Look closely at the corn row just above the ruler in the photo. Nearly every plant is about 6.5 inches apart. Assuming that was your desired spacing, it's a perfect stand.
Crop Watch 5/29: How soon will your corn be knee-high?
Now look at the top row near the top of the photo. Even without a ruler you can tell that one pair of plants to the left is closer than 6.5 inches apart. So the stand isn't perfect, but it's still reasonable.
Bob Nielsen, Purdue University Extension corn specialist, spent many years demonstrating the importance of even spacing of corn plants. He found yield losses up to 5% in his tests based on spacing alone.
Nielsen refers to what is called standard deviation. In layman's terms it's a measure of how far off the stand is from being perfect, like the row above the ruler. He says just due to less than 100% seed germination and other factors, the best you can hope for is a standard deviation of about 2.0. That's what he compares other stands to when determining if there is a loss.
To determine how accurate your stands are, measure spacing in 1/1000th an acre – that's 17 feet, 5 inches in 30-inch rows. Actually, Nielsen often measures plant-to-plant spacing in 25 feet of linear row within several rows at several locations. He records spacing in inches between each plant. If you have access to a computer spreadsheet that can calculate standard deviation, you can determine how well your planter performed.
Yield hits get stronger if you start getting standard deviations above 3.0. What it means is that spacing is more erratic than it should be. Nielsen has determined that can affect yield.
Erratic stands can be due to a number of factors, often starting with worn parts or wrong settings on the planter. Nielsen was an early proponent of getting planter units checked and replacing worn or broken parts to improve planter performance. He used his data from his spacing trials to make his point.
How far should plants be within a row? That depends upon row width and desired population. According to the Purdue University Corn & Soybean Pocket Guide, in 30-inch rows at 32,000 seeds per acre, that's 6.5 inches between plants.
Crop Watch 5/25: Sidedressing underway once corn is tall enough
That's about the spacing of the plants above the ruler. This was shot in the Crop Watch '15 field. The farmer was aiming for 32,000 seeds per acre in this part of the field.