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MaDSheeP
04-08-2004, 04:35 PM
Hey everyone, This question covers a few applications so I wasn't sure what forum to toss this post in, so any mods, feel free to move this if you like

ok, here is my question, for a class assignment, we need to take a picture and in 3ds max, match the focal length of the picture that was taken with a free camera, and then build geometry that will cause our character to appear to interact, or be standing behind the objects...

To find the focal length we use the photoshop file browser, and when you select a picture from a digital camera, it says the focal length, but many, if not all of the students, are getting the incorrect focal length numbers...

So does anyone by chance know what is causing photoshop to report the wrong focal length?

and does anyone know of a third party application that can report focal length from a photograph?

Thanks for any help :)

stephen2002
04-08-2004, 04:58 PM
What kind of camera are you using? Is everybody using the same camera?

I didn't even know that the focal lenght would get recorded into the file. That's a pretty cool feature. I took a look at some of the photos I took with my Sony MiniDV camera and for the close-up shots the focal length looks like it is recorded correctly. However the stuff that was shot at a great distance, like a landscape, the number is still in the mm range. Mabye I don't really understand what that number represents because it seems pretty random as I look through my pictures.

Shots taken with my old point-and-shoot HP camera don't have any of that extra data.

MaDSheeP
04-08-2004, 05:06 PM
we are all using the cameras that we bought for our digital photo class, top brands, canon G3s, G5s, Nikons, a few sonys...

and almost everyone is having problems =(

aparently one of the students last year found a third party application that correctly extracted the focal length... :shrug: :)

beaker
04-08-2004, 06:28 PM
This is because 3dsmax is probably guessing that your camera is 35mm. The camera's film back is different on digital cameras then on 35mm. Your FOV is going to change depending on the film back. You each need to find out the actual dimentions of the ccd in the camera in order to set it up properly in 3dsmax. To make thinks even worse there is the ccd dimentions and then there is the actual part of the ccd that is used(95-65% the size of the ccd depending on it). Also some are square, others are rectangular. Different camera makers use different size ccd's too. So you guys have your work cut out for you.

bartrobinson
08-14-2004, 06:01 AM
I didn't even know that the focal lenght would get recorded into the file. That's a pretty cool feature. I took a look at some of the photos I took with my Sony MiniDV camera and for the close-up shots the focal length looks like it is recorded correctly. However the stuff that was shot at a great distance, like a landscape, the number is still in the mm range. Mabye I don't really understand what that number represents because it seems pretty random as I look through my pictures.

The focal length is not the focus distance. The focal length is usually the length from the lens to the film back or CCD measured in millimeters. Most newer digital still cameras record this into EXIF data which can be read by many imaging programs. The problem of the 3D Studio Max camera not matching the real camera is probably like Beaker said, because Max expects a 35mm film back based focal length and isn't getting one.

Now, you can probably change that, but I have additional questions and thoughts. Isn't there a conversion formula? Also, to solve poor accuracy in CCD size and how many pixels from the entire CCD are being used can't someone calibrate the camera using a chart, measuring the chart and distance from chart to CCD in order to triangulate the size of the active CCD?

Simon
08-14-2004, 06:19 PM
You should be able to use a formula is you can find you ccd size. Unless you have a top of the range 14 megapixel camera.

However thats would be in a perfect world everythings done in equivalents. Also different lenses all produce different images even though they should be the same.

Theres no total fix im afraid. But you could photo some big cube, then model is and try and make an identical image. Using that u can find your focal length I suppose.

To be fair I match my composites by eye anyway.

simon

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