Comparative Measurement does not equal one-to-one.
All drawing from life involves comparison. When in Sight-Size that comparison is direct. When scaling via Comparative Measurement the comparison is indirect. Both skills are important for you to learn but it does matter in which order you learn them.
What is Comparison?
To visually compare one thing to another is to consider the similarities between them. And that’s exactly what you do when drawing what you see, be it from life, from another drawing, or from a photograph. Notice that the definition is fairly broad. Nothing specific is mentioned about scale, proportion, or even intention.
Direct Comparison in Sight-Size
Before we get to the scaling aspects of comparative measurement, let’s look at the Elements of Sight-Size diagram once again. If you’ve not seen it before, first head over to the What is Sight-Size? article.
2. Consistent Vantage Point
When your subject and artwork are visually side-by-side in Sight-Size, from the proper vantage point you can see both in one glance and in a one-to-one relationship.
When comparing in Sight-Size, the comparisons are direct: one-to-one, this equals that. As you can see in step 3, the source and the drawing of it are the same visual size. It is only the Sight-Size setup that allows this kind of direct comparison.
A Proportional Sum
The comparisons made when using Comparative Measurement are not one-to-one. Instead they are a proportional sum: this does not equal that. Instead, this might equal one-forth, plus one-forth, plus one-forth, plus one-forth, and so on. Comparative Measurement puts a double demand on your cognitive abilities because you must concurrently hold both scale and accuracy in your visual memory.
Of course proportion is equally important when in Sight-Size, but it’s of a different type than the scaled relationships of Comparative Measurement. Rather, Sight-Size is all about direct relationships. Those who use Sight-Size pay very close attention both to the whole and the relationships of the parts to it. For example, the overall width of your drawing should match that of your subject and so should the widths of its constituent parts. See the Intervals article for more information about that important subject.
When scaling your artwork in Comparative Measurement each interval has to be accurately scaled as well.
The Elements of Comparative Measurement
2. An Initial Size
On your artwork, choose an initial size for the common measurement so that you can proportionally draw it.
1. Determine a Common Measurement
Comparative Measurement often uses at least one common dimension. For a figure, this tends to be the size of the head. In the example above, the figure is three heads tall. The common dimension aspect is why Comparative Measurement is also known as Proportional Measurement.
2. Decide on an Initial Size
To begin the drawing you would arbitrarily decide what size to make the common dimension. This step is where comparing dimensions in Comparative Measurement deviates from comparing in Sight-Size.
To fit the entire subject on your paper you will have to scale the common dimension. The last step is to draw, to that scale, using the common dimension as well as increments of it.
The Domino Effect
Small errors grow into large ones before you know it. That happens with all forms of drawing and is a part of learning to draw what you see.
Comparative Measurement depends upon a scaled proportion so each new dimension relies on those which precede it. That’s an issue because an unnoticed mistake from a few days ago will negatively affect everything you do from then on. The trouble is noticing those mistakes early enough in the process to make the corrections before the dominoes fall.
The ideal, of course, is to never make a mistake when scaling. But the reality is beginning students always make mistakes, and seeing them in a scaled format is difficult. This does not equal that. Instead, this equals a proportional sum of that.
So, which part of the sum is in error?
Conversely, the one-to-one comparison made possible through Sight-Size eliminates the problems of scale. Errors are more easily seen and if they go unseen their aftereffects are less tragic because today’s effort does not depend entirely on yesterday’s.
Comparative Measurement by sighting. From the Elements of Figure Drawing (1903).
Some artists have mensuraphobia, which is the fear of measuring. But all visual comparison, whether in Sight-Size or through Comparative Measurement, does involve some form of measuring. That measure may be by the naked eye, or it may be through mechanical measurement.
In either case, the higher goal is to learn to use your naked eye and to always make use of the guess and check.
Due to its arrangement, doing that when in Sight-Size is straightforward because your eye registers one-to-one comparisons quite easily. The trouble is that your eye also suffers from a bad case of cognitive bias (if you drew it, it must be correct!). In fact, much of learning to see is really about mitigating that bias.
Given that I am a firm believer in the benefits of Sight-Size it might seem surprising that I also teach Comparative Measurement to my students. However, Sight-Size is always first. Only after the student demonstrates skill in the rudiments of seeing through Sight-Size do I begin to incorporate scaling. On the whole it’s not an either/or proposition. Rather, it’s a question of order.
If you would like to learn how to accurately draw what you see, the best way is through Sight-Size. And you can begin right now with a free guide! Then, when you want to use that trained eye to accurately scale using Comparative Measurement, check out the course book, A Comparative Eye.
There is much more about Comparative Measurement in the article Train Your Comparative Eye.