Comparing isometric with cabinet oblique for an object whose front face carries several circular holes, cabinet oblique
Answer: A
Keeping the front face parallel to the plane of projection leaves circles on it as true circles, which saves a great deal of ellipse construction. The penalty is a less natural appearance and a depth that must be halved to look right.
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In a true isometric projection, the outline of a sphere is drawn as a circle whose diameter is
Answer: A
An orthographic projection of a sphere is a circle of the sphere's own diameter whatever the direction of view, since the silhouette is always a great circle seen face on. So the sphere is the one feature of an isometric projection that is not foreshortened, every isometric edge being reduced by the 0.816 factor.
In an isometric drawing, where all lengths are set out enlarged by
\[ \frac{1}{0.816} \;=\; 1.225, \]
the sphere is drawn at 1.225 times its true diameter so as to match the enlarged edges around it.
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The ratio of isometric length to actual length of an object is __________.
RSMSSB JE - 2025, RSMSSB - 2025,
Answer: C
The ratio is 0.816.
In an isometric projection the three axes are equally inclined to the plane of projection, so every edge parallel to them is foreshortened by the same factor:
\[ \begin{aligned} \frac{\text{isometric}}{\text{true length}} &= \sqrt{\frac{2}{3}} \\ &\approx 0.816 \end{aligned} \]
This is why an isometric scale is drawn with lengths reduced to about 82 per cent. 1.23 is roughly the reciprocal of 0.816, the enlargement you get by drawing true lengths instead, and 0.5 and 2.0 are not related to the isometric geometry.
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