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There's been a great deal of discussion over the perceived weakness of the 49ers pass defense. The argument says that the 49ers won't be able to keep up with gunslinging teams like the Patriots and the Packers. I don't think that this will be an issue at all. I think the 49ers pass defense is up to the task of defeating these types of teams.
Let's look at some defensive statistics to show my point:
Points per game
I'd argue that this is the single most important statistic when it comes to evaluating a defense. I don't care if the defense allows 500 yards per game on offense if they can keep that offense from scoring it has done it's job.
The 49ers defense is ranked first in the league with only 15.3 points per game given up. They're followed by the Ravens (15.5 ppg), Steelers (17.4), Bengals (17.6), and Texans (18.1).
Before arguing that the reason the 49ers have such a good ppg average is due to them playing in the NFC West, consider this:
The 49ers have only played one team from the NFC West. They're the only team in the top 5 who's opponents have a winning record (the record of 49ers opponents is 26-24). For example the Ravens have faced teams with a total record of 23-29 and the Steelers opponents record is 24-30.
Passes defended
I think that this is the second most important category in judging a passing defense. A pass defended counts as a stopped play. It's an offensive failure and there's nothing but good about that. The 49ers are ranked #1 in the league. Following at #2 is the New Orleans Saints at 47, the Oakland Raiders at 46, the Houston Texans at 45, and the Baltimore Ravens at 43.
Completion percentage
Interceptions are the gaudy statistic by which to measure pass coverage but they often happen on plays where the defender is gambling on the route. If he doesn't get the interception the route often ends in a huge gain for the opposing team. I'd rather look at pass percentage completion as a more reliable statistic.
The 49ers rank 10th in the league in percentage completion with 58.5% of the passes from the opposing QB being completed. Not bad but not fantastic either.
Long passes
The next thing to look at when judging completed passes is to see how many long passes were completed. Did the defense allow a big play. Sadly the answer is yes. The team is ranked 19th in the league at passes of over 20 yards with 24 given up. 19th isn't awful, but it's below average. It's even worse when we look at completions of over 40 yards. In this the 49ers rank 29th in the league. Only the Broncos and the Vikings are worse. However, I don't think that 40 yard passes are a true indicator of a team's defense, since there's at best a weak correlation between 40 yard passes and team record.
Sacks
The 49ers are ranked 7th in the league in sacks with 21 on the year. They also have 30 QB hits and 98 QB Pressures (according to Pro Football Focus). That's pretty impressive.
Interceptions
The 49ers are tied for 7th in the league in Interceptions with 9.
Scoring
The 49ers are tied for 11th in the league with 10 passing tds given up. That sounds bad at first, until you realize that the 4th ranked defenses are the Washington Redskins and the New York Giants with 7 passing tds. In other words there's only 3 passing tds separating the 49ers from being 4th in the league in passing defense.