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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: enter image description here Orthographic View: enter image description here 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: enter image description here 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:
enter image description here

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:
enter image description here

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:

https://www.dropbox.com/s/spr839j2g3phti4/simple%20face.blend?dl=0

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.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Robert Gützkow
    Commented Jul 3, 2021 at 9:41
  • $\begingroup$ If you want to sculpt in ortho, sculpt in ortho-- but you'll get your "monster problem" when rendering the object in perspective, which is generally how you'll be rendering. Using 1000mm focal length is basically the same as ortho, and if you want to do that, you might as well use ortho. Focal length you should use is that which lets you sculpt something that looks right in a scene. That is highly personal, and depends on how you're imagining your 2D screen as behaving. Recommended: put some scenery in, adjust focal length to look right for the scene, then sculpt in that focal length. $\endgroup$
    – Nathan
    Commented Jul 3, 2021 at 15:18

2 Answers 2

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I'm not offering solution, I just didn't want to extend already long commentary. Solutions will be (probably) answered by authors of comments.

How to set the focal length of the viewport to be 1000 mm?
Viewport Focal Length is natively limited to 250 mm (only Camera can be set higher). The limitation can be found in source/blender/makesrna/intern/rna_space.c on line 4120:

prop = RNA_def_property(srna, "lens", PROP_FLOAT, PROP_UNIT_CAMERA);
RNA_def_property_float_sdna(prop, NULL, "lens");
RNA_def_property_ui_text(prop, "Lens", "Viewport lens angle");
RNA_def_property_range(prop, 1.0f, 250.0f);
RNA_def_property_update(prop, NC_SPACE | ND_SPACE_VIEW3D, NULL);

I didn't get it - why to use such high value (1000) for sculpting? You already noticed that model is not seen naturally (like seen by human eye). View style is close to orthographic, that is understand by brain as "distorted" since brain is expecting perspective deformations.

It is like to sculpt a Cube in ortho to look natural to eye (it means with perspective). You can get such result from one point of view, but since you change view, you immediately see it is not a Cube anymore.

Probably FOV doesn't matter while sculpting, because your brain adapts to current deformations thanks to orbiting around your model and just recognise what is correct from previous experience.

FOV matters when sculpting from photo as reference (like used as overlay). In this case you have to match blender camera with parameters of real camera used to capture reference (photo).

Sidenotes:
Switching from perspective to orthographic automaticly for Num 1, 3, 7 can be disabled in User Preferences > Navigation (of course it doesn't solve your issue with limited FOL in Viewport.)

enter image description here

Also to keep observation object by Camera - enable Lock Camera to View ... so you can orbiting and zoom while sculpting with desired FOL. Like that you don't have to care it is not Viewport view.

enter image description here

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  • $\begingroup$ "Artists around the world" - I have a similar feeling about this, though it's Argumentum ad populum. Perhaps OP is actually right that 1 m focal length should be used. If not, he still might have a legitimate personal preference. $\endgroup$
    – Markus von Broady
    Commented Jul 3, 2021 at 9:59
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    $\begingroup$ @MarkusvonBroady Thanks for my "no-answer" extension :) WIth Argumentum ad populum you opened another Pandora's box :) I tried to edit answer to avoid ... but easy to fail into another argument fault :) BTW no problem with OP personal preferences, but with presented arguments in sounds like sculpting with current setup results in renders or 3Dprints deformed ... that is something I can't confirm from my experience. $\endgroup$
    – vklidu
    Commented Jul 3, 2021 at 18:53
  • $\begingroup$ Markus, I believe that you are an independent thinker. You may be really good at scientific research, which always need scientists to cast doubt upon the mainstream belief and persistent to seek truth. Regarding to this fantastic answer by vklidu, I feel glad, but I will have to hold my judgement right now until I pay my money to 3D print out that sculpted head to see if there is any shape difference between 100 mm perspective view and the actual shape came out from the printer. $\endgroup$
    – Blender Lake
    Commented Jul 3, 2021 at 20:08
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This doesn't address the question of manipulating the focal length, but it hasn't yet been mentioned that 3D printing isn't necessary to solve the ultimate mystery here: just use the reverse, that is 3D scans.

This sample is from embodi3D, and the Y dimension is 21.5 m. The picture on the left shows the orthographic view, the middle shows camera view with 50 mm focal length and about 27 m from the surface, and the right shows camera view with 250 mm focal length and about 160 m from the surface.

enter image description here

This illustrates why I always use orthographic view for modelling, no matter how many opinions are against it.

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