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In risk of being off topic for broadness and discussioness... but I think in the end it's a question that can't be avoided.

Can someone please finally explain the difference and why we need both?

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  • $\begingroup$ A little info on the two modes - Mode: All modes, Hotkey: 5 (numpad), Menu: View » Perspective / View » Orthographic Perspective and Orthographic Views $\endgroup$ – user1595 Jun 6 '13 at 9:54
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Orthographic is commonly used in engineering as a means to produce object specifications that communicate dimensions unambiguously, each line of 1 unit length (cm, meter..whatever) will appear to have the same length everywhere on the drawing. This allows the drafter to dimension only a subset of lines and let the reader know that other lines of that length on the drawing are also that length in reality. Every parallel line in the drawing is also parallel in the object.

If you are looking at a larger scene with buildings then orthographic rendering gives a clearer measure of distance between buildings and their relative sizes.

With perspective mode lines of identical real-world lengths will appear different due to fore-shortening. It becomes difficult to judge relative dimensions and the size of objects in the distance.

While modelling nothing stops you from switching between orthographic and perspective mode to get a better sense of the object.

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  • $\begingroup$ Ok this makes sense as to why it does not look so 3d in orthographic mode and that seems good for ui to me... $\endgroup$ – CrandellWS Nov 17 '17 at 18:04
  • $\begingroup$ In addition to the difference between the two in general, there's a practical difference specific to Blender where the orthographic mode allows for background images at views along an axis (functionally similar to a CAD program), while the perspective mode, at the same views, doesn't. $\endgroup$ – thepufferfish Mar 23 at 1:00
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Orthographic is (according to Wikipedia):

... a means of representing a three-dimensional object in two dimensions...

Bascially, Orthographic is a fixed depth. You can't zoom in or out. There is no distance from the camera.

Perspective has depth. You can zoom in and out. You can judge distances. Cameras can be distanced from the scene.

Simply put, orthographic is what would happen if you squished everything in the scene (along the cameras view axis) and stuck it right against the camera.

Perspective and Orthographic Perspective on the left, Orthographic on the right

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  • $\begingroup$ In essence, pseudo rendering? $\endgroup$ – JNF Jun 5 '13 at 15:53
  • $\begingroup$ Kind of. Basically it flattens everything to the camera. $\endgroup$ – CharlesL Jun 5 '13 at 16:06
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Orthographic view is a view that is not real world. It virtualy displays objects, such as a cube, in a way that it can be seen with out a paralax error. Paralax error is where you can see an object from a certain angle and it look like it is the same height as another object. Optical illusions take advantage of this concept. To clarify, if you were to stare at a clock in orthographic view you would see every thing lined up perfectly, no paralax error, you can see clearly, exactly what time it is. If you looked at it from Perspective you would be able to see under the hands, and if look at from a different angle other than head on you would see a different time other than what it really is.

So Why do we need those views?

Well, say you are creating an architectural visualization, and you needed to ensure that two sides of this building were exactly the same height. If you were in perspective you could look at it from any number of angles and it would appear even. But if you were in Orthographic you would be able to see it without, parallax you would see it straight on virtually no dimensions except for up and down, and left to right. This way you would be able to see that oh, they are not really lined up, or that they are. Hope this helped!

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Orthographic - This is without perspective distortion. I use this when modeling. You can model more accurately, and align things in the scene easier.

Perspective - It will change depending on the 'focal length' you use for the camera when rendering or during animation, so this is best not used when modeling, but it can help you get a feel for the object while modeling.

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  • $\begingroup$ This is true for many things, however one could argue that it is better to model in perspective view because that is how the model will be seen (usually) in the final render - thus allowing you to judge appeal, silhouettes and proportions the same way they will be perceived by the viewer. This mostly applies to character work though, I'm sure orthographic is much better when it comes to architecture and prop modelling. $\endgroup$ – Greg Zaal Nov 17 '14 at 12:19
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Ok. It is pretty much everyone said but with an example. (Correct me experts if I am wrong).

Imagine you are standing on a middle of the Train Tracks facing the direction of the track and looking as far as you can on the origin of Track. (Imagine without train as well :) ).

Track looks like it is coming from a single point in your PERSPECTIVE. (http://www.animationbrain.com/one-point-perspective.html)

But Orthographic(ally) they are parallel lines as everyone knows.

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