I never actually know about this, it is almost always as long as it looks right, but is there an actual ratio ?
From the info on the Net, average diameters are:
eyeball - 24mm,
iris - 12mm. Plus/minus up to 2mm.
Can’t find an actual source though like an encyclopedia or something, sorry
If you start measuring and looking for similarities between ratios and analogies among different parts of the body. You will be amazed on how many of them you can spot.
As for example if the large rect in the image is scaled as [1.0], then this means that the iris is [ eyeball_perimeter * (0.618*0.618) ]. You mostly use 0.618 to scale Phi downwards or 1.618 to scale Phi upwards. However it took me 4-5 trials and errors to get the proportion right or at least to a point where it looks OK.
There might be hundreds of thousands of combinations that are undocumented. However you figure out only those that are useful for you and can remember and document. There are indeed some plain and generic lists or templates you can follow (such as 7-heads tall characters), but in 90% of chances they can’t cover all of the possible situations.
0.618 shows up again…mind blown.
Yeah is really cool, however the only downside is that 1.6 measurements are more difficult to get and require hard measurements.
On the other hand 1.5, is much more easy in design terms, because you can easy eyeball it in half (0.5) and get results faster on the fly. (No pun intended)…
Also it is a bit of controversy whether or not 1.5 or 1.6… make perfect design in terms of results. However indeed 1.6… is superior in terms of accuracy (referencing the real world) but 1.5 is faster and more efficient in terms of usage.