“table of my heart; My body is the frame”
'Table' here means a flat writing or painting surface — a tablet or panel prepared to receive an image. The heart is imagined as a physical board on which the beloved's portrait has been painted, and the poet's body is the wooden frame around it. 'Table' in this sense is standard Elizabethan usage; the modern 'table' as furniture is a later narrowing.
“perspective it is best painter's art, For through the painter must you see”
'Perspective' carries two senses at once. First, it names the painter's art of rendering three-dimensional depth on a flat surface — the technical skill of foreshortening and vanishing points. Second, it names an Elizabethan optical device: a glass or anamorphic painting that looks distorted head-on but 'corrects' into a clear image when viewed from the right angle. The point is that to see the poet's true skill — and the youth's true portrait — you must look at the painter, not past him.
“Yet eyes this cunning want to grace their art, They draw but what they see, know not the heart”
The couplet is the sonnet's volta — its turn or reversal of argument. After ten lines praising the poet's eye as a gifted painter, the closing couplet concedes that eyes, however skilled ('cunning'), can capture only what is visible; they 'know not the heart.' The inner life of the beloved remains opaque. 'Want' means 'lack' (not 'desire'), so the line reads: 'eyes lack this skill to complete their art.' The compliment to the youth's beauty is real, but the sonnet ends on a note of inadequacy and uncertainty.
historical The sonnet form