This thread has been updated on 15/11/2021. For the detail of the update, please check the end of this post.
Many YouTube video tutorials said that you should sculpt organic creatures in the Perspective mode, rather than the Orthographic mode. But when sculpting a face, the default 50 mm definitely has a lot of lens distortion, however, so does 100 mm. Here is a comparison between the 100 mm Perspective view and Orthographic View. In this example, the head is entirely sculpted in the perspective view with focal length set to 100 mm, where the size of the head and proportion looks quite normal.
100 mm Perspective view: Orthographic View: However, the Orthographic view shows a great "distortion" on the exact same head. Here, I put a quotation mark on the word distortion, because I believe that this is not distortion, but actually is the physically incorrect proportion - the beautiful face is an illusion but it is actually and physically a monster or stupid face.
Why is the physically incorrect proportion, rather than lens distortion?
2000 mm Perspective view and the distance 317 meter between the camera and face: The last screenshot shows that the severity or the level of the "distortion" in a Perspective view is approaching the severity of the "distortion" in the Orthographic view. This Perspective view is generated by a customized camera with focal length set to 2000 mm, and the location of the camera is set to 317 meter away from the head. We know that in real life, if we use a telescope with a long focal length to observe a face 317 meter away from us, we should not observe such great "distortion" in real life! Therefore, the model has physically incorrect proportion, rather than lens distortion.
To further prove that sculpting in the perspective view will result physically incorrect proportion, we need to use the technology of 3D printing. I do not have a 3D printer. But if we really 3D-print out this head, I expect that this head will look like the shape in the Orthographic view, rather than the shape in 100 mm Perspective view. If you have a 3D printer, I highly recommend you to try and observe the proportion.
The article by Anton Orlov in professional photograph provides a chart, which indicates for a portrait shot the ideal focal length with respect to the film format:
I have no idea what film format is used internally for the viewport of Blender. If we take the size of the viewport as the film format, then this film format could be above 4 inch by 5 inch. so for a portrait about head and shoulder, according to the chart, the ideal focal length should be at least 300 mm. Following this chart, Anton has also demonstrated the lens distortion for using the focal length 180 mm in comparison with 265 mm:
So, does this mean that I should set the focal length of my viewport to be 300 mm to sculpt a face?
I want to say "300 mm is not enough". Since the 300 mm is used for a portrait about head and shoulder, not used for a portrait about anything smaller than that. When I sculpt a left eye socket as an example, I have to zoom in even more, so that the entire viewport becomes a portrait about the left eye socket, then the ideal focal length could be well above 1000 mm.
Why not just use the Orthographic View to sculpt the face?
The Orthographic View makes everything flattened, which is good for most cases, however, in sculpting some specific parts of the body or face, such as cheek bone and eye temple, I want to see a little bit of depth in order to sculpt them well. But the Orthographic View can not provide any depth of the image.
Now, back to the question:
Blender viewport does not allow me to set up the focal length higher than 250 mm, then how to bypass this limit, so that I can set the focal length to be 1000 mm for sculpting please?
Thank you for reading!
Update on 15/11/2021:
Initially, I want to hire somebody to 3D print out the head mesh to check if there is any shape difference between 100 mm perspective view and the actual shape came out of the printer. However, 3D printing in my country is an expensive service, and I currently have a trouble to afford this. So to those who possess a 3D printer and feel curious, here is the original Blender file with the head mesh:
No matter whether someone could print this out or not, for the purpose of my own research, I plan to print this head mesh out in the later months of the next year.