Same focal lenght - identical image in different software?

[linvanchene]

The basic rule of compositing is:

Use the same focal lenght in all software to composite an image.

Example:
If you create the background in software A and set the focal lenght at 35mm you need to set the focal lenght at 35mm as well when you render the foreground object in another software B.


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So far I always assumed that there would be some universal standards around that make sure that the cameras we import and export would always yield exactly the same image in different computer graphic software.

I believed with specifications like Collada or Alembic there would be quite specific rules how camera values should be calculated. I assume the same may be true for .ORBX


Example:

If you create a scene in 3DS MAX and then export an ORBX to Cinema4D will the scene really look exactly the same?

Could it be that the rules which data is sent to other applications may be fixed but the way each software then interprets those values and transforms them into a scene might be quite different leading to different results?

Are really all software applications that are supported by OctaneRender plugins using the same methods to calcualate focal lenght?

Or could it be that some applications use different focal lenght values that are just an approximation of real world cameras?

[riggles]

Interestingly, if you look in Octane Standalone, you won't find a focal length setting. It's defined by FOV (field of view) which is an angle measured in degrees. So presumably, it's this same value that being used internally in various plugins and being carried over through intermediate formats. Besides that, focal length values are derived by a simple calculation, so there isn't a need for approximation.

The biggest challenge to getting consistent output between apps is how the film gate is handled. That will mess with your camera for sure if you're not careful. In the MODO plugin, for example, the default film gate doesn't work the properly with Octane. So if my resolution aspect ratio and my film gate ratio don't match, I will get different results between my render output and what I see in the camera viewport.

[linvanchene]

Interestingly, if you look in Octane Standalone, you won't find a focal length setting. It's defined by FOV (field of view) which is an angle measured in degrees. So presumably, it's this same value that being used internally in various plugins and being carried over through intermediate formats. Besides that, focal length values are derived by a simple calculation, so there isn't a need for approximation.

The more I try to find out about focal lenght and how it is handled in different software the more differences pop up.

I found a converter online that transforms focal lenght to FOV.

http://www.bdimitrov.de/kmp/technology/fov.html

I can see that for computer graphics maybe FOV values have some use.
But when working with real life footage it may be helpful to work directly with focal lenght values without needing to convert them first.

The biggest challenge to getting consistent output between apps is how the film gate is handled. That will mess with your camera for sure if you're not careful. In the MODO plugin, for example, the default film gate doesn't work the properly with Octane. So if my resolution aspect ratio and my film gate ratio don't match, I will get different results between my render output and what I see in the camera viewport.

Does this work the same in other software?


Additonal information:

In real life one major point to consider when comparing focal lenghts of lenses is the sensor size and its resulting crop factor.

The focal lenght values in photography and film are be based on a 35mm camera setup.
Most professional cameras use "Full frame" 35mm equivalent sensors.

http://en.wikipedia.org/wiki/35_mm_equi ... cal_length

Nevertheless consumer level cameras often use smaller APS-C sensors.

http://en.wikipedia.org/wiki/APS-C

Depending on different sensor sizes you end up with a crop factor.
One of the most used crop factors is 1.5.
But some brands may use other factors on smaller sensors.

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

Updated / Edit:

I simply assumed and hoped (!) that all computer graphics software is working at a "Full frame" 35mm equivalent focal lenght.

Can anyone confirm or deny this for OctaneRender standalone?

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Can anyone share some issues they encountered when comparing images of the same scene rendered in different software with things just not matching up properly when trying to composite?

In theory with the .ORBX format it now should be easy to create a test scene in one software and then export it with .ORBX and render it in other software to compare differences.

If all works the perspective and the edges should all match up.

[riggles]

Octane doesn't use a film gate or camera back, which is why it also doesn't use focal length. It just uses FOV, which is a fixed value that operates independent of sensor size.

Are you having trouble with Octane renders from different apps not lining up?

[linvanchene]

Octane doesn't use a film gate or camera back, which is why it also doesn't use focal length. It just uses FOV, which is a fixed value that operates independent of sensor size.

Are you having trouble with Octane renders from different apps not lining up?

I am having trouble with a specific software that unfortunately seems to have connected the focal lenght values to the render aspect ratio values.

I will not name the application until they had time to investigate the issue and then come up with an explication if this was just a bug introduced in an update or a more serious issue that is tied in deeply with how all camera values are combined to calculate the viewport.

Examples:
With a render aspect ratio set to 2:1 a 50mm focal length is effectively reduced to 31.59mm in the viewport.
The user choose 50mm in the menu but the software outputs this as 31.59mm in the viewport.

Extreme case:
Focal Lenght of 50mm and a Render Aspect Ratio of 1:10 results in the viewport displaying an actual aspect ratio equaling of 5.53mm


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Because the OctaneRender plugin uses the values sent by the original application this means also the image in OctaneRender plugin viewport is completly distorted and not displaying the focal lenght the user originally choose.

IF only one plugin in the pipeline sends in unrealistic Camera values with ORBX this error will then be imported into the standalone or any other plugins down the compositing pipeline.


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So basically the goal of this thread is to get an understanding how widespread it is in the CG industry that some companies even in 2014 are still using only estimates of real world camera behavior that is not suitable for accurate compositing work.

To phrase it in a positive way:

I am trying to find Computer Graphics software applications that let me choose a lot of realistic real world camera properties so I am able to composite images based on real world camera values.

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But the longer I think about this the more I get the impression that this is only one small aspect of a larger one:

In order for the .ORBX standard to be fully useable finally all companies would need to work together to agree on more common standards how a virtual scene is set up.

The way the Focal length and other camera values are calculated is only the tip of the iceberg. Think of the orientation of x,y,z ; unit lenght; flipped UV coordiantes...

[riggles]

I am having trouble with a specific software that unfortunately seems to have connected the focal lenght values to the render aspect ratio values.

I will not name the application until they had time to investigate the issue and then come up with an explication if this was just a bug introduced in an update or a more serious issue that is tied in deeply with how all camera values are combined to calculate the viewport.

Examples:
With a render aspect ratio set to 2:1 a 50mm focal length is effectively reduced to 31.59mm in the viewport.
The user choose 50mm in the menu but the software outputs this as 31.59mm in the viewport.

Extreme case:
Focal Lenght of 50mm and a Render Aspect Ratio of 1:10 results in the viewport displaying an actual aspect ratio equaling of 5.53mm

Well it's hard to know for sure what's going on if you don't name the application. But if it's one of the big DCC app plugins, then you might be running into the film back/film gate issue I've been talking about. It's not the easiest subject to wrap your head around, so it would be worthwhile to research which film gate fit types are supported and how they work.

But basically, if your render resolution has an aspect ratio of 2:1 but your film back dimensions do not (standard full-frame DSLR is 3:2) then something is going to be altered. If your film gate fit is vertical, then the vertical FOV will be used. If your film gate fit is set to horizontal, the horizontal FOV will be used. This appears to zoom your camera in or out, even thought the actual focal length value has stayed the same. The greater the difference between the two aspect ratios, the more pronounced this effect will be.

[Vue2Octane]

good point