I think full sun means that the sun shines on the plants.

"which stops the sun from shining directly"......

I think you are describing the condition of shade. There are degrees of shade, from partial to total. Except perhaps in the far south of the country, one doesn't want any degree of shade on tomatoes.

I don't know about that...my neighbor has some trees blocking direct sun from her tomatoes, but hers are a LOT bigger/more productive than mine, which are grown in full sun.

Well what pn brown said is true....but...here in TX and other places in the south....our afternoon soon is so brutal that shade is a good thing...I have my garden...under a tree....At the time it was the only place to put it...but then when I realized that it was shading the killer late afternoon sun...I left it there...a little shade doesn't hurt if you get enough sun otherwise..

I think I read in a horticulture book that plants (full sun type) only need 4000fc for photosynthesis and that a summer day at noon in a lot of the US is more like 10-20k FC. I may be remember this wrong, I am a bit of a newb. But if I am remembering that correctly it seems like what might look "shaded" might still be fine for "Full sun" plants...I keep meaning to see what a light meter costs so I can see how bright all this is....

I believe the recommendation for growing tomatoes is a minimum of 6 hours of direct sunlight per day.

I regard as "sunlight" only when the sun is shining directly on the plant, or with high-efficiency reflection, such as from a mirror or one of those mirror-fabrics (sort of like shiny tinfoil) they make for garden purposes like increasing light.

I realize that planting against a white wall will create a microclimate of greater light on the plant than if it were planted against a black wall or just standing alone in a field. I've been using that principle to my advantage with sage and sunflowers.

There's light shade, partial shade and deep shade. To me, light shade is heavily dappled with sunlight, like sunlight through a tree that doesn't have a dense canopy. Partial shade means that the plant gets less than 6 hours of sunlight shining directly on it. Deep shade means the plant isn't even getting dappled sunlight, because the tree canopy is too dense, or the sun is blocked by a solid barrier (like a building). A plant can have both light and partial shade, or both deep and partial shade - like if it has 4 hours of direct sunlight shining on it and the rest of the day it has most of the sun screened by a tree.

One of the things I did for my garden area one sunny weekend was make a drawing of the garden plot, copy it 12 times, then sketch in the shade lines every hour. That way I could tell exactly how much sunlight a particular area got on a sunny day, at a certain time of year. I repeated it a couple other times in the season and now I keep those sheets in my garden notebook. It led me to abandon a bed that hadn't been doing well because I figured out why - it was fully shaded at the base, so the vining crops I kept trying to grow there weren't making it up to the sun. Interestingly the morning glory weeds were doing great there, so now I grow morning glories there. Maybe next year I'll plant asparagus there, because it can tolerate shade, especially at first, until it gets tall and ferny (which would be when it was in the sun).