When working with technical illustrations, you must consider perspective. The following article will provide you with some useful information on working with both parallel and true perspective.

Every technical illustration is done in a perspective. Usually, you will employ the perspective that best suits the information you wish to communicate within the depicted situation. Individuals who are new to Technical Illustration are confronted with a great number of questions when they must decide on the appropriate perspective. Quite often, they try to depict a body with as much realism as possible. Therefore, they choose a vanishing point perspective. 

Of the vanishing point perspectives, the true perspective is usually the one you already know from reality (e.g. from a look out of the window). All extensions of an object's edges meet in three points. The following picture shows the directions of the three vanishing points.

 
This kind of representation has a very realistic effect, but is associated with various problems at the same time:

The longer the object is, the smaller it becomes in the background
The intention of a technical illustration is to convey easy-to-understand information about a specific circumstance. An object that becomes smaller in the background does not allow the illustrator to estimate its proportions. This often causes elements that are located further behind the object to become unrecognisable. A good example of this is the hindmost tire of a truck when looking at it from a front perspective. Furthermore, you can only assume that all tires are actually the same size.

Creation of a true perspective is difficult
When creating a true perspective manually, you cannot work with real measures. You have to project every single measure you wish to employ into the perspective and the effort is disproportionate to the result. In various positions, ellipses have different diameters, values and angles.

It is not possible to re-use pre-existing objects
All elements (e.g. screws, tyres, etc.) have to be drawn anew for each view. Since the measures of all elements change with their views, you cannot re-use an object you have drawn before.

Apart from the true perspective, the parallel perspective is certainly the most efficient method of depicting an object spatially. In a parallel perspective model, you can employ the real measures. This implies that you can take these measures directly from technical drawings. Since the parallel perspective does not become smaller in the background, all elements can easily be recognized and visually compared with those in the foreground. Formerly created elements can be re-used when they are in the same installation position. The most well known parallel perspective is isometry. This variant offers the additional advantage of allowing the elements to be used in other views by rotating them 120°.

Conclusion
While true perspective is pleasing visually, it is less suited to technical illustration than technical advertising. An adequate field of application for a true perspective would be the colored cover page of a technical manual. The contents of this manual, however, are meant to convey technical details in an understandable way. For this, the parallel perspective is the most appropriate method. Always keep the legal aspect in mind: you are also liable for the information in your product's documentation and should therefore create the illustrations in a clearly recognizable way.