use Michael Breidt's excellent camera overscan script... http://scripts.breidt.net/
Set your camera to 35mm and set the render dialog settings to landscape, so 1536x1024, close the dialog box (important for the script to work propperly) run the script and put in 1024x1536, you'll now have a lens of something like 52mm which is the portrait equivalent lens for a 35mm landscape lens.

I'm pretty sure this is the only way to sort this issue, but let me know if it's covered elsewhere

[skogskalle]

Thanks Dave!

I only used landscape shots on my project last week, but this is great info for future projects...

[rikou]

[QUOTE=stef.thomas;429796]Film gate refers to the width of the sensor in the camera. The focal length multiplier found in the list I posted allows you to calculate the focal length in it's 35mm equivalent which is the standard found in film cameras. The width of a standard full frame digital sensor is 36mm which is why this is the default value for the Vray camera.

Hi,
just want to be sure to understand what you mean.
If I check your list, it's for "the computaion of the 35 mm equivalent focus"

You said : "Going by the value for yours the focal length of 7.7mm equates to 37.28mm in max, assuming your film gate has been left at the default 36mm"
So ? Do I have to change your film gate to 35mm in you vraycam ? Can you enlighten me please ? Thx.

[stef.thomas]

Film gate should be left at 36mm, which is the width of a full-frame digital sensor and 35mm film image. In actual fact 35mm is the width of the film but this covers the short width of the image plus the sprockets. The recorded image is 36x24mm and lies along the length of the film. Slightly confusing, I know!

Some references: http://en.wikipedia.org/wiki/135_film http://en.wikipedia.org/wiki/Full-frame_digital_SLR