Here’s something that isn’t new to you: It’s difficult to hit a home run in San Francisco. It’s difficult to hit a home run in any ballpark, but, in particular, San Francisco makes it tough. This information is well understood.

So why don’t we freshen things up with a twist? Not only is it hard to hit a homer in AT&T Park — AT&T makes homers more hard than Coors Field makes them easy. That’s clunky, so to put it a different way, AT&T has the most extreme home-run park factor in the league. The most extreme of the pitcher-friendly places, and the most extreme of the hitter-friendly places, just speaking in terms of distance from the average. We all know the ballpark holds most fly balls, but while we were paying only indirect attention, AT&T became the toughest dinger ballpark in 25 years.

This is based on FanGraphs’ own park-factor calculations, and here’s a glimpse of the Giants’ historical year-to-year home park factors. The way these work is that 100 corresponds to the average, and any number under 100 means the park reduces that statistical category. That page shows plenty of different factors, but the one we’re interested in here is, of course, the “HR” column. You can see that homers have been tough in San Francisco ever since they moved into their then-brand-new yard, but that 84 factor has stood up four years in a row. A home-run factor of 84 is a remarkably low home-run factor.

Last year’s highest was 113, a tie between Coors Field and Great American Ball Park. That’s 13 points from the average, while AT&T comes in at 16 points removed. Second-lowest, Marlins Park comes in at 88. It’s not like San Francisco is the only place that makes this tough, but that’s the only park I’m going to put in some context.

We have these numbers on FanGraphs going back to 1974. Since then, the only teams with lower home-stadium homer factors were a bunch of Astros clubs from the 70s and 80s. The Astrodome, in turn, developed a pitcher-friendly reputation. Other than those Astrodome years, which had homer factors ranging between 76 and 83, the lowest home-run factors we have are 84. It’s a tie between present-day AT&T Park and Royals Stadium from the years 1990 – 1991. That’s the most recent comparison. By our own math, AT&T now is the most homer-suppressing park in two and a half decades.

It might seem a little bit strange, at first thought, that AT&T has gotten tougher. The stadium itself hasn’t changed very much. The home-run factor used to bounce around closer to 90. But then, it’s worth remembering that all of this information has to be considered relative to the average, and it’s the average that’s changed. It’s the ballpark context in which AT&T has existed. It’s true that Coors Field made a small tweak to try to cut down on a few dingers, but on the other hand, Citi Field has moved in the fences. Safeco Field has moved in the fences. Marlins Park and Petco Park have moved in the fences. Progressive Field underwent renovations that caused certain balls in the air to take off. One by one, pitcher-friendly environments have adjusted themselves, mostly in the name of fairness. The greater home-run landscape has changed, while San Francisco has stayed where it’s been.

And so we can say this: Over the past four years, there have been 446 home runs hit in Giants home games. That ranks 29th out of 30. At the same time — or, I guess, not exactly at the same time — there have been 650 home runs hit in Giants road games. That ranks 13th out of 30. Just based on these very basic numbers, AT&T has reduced homer output by 31%. Marlins Park is at 26%, with PNC and whatever Oakland is calling it now tied at 20%. This is, yet again, just detail to cement what you already knew, but this is where the home-run spike has gone to die. League-wide, home runs were showing up in modern-day record numbers. Not so for the Giants, not half the time. Overall run-scoring has been much more reasonably reduced, and it’s clear the Giants have been able to survive, but to an increasing extent, Giants home games have featured a different sort of baseball.

Brandon Belt might have my favorite relevant splits. Like many players, Belt has performed better at home, with a career .363 wOBA. In all other ballparks combined, he’s posted a wOBA of .347. However, in San Francisco, Belt has knocked 26 dingers. Elsewhere, he’s knocked 54. He’s had double the rate of homers per fly ball in other environments, and here’s a glimpse of all the batted balls in the air, from Baseball Savant:

Both of those are overlaid onto AT&T, even though, obviously, the road games haven’t taken place there. You can see where Belt’s would-be homers have suffered around Triples Alley, but he’s been productive — more productive! — nevertheless. Psychologically, I’m sure it’s been something to get used to, since hitters want to chance to just go for a jog, but by now I imagine Belt knows he’s hit just fine at home. He’s just had to sprint for it a little more. The Giants probably talk about this a lot.

Home runs are only one part of the action, and as mentioned, AT&T isn’t overall way too extremely pitcher-friendly. It does keep runs down, but no more than Los Angeles, San Diego, or Anaheim. Even the parks that have moved in the fences have, in turn, cost themselves non-homer base hits. Park factors are complicated, and not an easy thing to selectively change. The thing about AT&T is just that we haven’t seen a homer-suppressing park like this in a while. Not by FanGraphs numbers, not relative to the greater league average. It can cause certain Giants hitters to feel underrated, just because the power numbers might not be there. For the most part, they’re there. You just have to think of them right. It’s a slightly different baseball the teams get forced to play.