Dome Light Background, Sensor Size, and Focal Length

[GregBollella]

Maybe better to post to RS forum but thought I'd try here first.

Using an HDRI as a background for a dome light I want to have a close interior shot of a scene but through openings to the outside I'd like a much wider angle-of-view.

To get the interior and exterior in such balance I would need a focal length of about 20 mm. However, this creates, as one would expect, distortions in the interior parts of the scene. I see that changing the angle-of-view simply changes the focal length.

What seems to work to increase the first (x?) value of the Sensor Size until the scene has the proper view of the HDRI and the interior. Doing so does not change the focal length so no, I believe, interior distortions.

My question: is changing the Sensor Size a proper way to accomplish the effect or is there another way?

Thanks,
Greg

[Dr. Sassi]

Hi Greg,

Let me get the practical stuff out of the way; I have shot for nearly 25 years 360ºs. So I might go deeper into this than you might want, but some points need to be clear to get things right.
Besides, I have shared most of this in a few series. Here is the panorama trailer for the series.
https://youtu.be/qUQLeV6sATo

The focal length is often seen in conjunction with the sensor size, as both create a result. The optical (or its simulation) measurements are the same. The projection size of a lens construction makes it usable for different sensor sizes while keeping the focal length the same, for example. How large the sensor determines what is used from that lens. What is used from the provided lens results in the Field Of View.
In 3DCG, we typically have very little to do with the limitation of Glass. In the real world, there are quick limitations, like lenses for smaller sensors often have a smaller projection area and cover, so no full frame or larger formats.

You mentioned a 20 mm lens, and in Cinema 4D, the Ssensor size can be pretty large, capturing more and more of a scene while increasing it. Since it is a perfect Rectangular (vs. Spherical) result of the scene, the wider you get, the more distortion will be introduced as you look more tangential than straight to it.

Besides, the only point from which the 360º HDRI Panorama is not distorted is from its shooting point, which is precisely in the "Sphere's" center that the image describes. If you have a small room showcased in the image, moving away from that point will change the result drastically with landscapes that is less the case, as things are more distant.

As a rule of thumb, as long as the subject nor the camera moves, the perspective lines will not change. The only exception is a spherical lens (Fisheye) or if shot with a panorama head and then stitched. Here straight lines are then shown as curves.

I hope that lays the basics.

I have real trouble understanding the idea of
"Using an HDRI as a background for a dome light, I want to have a close interior shot of a scene, but through openings to the outside, I'd like a much wider angle of view."
while keeping things correct. That is impossible, and you must fake it - with some risks.

If you do not move the camera, the relation to a window and its opening will not change, no matter the settings. If you like to see more from the "Outside," the only way is to move the camera closer to the opening.

Besides that, there is only faking a wider view by setting up a tile of the HDRI, showing it twice for 360º, but that might look wrong and awkward. There is no option in Dome Light to Tile, so it needs to be on a larger sphere, to get there.

All the best

[GregBollella]

@Dr-Sassi Based on the information in your response I see that 100mm FL with a large sensor size is about the same, wrt, distortion as a 20ml FL with default sensor size.

Below is a link to the c4d file on DropBox. Hopefully, I have the permissions set ok. LMK.

https://www.dropbox.com/s/efq5dkzpifm6pn9/AbstractPastels48-Mar23ForCV.c4d.zip?dl=0

I have, in other projects, used a very large sphere with an abstract texture on the sphere. A dome light to illuminate it. This gives me more control of the background image depth/size by moving the scene objects, the camera, or changing the size of the sphere. Although, I have only used this technique for abstract backgrounds not image HDRI. Might give that a try.

Thanks, as always!

Greg

[Dr. Sassi]

Hi Greg,

Thanks for the file. No textures. (Save With Assets…)

You can always use an EXR file from the Asset Browser, and then there is no need to include it. (I typically avoid Radiance/".hdr" as they are not real 32/bit float images nor have any color-management options.)

I checked your file, and if the image is just for the background, perhaps compositing might be an alternative.

You have not switched on Depth Of Filed (It's called Bokeh in the interface, but that is misleading).
If you work with DOF, then focal length (and sensor size ratio) matters. As with photographic lenses (vs. Film), the expression is in fracture (hence the F in the lens). This is a front element vs. relative aperture ratio. This means a short lens has a smaller aperture "hole" than a longer one with the same Aperture number, e.g., F/2.8. The larger than "hole" is in the Aperture, the more blur in out-of-focus areas is to be expected. Hence, many pro photographers use middle (or large) format cameras, not full frame or crop sensors.

Image: A/B of a 26 and 100mm, while the "sensor" is adjusted to have the same field of view.

All the best

[Dr. Sassi]

Hi Greg,

Finally I can talk about. Please have a look at the new feature in 2023.2!

https://www.maxon.net/en/article/spring-2023-maxon-one-release-adds-extraordinary-value

Redshift
Custom Backplates are now available in Redshift for C4D, offering amazing compositing control. Artists can choose whether background footage is incorporated into the render or added before and after post effects. Renders can be matched to an existing backplate thanks to the Post-Composite mode.

Perhaps worth a shot, or two.

Cheers

[GregBollella]

@Dr-Sassi Yes! I saw that demoed by Elly on Thursday and incorporated it into my current work. Excellent feature!

Greg

[Dr. Sassi]

Thanks for the feedback, Greg.

Another thank you for watching the show!

Enjoy your weekend