ZBrushCentral

Perspective focal length: a little confused

ZBrush Usage Questions
#1

Ok, I have photos I shot with a lens at 200mm, far away of a head. I set the perspective value to 180, as it can not go to 200. At 180mm in zbrush, it acts as if its an extreme wide angle. :confused:

Being a 200mm photo I can go with orthographic fine Iā€™m sure, but this is a little confusing to me. When I go to 5mm, it gets closer to orthographic. Is this a bug? clarification would be great. Iā€™m running 3.12 mac. LOVE this program besides the many bugs. Its expected since its a brand new version and its a 3d program, they are all buggy to some extent.

#2

"A side effect of using lenses of different focal lengths is the different distances from which a subject can be framed, resulting in a different perspective. Photographs can be taken of a person stretching out a hand with a wideangle, a normal lens, and a telephoto, which contain exactly the same image size by changing the distance from the subject. But the perspective will be different. With the wideangle, the hands will be exaggeratedly large relative to the head. As the focal length increases, the emphasis on the outstretched hand decreases."

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

-Dustin

#3

Not sure what the math is behind ZBā€™s virtual camera, but I doubt the settings in the focal length parameter directly correspond to a real cameraā€™s focal length. To match CG cameras to shots produced by real cameras youā€™d need a lot more info that just a focal length. A very important number for instance is the size of the film plane (meaning the film or sensor size). And as there is no way to input that in ZBrush, trying to match camera settings accurately is impossible. Youā€™ll have to eyeball it instead. Donā€™t worryā€¦ itā€™s good to train the eye that way. :slight_smile:

#4

Very true about the sensor size. I am using a DX lens, and from my understanding is the millimeter reading on the lens is in 35 mm format. So 18mm on a DX lens would be the same as 18mm on a 35mm lens, but you would need a different lens. I hope that makes sense.

But thats not quite my question. It seems that a larger millimeter in zbrush introduces more perspective distortion, which is the opposite of the way a real camera lens works. I guess I will just eyeball it like you said lone deranger. I was just wondering if there was any reason it was reversed. Since 200 mm is pretty close to orthographic, it shouldnā€™t be too hard to eyeball, I might even be able to leave it as orthographic since thats the way my image planes are lined up.

Originally I had something like 30 mm when I took the photo, and I just couldnā€™t get it looking right. NOw with the new 200mm photos, I may just add a tad bit of distortion. Maybe 10mm in zbrush units, since it seems to be reversed.

Thanks for the replies.

#5

ragonamuffin - The slider in the same location in ZB 3.1 was labeled ā€œFocal Lengthā€, which is what you are referring to in respect to real world lenses.

In ZB 3.5 rX the slider is now labeled ā€œFocal Angleā€ which I am guessing means that it sets the viewing cone to the specified value. So, 5 is a very narrow cone and would result in a near orthographic view ( 200mm lens ), while 180 lets you see half a sphere ( 5mm lens ).

What is misleading is that the slider is in the same spot as the last version and the label text is close enough that we could easily gloss over the change. I did. Also the help text would indicate the opposite, it even uses the term ā€œfocal lengthā€.

focal_angle.jpg

#6

The slider (on 3.2 at least) behaves exactly opposite from what would be expected in a real world zoom lens. With a real lens, the higher the focal length, the more compressed the foreshortening effect of perspective, the shorter the focal length, the more pronounced the effect of perspective. In ZBrush it works counter to reality. Sounds like theyā€™ve relabeled the slider in 3.5 to make it a little less nonsensical. Too bad they couldnā€™t have corrected the problem so that we could have some sort of real-world analog.

#7

Datto - I agree it is confusing and that is what led me to search if anyone else was confused as well.

In ZB 3.5 r2 the slider was labeled ā€œFocal Lengthā€, but it was the opposite of what you would expect. In ZB 3.5 r3 it is now labeled ā€œFocal Angleā€ with lower numbers approacing an orthographic view, while larger numbers are a wide angle. Both of the ZB 3.5 versions are opposite of how it behaved in ZB 3.1.

The confusion for me lies in the help text that has not changed from ZB 3.1. Nor has the online documentation.

If you look at the ā€œWhatā€™s new in ZBrush 3.5ā€ web page, the Perspective page mentions focal length.

The slider no longer sets a focal length, it sets it indirectly.