OTOY Forums

I got a lot of work in the last weeks. Photographers sent me their compositings and wanted me to create 3d objects for their scene. Like for example glasses, tables, chairs, windows and so on!

When i get their pictures i can see with which focal lengts and aperture it was made. In other render engines it is very simple. You take the same aperture and focal length, like the customers picture and the render fits absolutely perfectly in the made photo!

In octane there are only strange numbers and its a guess play until the aperture and the focal lengts and the perspective fits as good as possible in the scene.

So my question is - how do the numbers work and how can i make sure that my render has, for example, aperture of 8 and a focal lengts of 85mm? How is that possible?

Thanks for your help -

Regards Chris!

angle of 85mm lenve mounted on full frame should be between 28 or 29 degrees.
I don't know exact math, but You can check this on any lens site =)..

as for aperture as far as I remember the number is not written like in other engines..
if You want to get let's say 1/4 in octane You have to write not 4, but 0,25..

haven't used Octane for a while, 'cos sitting on iPad only =) so the thing about aperture,
might be not correct..& can not check that, but it's an idea to try for You.

cheers

Hi Glimpse!

Thank you very much for your answer.

So you mean that the numbers in the FOV slider, in your example 28/29, are the degrees which the "lens" should display in the real life?

That can not be correct. i tried it with FOV 1, 100 and 180 - please have a look at the pictures i hang on!
If a lens can make a picture of 180 degrees, for axample a fisheye objective the picture must look like this:

http://www.google.at/imgres?imgurl=http ... DQ&dur=823

I also made 6 pictures here in my shop. 16mm, 24mm, 60mm, 85mm, 200mm and 400mm. All made on a Fullframe 5DmkIII DSLR Camera and calculated how much "angle" the Focal length should display. For example 24mm display 84 degrees on a full frame camera. I entered the same numbers in Octane and the results were completely different. The perspective was completely different and the dissortion too. Try it by yourself. Look in google on a 24mm picture and enter 84 in octane. The results are very different.

I then entered the same values in the render engine Arion. I used the same scene. The results matched nearly perfectly.

So - what do these numbers in FOV and aperture really mean and how shall they be used if i know the aperture and focal length of a DSLR Full frame made picture?

Regards Chris!

well, I might be wrong, but I believe because of the reason, that Octane render curently has no option to make spherical and "curved plane" images that FieldOfView parameter just gives ability to regulate angle of perspective - this has conection, but.. because Octane doesn't take into acount other lens parameters You will not get what You have in real world. so in this case You probably need to cheat a bit =)

Hi Glimpse!

I made a view samples to make more clear what i mean!

I found on wikipedia a great table, where the angle is correctly calculated to every focal length for a Full Frame Sensor. It is the picture 2:

Also on the same side is a good example, that shows how focal length (not focal depth - focal LENGTH) affects the perspective. It is picture 21:

So i quickly drawed these 2 bottles in Rhino and made a quick render for comparison. In picture 22 you can see the comparison between the picture made with 18mm and how octane shows the perspective at 18mm=100 on the FOV slider. Here you can perfectly see what i mean. I tryed to match the scene as good as possible as on the picture, but it looks completely different. I allways added the picture in the render view, so that all of you have a direct side by side comparison!

23 shows the same at 34mm focal length. 34mm=65 on the FOV slider:

and last but not least the same with 55mm. 55mm=43 on the FOV slider:


As you can clearly see in all 3 pictures, it looks completely different than it "should" be. So how can i make a picture that looks liek it should be? I need that cause of the many renders i have to do for the compositings for the other photographers. I really start getting desperated

Regards Chris!

hm. for my plugin i need to calculate the octane fov from another camera model, which works ~ like a real camera. if i have focal lengths of 18/34/55mm, the output angles are 111.1/75.3/51.0, given an image aspect ratio of 3:2.

since the original camera model uses the vertical axis, whereas octane uses the horizontal axis to define the field of view, the calculation needs to also take the aspect ratio into account. and the aspect ratio needs to be the same for both models of course. don't know what the system axis for a dslr is, but this is clearly important.

apart from that, the images you did in octane are indeed extremely distorted. my guess is, your scene scale is wrong. using those angles i only get such a distortion if my objects are tiny - a few millimeters "tall"...

[edit]: looking at the numbers of your camera position (and target) i'm pretty sure, that the scale is wrong - since those numbers numbers are in the millimeter range... your camera is between ~ 2mm and ~ 4mm above the ground, what apparently doesn't match the photos...

Hi t_3!

Thanks for your answer!

I modeled the objects in Rhino in mm and then imported them in octane - also in mm! In my opinion there should be no difference, cause if i photograph a jewelry with 15mm of size with 18 mm focal length, it looks as distorted like a building that is 20 meter high, when i make a full frame 28x36mm picture at 18mm. Only DOF is affected which is a question of aperture and not of focal length. But to make a direct comparison and to be sure, i scaled my objects up to 280mm which is about the size of the bottles should be in real life.

I made an object and put it in octane again and the results were the same. Try it yourself. I added the scene as an obj with the 280mm bottles. Try to get the picture and the perspective like in the photo 21 in the earilier post. I really hope you reach the goal and tell me how you made it! I am desperated here!

Also......how did you get this strange numbers? 111 and so on. Which plug in do you use? When i google and calculate the Full Frame 28x36mm Sensor at 18mm it gives me 99,34° angle, not 111,1. But nevertheless - 99 and 111 still make not much difference.

Please try it and tell me whats the problem here.

Thanks for your help - Chris!

Your last renders look right. Tilt the camera up and down a bit and you can see the perspective change accordingly. If the camera view direction is fully horizontal the perspective will be the same as in the pictures.

--
Roeland

in software like maya you can set fov in degrees or mm, just choose what you prefer

Not sure I've caught everything in this thread - however....

1) Octane fov is horizontal (not vertical)

2) In the Poser plugin, to get "fov" from the focal length, I used Paul