Focal length changes exposure. And camera does crazy stuff...

[greymiura]

Every now and then I run into issues with my Physical Camera and I just can't figure out what this is...

I created this scene from scratch in R25.117 (Win 10) and with the newest version of V-Ray (v05.20.02).

https://we.tl/t-YEIJkU15jr

It' s very basic and uses just default settings: Two primitives with a V-Ray material, a V-Ray Sun and Sky and a Physical Camera. The Camera Type is Standard, Auto Exposure is deactivated (same with DOF, Auto White Balance and Override FOV), the only thing I changed was turning off the Physical Camera's vignetting.

If I render the scene with a focal length of 400 mm and Specify Field of View deactivated, I get a very bright image that is rotated 180° (image 01)

Activating Specify Field of View fixes the orientation and the image gets way darker (image 02). And if you change the focal length the exposure changes once again (image 03).

I didn't dig deeper into it, but I always thought that the focal length isn't supposed to change the exposure value, is it?

[shaio]

Hey greymiura ,

The inverted image is a result of the physically based properties of the camera where usually the focal length is connected to the field of view just like in the real world lenses. Focal length of 400 is a too high value for having objects with such small size (10cm in your case) without being at enough distance from the camera to the object, resulting the image being inverted. When the Specify Field of View is enabled then V-Ray disconnects the connection between the focal length and field of view and takes the unrealistic field of view value to compensate for the small distance between the camera and the object giving you the correct result without the image being inverted and at the same time match the viewport editor. Unfortunately Cinema 4D doesn't allow us to have separate camera objects instead of tags to use as V-Ray cameras and give us the control over displaying the same results in the viewport and the rendering to avoid such confusion as the current one.
You're getting lower exposure results because the V-Ray camera uses physically based exposure to automatically set the exposure based on object distance and the close to the object you're the lower the exposure would be. Setting the Exposure inside the physical camera from Physical Exposure to No Exposure would disable this effect.

I'm attaching a video showcasing how the the image becomes inverted and how to use the alternative exposure methods to ignore the image being under or over exposed. Let me know if this helps.
Focal Length and Exposure - video

[greymiura]

Hey shaio,

thanks a lot for your kind explanation and the video! But to be honest, I wasn't able to get your points completely. Lenses with a focal length of 400 mm or even more may be special, but they aren't quite exceptional, are they? You'll find them at every sports event. And the sphere is roughly the size of a soccer ball, shot from a distance of about 6.5 meters. Nothing too unrealistic in my view...

And regarding the changing exposure: I always thought that in real world photography you use the aperture value in combination with the shutter speed and the ISO value to get the correct exposure, nothing else. If you measure the light at the location of an object with one of these fancy light meters that professionals use, you'll get certain sets of values for all three parameters regardless if the lens you use is a 400 mm or 35 mm, don't you?

Since I'm very much used to working with the Physical Exposure method in V-Ray and the No Exposure option would mean to readjust all the lights I think it's easier for me to carry on with Physical Exposure and rather do the minor adjustments when changing the focal length. I was just wondering if this was some kind of issue or intended behavior...

[GMTJay]

Changing your camera overrides from Standard to Pinhole will fix the issue with the image being inverted. It's been pointed out that doing so means the camera no longer matches a real-world camera exactly, in case that's important to you.

[shaio]

And the sphere is roughly the size of a soccer ball, shot from a distance of about 6.5 meters. Nothing too unrealistic in my view...

You are right but something felt off when I was looking in to the scene that made me think that the distance is one of the issues and still could be. I'll check the scene once again and see if there isn't something else related to this.

[greymiura]

shaio,

thanks for your reply! Of course I'd appreciate any further research, but this is not an urgent matter. I just feel better when I understand what I'm doing And if you tell me that this is how it's intended to work, I can cope with it.

GMTJay, thanks for this, too! Yes, that's what we were doing in the old days when we needed a one to one match with the editor view or results from other enginges. But in my opinion there isn't any need for the Pinhole overwrite any more thanks to the new Specify Field of View checkbox (I can't tell you how much I love this little checkbox). I haven't used the Pinhole mode recently, but if I remember things correctly there were a lot of shortcomings like a missing vignette option and divergent DOF results...

[deyan.hadzhiev]

Hey greymiura, we investigated the scene a bit further and it turns out the problem is caused by the combination of the changed focal length and the project scale in millimeters. The problem on our side is that when there is no target object and no specific focus distance used for the camera, a default value is used, but unfortunately, this default value does not consider the scene scale. We will improve this behavior by taking into account the camera target distance instead.

Until this is fixed, you can explicitly set the focus distance in the physical camera settings (with a distance greater than the focal length).

[greymiura]

Deyan, thank you very much for your time and effort! That makes sense. Unfortunately, it's not the first time (and not the first render engine) I ran into issues while using millimeters. My apologies for not mentioning it. It's a bad habit of mine - I'm simply not able to think in centimeters...

[greymiura]

I'm not sure if this is a related matter or a whole new topic, but may I ask you for your help once again? Today I started animating a camera that I used for my lighting setup and in the middle of it the exposure slightly changed. After a few tests I realized that the reason was a target tag I added.

You should be able to reproduce the behavior in this little scene:

https://we.tl/t-Qklo6xXCOt

If you start interactive rendering and delete the target tag of the camera the image becomes darker, but why? The focal point isn't linked to the target object and deleting the tag wouldn't change the camera settings anyway, would it? So I assume that this is rather unrelated to the issues above...

Next I thought, well, let's try a workaround and put the camera in a null object. I undeleted the target tag, selected the camera, hit alt-g - and the image got black. Moving the tag to the null fixes both issues, so I think this workaround will porbably do it for the time being. But once again I'm a bit clueless about what is happening...

[deyan.hadzhiev]

Hi greymiura, the issue you observe is again related to the focus distance. We currently erroneously use the target tag object distance as focus distance, regardless of the checkbox for its use in the camera object. We will be fixing this in a future release as well, but until then, you should be able to keep the focus distance consistent with the explicit "Specify Focus" checkbox in the Physical Camera tag, that I showed in my previous post.
Sorry for the inconvenience and thanks for the report.

[greymiura]

Deyan, once again many thanks for your help, you're much appreciated! And I have to apologize for my misconception above; of course the exposure in V-Ray doesn't change with the focal length but with the focal distance and that's obviously the correct physical behavior.

However, wouldn't it be nice to have the possibility to disable this link, for example to keep a consistent exposure when doing large scale focus shifts whithout completely disabeling the exposure options? I understand that V-Ray's Physical Camera is supposed to work physically accurate in many aspects, but sometimes - as with the Specify Field of View checkbox - a less accurate behavior might be easier to use. This would be a point on my wishlist...

[greymiura]

Dear @deyan.hadzhiev

please excuse if I bother you with this topic once again, but I did a few tests in V-Ray 6 beta today and it seems as if the exposure still changes with the focal length. The Specify Focus option of the Physical Camera disappeared, so I assume that V-Ray now conciders the value specified in the C4D camera, but it happens as well if you assign a focus object. Or am I mistaken? This wouldn't be a really dramatic issue but I thought maybe it could be a good occasion to consider it for the final release of V-Ray 6.

And while we're on it - may I ask you to reconsider an option to totally unlink the exposure from the Focus Distance parameter? That would be quite helpful when working with small scale scenes where the changes in exposure can be dramatic.

[shaio]

Hey greymiura ,

The Specify Focus option of the Physical Camera disappeared, so I assume that V-Ray now conciders the value specified in the C4D camera

Yes, from now on V-Ray considers the Focus Distance value in the CInema 4D camera.

And while we're on it - may I ask you to reconsider an option to totally unlink the exposure from the Focus Distance parameter?

As I mentioned and showed in the video in my first post above the exposure is calculated based on the distance to the object. If you want to turn of this behaviour you need to go to the Color and Exposure tab inside the Physical Camera tag then set the Exposure from Physical to No Exposure or Exposure value which will unlink the exposure from the distance to the object. With the No exposure option the exposure of the scene will be controlled by the ISO, Shutter speed, F-Number etc. With the Exposure value selected the exposure is controlled by the Exposure value as well as the iso, shutter speed, f-number, etc. Changing the distance to the object or the focus distance or even when you move the camera shouldn't affect the exposure anymore.
Let me know if this helps.

[greymiura]

Hey shaio

thanks for your reply! Am I getting this so terribly wrong? Here are two examples from a very basic scene. All default values except from the activated Specify Field of View option and the deactivatd Vignetting.



Each image is a simple Photoshop overlay of a view with a focal length of 40mm and another one with a focal length of 400mm and 1/10 of the resolution. The latter is the small frame directly surrounding the sphere and is considerably darker than the 40mm view both with Physical Exposure and Exposure Value. But the camera and object positions and therefore the distances don't change between the 40mm and the 400mm views and I didn't touch the Focus Distance.

This is the (V-Ray 6) scene:

https://we.tl/t-RWm0ivvS4D

[greymiura]

Dear shaoi,

please believe me, I'm so terribly sorry. Honestly. I mean, this is the most basic stuff and I just can't wrap my head around it...

This is mostly unrelated to the topics discussed above: You suggested switching to Exposure Value or No Exposure, but when doing so the ISO parameter gets greyed out on my side and the Aperture and Shutter Speed values don't affect the exposure in the least. I tried pretty extreme values which should result in a 23 EV range.

Here are the examples for Exposure Value



And for No Exposure



Are things really supposed to behave this way? Maybe it's time for me to start looking for another job...

Thanks a lot for your patience & best regards