One of the biggest skills you need when drawing is active seeing. The idea is to see and evaluate visual relationships.
Rather than simply looking at the subject, you should pay attention to the size, tone, colour, scale and angles of your subject. By doing all these, you can increase your general observational skills and this will have a huge impact on your drawings.
Also, it is recommended to compare size relationships. You can ask questions such as ‘How far is the object from another?’ and ‘How tall is one object in relation to another?’ and so on. Override the prejudices that you will have about a subject and observe what is in front of you.
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